Cable Trunking

To put cables in such trunking one normally takes the lid off , lays the cables in and replacs the lid , but it is possible over short distances or straight length to pull the cables in as one does with conduit . WHichever method is adopted , the number of cables and sizes of trunking must be such that no damage is caused during installation ,. The same considerations apply as in the case of conduit .
  Being so mucj larger than conduit , trunking can quiete clearly not be buried in the walls of a building . It has to be run on a surface . THere are occasions when there are other services such as heatingm pipes or gas . THis is one situation in which cable trunking is an ideal way of installing the wiring . It can be similarly used in false ceilings . In both these cases , there must be sufficient doors or access traps to enable electricians to reach the trunking for rewiring .
  Buildings such as assembly halls and gyms often hae exposed steel lattice framework supporting the roof . It is then possible to run cables trunking neatly through the spaces of  the lattice . Sometimes the architect will permit the cable trunkiing to be fixed under the beams and along them . Either method is simpler and neater than fixing several conduits parallel to each other on the surface of the ceiling , and has the further advantage that during the life of the building , the wiring can be altered very easily . Lighting trunking has additional folds in the cross setion which makes it more rigid than conventional trunking and is able to span greater distances .
  In workshops and laboratories there is usually a large number of machines and other equipment which have to be served with electricity , A conduit system can then becoe complex and , therefore , expensive . A simple and neat method of wiring these areas is to run trunkin round the walls and to install all the circuits inside the trunking  . In rooms of this class , this is quite acceptable and no one objects to the appearance of trunking visible on walls . Again there is the advantage that when machines are replaced or when new machines are installed , the consequent changes to the elctrical service are easily made . The same consideration applies  , but with added force , to factories .
    When machines are place in the centre of a room a good method or serving them is to run trunking at high level under the ceiling and drop to each nmachine with a length of conduit , It is , of course , possible to install conduit withint he floor with an outlet near each machine , but there then has to be either rigid or flexible conduit at floor level , and if machines are moved or additional ones brought in the floor has to be dug up before the conduit can be extended to the new position . For the initial installation the elctrician would have to known the exact positin of the terminals of each machine before the floor is laid , and it is very seldom that either the buildier or the final occupier of the factory can provide this infdormation so early . THe overhead system avoids the difficulty of locating exact position of machines too early in the construction process and makes future changes more easy .

71

CaBle Trunking

Where a large number of cables has to be run together , it is often convenient to put them in trunking . Trunking for electrical purposes is made of 18-gauge sheet steel , and is available in sizes ranging fronm 50mm x 50mm , 75mm x 100mm , 150mm x 75mm and 150mm x 150mm although 50mm x100mm and 100mm x 100mm ar also availabl . It is usually supplied in 2m lengths and one complete side is removable , as shown in Figure 3.15 . The removabe side , or lid , either screws on or clops on with a snap action . The latter arrangement is cheaper but a little more awkward to handle .
  A variety of bends , tees and junctions is available from all manufacturers of such trunking . SOme of these are shown in Figure 3.15 They enable the trunking to be taken round corners , to reduce in size as the number of cables is reduced and to allow a main run to serve a number of branches .

Dry partition

New buildings have internal partitions constructed of timber studding with light plasterboard facing , as illustrated in Figure 3.12 , or preconstructed partitioning . THis is particulrly the case with proprietary industrialized system of bulding . Such a partition has a void within it which is used for engineering services , and this is also a situation in which PVC sheathed cable without further protection is the most suitable system of wiring , provided that the cable is at least 50mm from the surface . Voids of this sort , in which PVC sheathed cable is run , are usually sufficiently accessible to make rewiring , if not easy , at least possible . Fixing of accesoris is made easy by the use of dry liner boxer as illustrated in Chapter 1 ( Figure 1.1 ) .
  It may happen that the building structure is such that PVC sheathed cable on its own is the most suitable system to use , but that there are few places where cables has to drop in plastered walls or run across floors . It is then desirable to give the cable additional protection at these places by runinning it inside conduit at these places olny . This has the additional advantage of makiing rewiring easier . As the conduit is used olny for short lengths for local protection , both light -gauge steel and PVC conduit are suitable .
  PVC sheathed cable may be buried in plaster without damage . There is , however , the possiblity that nails may be accidentally driven into the cables when pictures are being fixed to walls . Ideally , the cable should therefore be protected by conduit , but if there is not sufficient depth of plaster to make this possible it can still be given protection by shallow rigid PVC or galvanized metal channelling as shwon in Figure 3.13. many authorities feel that even this is not necessary . BS 7671 specified aeas within  a wall in which cables are installed at a depth of less than 50mm , where the cables do not need additional protection . Guidance is also given in the IEE Guidance Note .1 and the IEE On-Site Guidance . The basic rule is that cables can be run under plaster at a depth of less than 50mm without protection , in straight lines between accessories , also withint 150mm of a change of direction of the wall .
  There are situations where the appearance of an installation is of secondary importance , and where at the same time a surface system wil not receive rough usage . Such a case might occur in an old building used for commercial purposes or in simple huts at a holiday camps . PVC sheathed cable may then be run on exposed surfaces without further protection ,. Since it is visible it will not be damaged acidentally by people trying to the walls.
  PVC sheated cable is fixed with moulded plastic clips . An example is illustrated in Figure 3.14 . The clips should be spaced appropriate to the size of cables , IEE Guidance Note 1 Selection and Erection of Equipment and the IEE On-Site Guide give guidance on the spacing of cable clips .

PVC shethed cable

There are many cases in which wiring can be installed in PVC /PVC without further protection . For example , this may be done in any voids in a building such as false ceilings and wooden floors . WHen the cable runs parallel to joists in a wooden floor , it can be clopped to the sides of the joists . WHen it has to run across them it is better to thread it through holes drilled in the neutral axis of the joists than to notch the top of the joists . Holes drilled on the neutral axis weaken the joist less than notches cut in the top , and because the cable is further from the floorboards on top of the joists , it is safer from nails driven into the floor . BS 7671 states that a cable passing through a timber joist must be 50mm from the top or botoom of the joist , or be mechanically protected .

67

Plastic conduit

PVC conduit is being increasingly used in place of heavy gauge steel conduit . its advantages are that it is cheaper and more easily installed than steel conduit and that it is non-corrosive and unreactive with nearly all chemicals . Although it is imcombustible , it does soften and melt in fires and cannot be used at temperatues above 65 degree celcius . At low temperatues , it becomes brittle and should not be used where it will be exposed to temperatures below ) degree celciius . Most specifications call for high-impact grade heavy-gauge PVC , which is tough enough to withstand the ill-treatment which all material receives on buildins sites . it will protect cables insid it just as well as steel conduit from nails accidentally driven into the conduit , but it is not qute as resistant as steel conduit to heavy blows and to crushing .
  Heavy gauge PVC conduit is not resistant to blows , but has a slightly higher temperature range than impact grade , and is suitable for many types of industrial installation . Light-gauge conduit is cheaper but not so robust and may not always withstand the conditions existing on a building-site .
  As the conduit is made of an insulating material , it does not provide a means of earth continuity  A separate circuit protective condutor must , therefore , be pulled into the conduit along with the other cables . A PVC insulated cables of adequate cross section may be used for this the circuit protective conductor as well as for the phase conductors . WHen a separate-circuit protective conductor is used , it may be necessary to connect lengths of it at wiring points . THerfore , PVC conduit , fittings are supplioed with an earth terminal
  As explained in the descriptions of steel conduit , there are situations in which flexible conduit has to be used . In a steel conduit system , the flexibles do not provide earth continuity and a separate circuit protective conductor has to be run along each flexible length . If a large number of such connections occurs , one of the chief advantages of a steel conduit system , namely the way it gives earth continuity , is lost . In that case , it may be as well to use PVC conduit with a sparate circuit protective conductor throughout .
  lengths of PVC conduitc are joined mby an unscrewed coupler which is cemented to the two pieces of conduit to be connecte by means f a special solvent . The solvent used is made particularly for tis application and is supplied by the makers of the conduit . PVC conduit boxes for use with PVC conduit have short sockets which make it possible to connect the conduit to the box with a coupler of the same type as is used for connecting lengths of conduit .The PVC can also be threaded , and push fit to threaded adaptors are made with the aid of which connections to boxes and equipment can be made in the same way as for steel conduit .
  PVC has a high coefficient of expansion and provision must be made for thermal expansion wherever there is liable to be a temperature changes of 25 degree celcius or more and also wherever a run of more than 8m occurs . THe necessary allowance is made by means of expansion couplers . An expansion coupler is a coupling of ectended length , one end of which is bored to a standard depth and the other end of which has a sliding fit over a longer distance than the standard coupling depth . Expansion is liable to make PVC conduit sag more readily then steel conduit and it need fixing at closer intervals . Saddles should be fitted at a spacing of about 900mm .
  Bends can be made in PVC conduit as in steel conduit , but it is essential to use a bending sprin inside the conduit to prevent the cross section becoming reduced in the bending prcess . THe smaller sizes can generally be bent cold , but 32mm conduit and larger must be gently heated for a distance of about 300mm on either side of the intended bend .
  One has to remember the susceptibility of PVC to high temperatures if one proposes to suspend luminanires from PVC conduit boxes . The heat from the lamp is conducted through the lfexible cable and through the fixing screws , and it could happen that it softens the PVC box . To oevercome this problem , it is possible to provide the boxes with metal insert s.
  PVC conduit is made not olny in the normal circular cross section but also with an oval section . THe reduced depth of an oval section enables it to be accommodated within the thicknes of plaster in places where the use of round conduit would make it necessary to chase the brickwork behind the plaster . This makes the oval conduit very useful for switch drops and for small domestic installations . In the latter case , it makes it very easy to add new wiring in an old house . The electrician can cut away and repair plaster whereas help might well be wanted from another tradesman if bricjkwork needed cutting .
  The same PVC material is made as rectangular and semicircular channelling . This is intended primarily as a protection over PVC insulated pVC sheathed cable where the latter is installed on the surface of walls . It can also be used as a protection to PVC/PVC cable when the latter is burried in plaster , the justification for this use being that it saves depth and that the side of the cable next to the structural part of the wall does not need protection .

  Flexible PVC conduit is available in two types , In the one ,. flexibility is conderred by a corrugated construction . In the other , the PVC itself is a plasticized grade so that the flexibility is a property of the material itself . Flexible PVC conduit can be used to negotiate awkward bends and in situatins where rigid conduit would be difficult to install , and it is sometimes resorted to for the solution of unforeseen problems which so often seem to arise in the course of building work . THere is , however , a danger to using it in this way . It is possible to take such advantage of the flexibility that the conduit curves so sharply that is is impossible to pull cables through it . If this happens the problem of instaling the conduit has been solved olny by the creation of a more dicciult problem for the next stage of the erectin process . Flexible conduit should , therefore , be used with caution .



67
 

conduit fixings

  Electrical conduit is to thick enough to support its own weight over long distances without sagging . The support must , therefore , be at quite close intervals , and the maximum distance which should be allowed between supports are as follows :
 20mm conduit  -horizontal support (1.75m ) - vertical support (2.0m )
25 and 32mm conduit - horizontal support ( 2.0m )- vertical support 2.5m
40mm and over  - horizontal support (2.25m ) -vertical support (2.5m)

The IEE Guidance Note 1 and the IEE On-Site Guide both give guidance on the maximum spacing of conduit fastenings .
  The cables are drawn into the conduit with the help of a steel tape and a draw cable . The steel tape has a hemispherical brass cap on the end which prevents its sticking on irregularities at joints of the conduit and also helps guide it round bends  THe tape also has a loop at its other end and a steel draw cable is attached to this . The cables themselves are then attached to the other end of the draw cable . The electrician attaches  the cables by threading them through a series of loops in the draw cable . THey should not all be attached to the same point otherwise there is a significant . sudden enlargement in the bunch of cables and this presents and edge whch can catch in the bore of the conduit and which will be difficult to negotiate bends . When each cable is looped through the draw cable , it is folded back on itself and the end is taped . This gives a smooth surface to sticking out and catching the insid of the conduit . THe method of connection is shown in Fiogure 3.11 .
  Pulling cables through conduit is a job for electrician and mate. One pushes the steel tape with the draw cable attached to it from one draw-in box to the next : as soon as the tape appears at the receiving boc , the other takes it and pulls gently from that end . The lattter then pulls the draw cable and finally the bunch of cables while the former feeds them into the conduit . The person feeding the cables in must do so carefully and must guide the cables so that they do not cross or twist over each other as they enter the conduit . IF they are allowed to twist , the whole bunch may stick nd even if they can be forced in , it may be impossible to withdraw some of them later . The whole bunch may stick and even if they can be forced in , it may be impossible to withdraw some of them later . The whole job require great care and needs cooperation between the two people at opposite ends of the run . It is a help if they are within sight of each other and essential that they should be within earshot of each other . On the rare occasions when the run of conduit through the building from one draw in box to the next makes it impossible for shouted directions to be heard from one end to the other , a thrid person will have to be called in to stand halfway and relay messages , or use an intercom .
  In hot weather , the insulation of the cables is liable to become soft and tacky . Drawing it through the conduit may be made easier by rubing French chalk on the cables . In other circumstances , when friction between the cables and the conduit is high and makes pulling in difficult , it may be advantageous to apply him a thin coating of grease or tallow wax to the cables .


65

conduit entries to equipment

provided that the lower voltage circuits are insulated for the highest voltages present , it is better not to do so . BS 6701 :1994 recommends that there be a minimum distance between mains (Bands II ) and telecommuinication circuits ( Band I ).
  THe next matter to receive our attention is how to fix conduit . FIgure 3.10 shows various devices for fixing conduits . THe pipehook or crampet , Figure 3.10 is a satisfactory and simple fixing , but is too unsightly to be used on surface work . It can be driven into timber , brick or masonry , but is more likely to be dislodged than a screwed fixing ; where the conduit is to be buried in plaster after it has been fixed , this does not matter because the plaster will hold the conduit in place , but where the conduit is to remain exposed a firmer fixing is desirable . THe saddle hook shown in Figure 3.10b is by far the commonest fixing . It passs round the conduit and is secured to the wall by two screws . THe olny advantage of the clip shown in Figure 3.10c is that it saves one screw . It is not as secure as the saddle and the cost saving is not sufficient for a good engineer to use it .
  Sockets and other conduit fittings necessarily have a larger outside diameter than the conduit itself . If these components are tight to a wall , the conduit must be slightly proud of the wall . Because of this , when an ordinary saddle is tightened , it will tend to distort the conduit . THis can be prevented by the use of a spacer saddle , Figure 3.10 d . which has the same thickness as the sockets . The spacer saddle has the further advantage that it prevents the conduit from touching damp plaster and cement which could corrode ad discolour decoration .
  When conduit is fixed to concrete , the time taken to drill and plug holes in the concrete is a very large proportion of the installastion time . A spacer bar saddle has olny one screw to be fixed to the wall and the saving in tie can be greater than the extra cost of the material .
  The distancesaddle shown in Figure 3.10e holds the conduit about 10mm from the wall . THis spacing eliminats the ledge betweent the conduit and the wall where dust can collect and makes it possible to decorate the wall behind the conduit . It also makes it impossible for minute drops of moisture to collect in the crack between conduit and wall and thus reduces the possibilities of corrosion . For these reaseons , distance saddles are almost invariably specified by hospotal boars and local authorities for surface conduit .
  Conduit often runs across or along stel girders or joist , either exposed or within a false ceiling . It is not desirable to drill and tap structural  steelwork and it is better to use girde  clips of the type illustrated in Figure 3.10f . Whilst standard girder clips can be bought from conduit manufacturesrs , it is usually simpler to make special clips to suit individual condition on each job . FOr multipple run and runs with other services , uni-struct is often used .         

63

conduitm chased into walls

Many mordern buildings have floors and even walls of concrete with very little or no finish on top of it ; this is particularly true of industrialized methods of buildings . In such buildings , the olny practicable alternative to putting wiring on the surface is to bury conduit within the structural concrete . This needs considerable care . The exact position of the conduit within the depth of the slab must be aggreed with the structurall and close supervision is required of the work on site to ensure that the conduit is correctly placed . It have to be fixed in position immediately after the steel reinforcement has been laid in the shuttering and before the concrete is poured . If it is not well tied either to the reinforcement or to the shuttering , it may be dislodged as the oncrete is poured and vibrated . Opend ends of conduit which  may have to be left at the end of the section of concrete being cast , ready for connection to the next piece of conduit , must be covered with metal or plastic caps to prevent cement or stones getting into the conduit . Every electrician of any experience can tell a horror story of a blocked conduit . THe conduit boxes must also be folled with a material which will prevent cement and stones entering but can itself be easily removed once the concrete has set and the suttering has beens struck . THe most commonly used material for this purpose is expaded polysyyrene .
  Once conduit has been cast inside a concrete slab , it is totally inaccessible for repair or replacement . THE rules for installing it in such a way that the drawing in of cables is easy are , therefore , or exceptional importance . It is advisable for the conduit to have plenty spare capacity for the number of cables to be drawn into it , for bends to be easy and for there to be plentry of drawn -in-boxes .
  WHen conduit is placed on top of a floor ready to be screeded over , workmen are liable to walk over it after it is laid and before the screed is poured . Light gauge conduit is not robust enough to stand up to this . THe use of lightweight conduit is . therefore , usually confined to small domestic installations .
  In wooden floors , conduit can be run under the floorboards . Where it has to run across joists , the latter must be slotted for the conduit to get through underneath the floorboards . The agreement of the structural designer must be obtained before joists are cut. This method is not , however , much used now ; in wooden floors it is more usual to employ PVC  sheathed cable run 50mm from the top or the bottom of the joist , to prevent damage from the floor fixings . It does not need further protection .
  Soem thought has to be given to the relative position of conduit and boxes . THe position of the conduit is determined by the route it takes through the structure . THe outside of the box has to be flush with the finished surface , or in the case of a surface syste, the back of the box must be on the surface . THe positions of the conduit and box being fixed independently of each other by different considerations , it may happen that the conduit is not in line with any of the outlet holes in the box , and some method has to be devised to overcome this mismatch .
  Figure 3.9a shows surface conduit with a set in it to enter a surface box .
  It is difficult to make this look neat and it is better to use a distance saddle and a special box which makes it unnecessary to st the conduit . Such a box is shown in Figure 3.9b . When the conduit is buried in the structure , it may have to be set as shown in Figure 3.9C . If the conduit is far enough inside the surface , a back-entry box can be used as in Figure 3.9d , but it must be remembered that this introduces a fairly sharp bend in the conduit which could make it harder to pull in the cable . Anoher possibility is to put the box in line with the conduit and fit an extension ring to the box to bring the cover forward to the surface . THis is shown in Figure 3.9e
  When buried conduit has to feed surface distribution boards or switches , the conduit must be brought into a flush recessed box so that the cables enter the surface board or switch through the back . If necessary an extension ring has to be placed between the box and the surface . Figurre 3.9f shown and an example of buried conduit feeding a fuseboard on the surface .
  Most buildings larger than a single dwelling have  THREE-phase supply , although nearly all the equipment in them is single phase . Although the regulations allow one to run television and telephone cables in the same conduit .     62

Earth clamp 2

Care must be taken in the making of bends to avoids rippling or flattening of the conduit . THe smallest sies of conduit ( 16mm and 20mm ) can in fact be bent over one's knee. THis is not , however , to be recommended because it is unlikely that a neat bend without kinks or flattening the conduit will be produced . A bending block , as shown in Figure 3.6 , is a better device . The bottom edge of each hole should be bevelled so that the conduit is not pulled against a sharp edge. THe conduit to be bent is inserted in the hole and hand pressure is brought to bear to bend the conduit slightly . The conduit is then moved through the hole a short distance and the process repeatd . Practice is necessary to make a good bend without kinks and not all electricians possess the necessary skill .
  For larger conduit , a bending machine is essential , and is to be recommended for all conduit . It is the olny truly reliable way of making a good bend without reducing the internal cross sectional area . A bending machine is shown in Figure 3.7
  A allow ease of wiring and avoid damage as cable are drawn in , the number of cables in each conduit has to be limited . IEE Guidance Note 1 Selection and Erection or the IEE On-Site Guide gives guidance and methods of calculating requires diameters of conduits for various numbers of cables . For cases not covered by these tables it is useful to employ the concept of space factor . This is defined as total cross sectional area of cables divide by internal cross sectional area of conduit multiply by 100 . Note that the spac relates to the space taken up by the cable and not the unoccupied space .
  It is harder to pull several small cables together than one large cable , and when a number of cables have to go in the same conduit , it is advisable to keep the space factor well below 40 percent . Space factors of less than 20 percent need not be considered at all extravagant . For the same reason , it is often better to use two size 25mm conduits side by side than a single 32mm or 50mm even when in theory the latter is adequate .
  Many types of insulation deteriotate if they become damp . It is , therefore , important that moisture should not collect in the conduit system . Moisture can occur through water entering during building operations and also later on through condensation of moisture in the atmosphere . A conduit system must be laid out so that it is well ventilated , which will prevent condensation , and so that water which does enter will drain to one or more low points at which it can be emptied .
  It is good practice to swab through the conduit after it is erected and before cables are drawn in to remove any moisture and dirt which have collected . THis is done simply by tying a suitable size of swab on the end of draw cable and pulling it through the conduit from one draw-in box to  the next .
  To avoid damage to cables as they are drawn in , burrs on cut ends of conduit must be removed with a reamer before the lengths of conduit are joined .
  There are a number of positions in a buildings in which the conduit can be fixed . It can obviously be run on the surface of walls and ceilings , and when a buildings is constructed of fair-faced brick walls , surface conduit is usually the olny practicable wiring system which can be adopted . If walls are plastered , the conduit can generally be concealed within the plaster. There must be at least 6mm of plaster covering the conduit if the plaster is not to crack . Since plaster-depth conduit boxe are 16mm deep , the total thickness of plaster must be at least 22mm . IF the architect or builder proposes to use a lesser thickness than this  , it becomes necessary to chase the conduit into the wall so that some of the total distance of 22mm between face of plaster and back of conduit is in the wall and some in the plaster .
  In many modern buildings , internal partitions which do not carry any of the structural load are made of breeze-blocks about 75mm thick and in some cases as little as 50mm thick . If these have to be chased to take 25mm conduit , there is very little partitions left . Using conduit with such partitions is  a very real problem and the electrical engineer often has to abandon a conduit system in favour of one which is less robust but takes up less space .
  Horizontal runs of conduit over floors ca sometimes be arranged within the floor finish . Probably the most widely used floor finish is still the fine concrete screed . Provided the screed is sufficiently deep , the conduit is laid on top of the floor and just screeded over . If the screed is not deep enough to make this possible , it may be possible to cut chases in the floor itself so that the conduit is partly in the floor and partly in the screed . However , structural floors are nowadays designed to such close limits that the structural engineer may not permit the elctrical engineer to have c hases cut in the floor slabs . It is often necessary for conduits to cross each other in a floor and there are also other services , such as water and gas , which run in pipes laid in or over the floors . It is then almost inevitable that conduit has to cross one or other of these other services . Ity will be obvious that crossovers , whether of conduit and conduit , or of conduit and other services , are the places at which maximum depth is needed . It is these critical points which determine whether or not it is possible to accommodate the conduit within the floor finish . tHIS MUST BE DISCcused by the electrical engineer and the architect quite early in the design as the decision will affect the type of wiring which the engineer has to design .
  Conduit can also be buried within concrete slabs  .  pg 61

Figure 3.12 Dry partitionNew buildings have internal partitions constructed of timber studding with light plasterboard facing, as illustrated in Figure 3.12, or preconstructed partitioning. This is particularly the case with proprietary industrialized systems of building. Such a partition has a void within it which is used for engineering services, and this is also a situation in which PVC sheathed cable without further protection is the most suitable system of wiring, provided that the cable is at least 50mm from the surface. Voids of this sort, in which PVC sheathed cable is run, are usually sufficiently accessible to make rewiring, if not easy, at least possible. Fixing of accessories is made easy by the use of dry liner boxes as illustrated in Chapter 1 (Figure 1.1). It may happen that the building structure is such that PVC sheathed cable on its own is the most suitable system to use, but that there are a few places where cable has to drop in plastered walls or run across floors. It is then desirable to give the cable additional protection at these places by running it inside conduit at these places only. This has the additional advantage of making rewiring easier. As the conduit is used only for short lengths for local protection, both light-gauge steel and PVC conduit are suitable. PVC sheathed cable may be buried in plaster without damage. There is, however, the possibility that nails may be accidentally driven into the cables when pictures are being fixed to walls. Ideally, the cable should therefore be protected by conduit, but if there is not sufficient depth of plaster to make this possible it can still be given protection by shallow rigid PVC or galvanized metal channelling as shown in Figure 3.13. Many authorities feel that even this is not necessary. BS 7671 specifies areas within a wall in which cables are installed at a depth of less than 50mm, where the cables do not need additional protection. Guidance is also given in the IEE Guidance Note

New buildings have internal partitions constructed of timber studding with light
plasterboard facing, as illustrated in Figure 3.12, or preconstructed partitioning. This is
particularly the case with proprietary industrialized systems of building. Such a partition
has a void within it which is used for engineering services, and this is also a situation in
which PVC sheathed cable without further protection is the most suitable system of
wiring, provided that the cable is at least 50mm from the surface. Voids of this sort, in
which PVC sheathed cable is run, are usually sufficiently accessible to make rewiring, if
not easy, at least possible. Fixing of accessories is made easy by the use of dry liner
boxes as illustrated in Chapter 1 (Figure 1.1).
It may happen that the building structure is such that PVC sheathed cable on its own is
the most suitable system to use, but that there are a few places where cable has to drop in
plastered walls or run across floors. It is then desirable to give the cable additional
protection at these places by running it inside conduit at these places only. This has the
additional advantage of making rewiring easier. As the conduit is used only for short
lengths for local protection, both light-gauge steel and PVC conduit are suitable.
PVC sheathed cable may be buried in plaster without damage. There is, however, the
possibility that nails may be accidentally driven into the cables when pictures are being
fixed to walls. Ideally, the cable should therefore be protected by conduit, but if there is
not sufficient depth of plaster to make this possible it can still be given protection by
shallow rigid PVC or galvanized metal channelling as shown in Figure 3.13. Many
authorities feel that even this is not necessary. BS 7671 specifies areas within a wall in
which cables are installed at a depth of less than 50mm, where the cables do not need
additional protection. Guidance is also given in the IEE Guidance Note
1 and the IEE On-Site Guide. The basic rule is that cables can be run under plaster at a
depth of less that 50mm without protection, in straight lines between accessories, also
within 150mm of a change of direction of the wall.
There are situations where the appearance of an installation is of secondary
importance, and where at the same time a surface system will not receive rough usage.
Such a case might occur in an old building used for commercial purposes or in simple
huts at a holiday camp. PVC sheathed cable may then be run on exposed surfaces without
further protection. Since it is visible it will not be damaged accidentally by people trying
to fix things to the walls.
PVC sheathed cable is fixed with moulded plastic clips. An example is illustrated in
Figure 3.14. The clips should be spaced appropriate to the size of cables. IEE Guidance
Note 1 Selection and Erection of Equipment and the IEE On-Site Guide give guidance on
the spacing of cable clips.

Earth clamp

In casting need not , therefore , cause a rpoblem during assembly , although a very large misalignment will pull the flexible into such a sharp S that the site electrician will not be able to  pull cables through it . Flexibles conduit is also used to bridge electrical services from one to the other side of the expansion gap in structures  .
  A conduit system must be completely installed before any cables are pulled into it . It is , therefore , essential that it is set out so that an electrician can pull cables into it without difficullty . Conduit system are intended to be rewirable ; that is to say the intention is that 20 oe 30 years after the building has been erected , it should still be possible to pull all the cables out of the conduit and pull new ones into it . IF this is possible , then quite regardless of what happnes when the building is first constructed , the layout of the conduit must be such that cables can be drawn into it when it is complete and finished .
  The original reason for wanting to have electrical system which couild be recabled during the life o the building was that VRI cable deteriorates in about 20 year to the stage at which it should be removed . PC cable appears to last indefinitely so that all modern installation which use this cable should not need rewiring . The use of electical  appliances has increases greatly in the last 50 year , and when old buildings which had VRI cable are reqired th opportunity is invsriably taken of modernizing the installation by adding extra outlets and circuit . New cables then have to be run where there were no cables previously and the original conduit has at best to be added to and at worst abandoned altogether . Rewireability is then no help and in fact the need for a rewireable system is not as great as is often supposed .
  On the other hand , there is always the possibility that a cable may become damaged during the construction of a building , and it is obviously and advantae if it can be replaced without difficultu=y after the building  has been finsihed . If the conduit is installed so that the system is rewireable , reparis will always be possible . The requirement for reqireability should therefore be kept to as far as posible , but the engineer in charge should have discretion to relax them if exceptionally difficult circumstances are encountered .
  To achieved rewireability , drawn-in boxes must be accessible from the surface , or in other words their covers must be flush with the finished surface . The covers can then be removed without any cutting away of plaster or brickwork . in addition , the length of conduit between successice drawn -in boxes should not exceed about 10m and there should not be more than two right -angle bends between successive boxes . A further reuirement is that the bends themselves should be made with as large a radius as the position of the conduit within the building permits . This is the reason that specification often insist that bends shall be formed in the conduit itself and prohibit the use of factory -made bends . The latter are necessarily of small radius and could damage insulation if cables have to be fored through them . Inspection bends do not provide adequate room for feeding cables through in a neat and workmanlike manner and the conduit should be so laid out that they are not necessary .

Conduit entried into boxes

Connections to distribution boards and switchgear are made in a similar manner .
  In addition to the boxes described in Chapter 1 , other fittings are made for use with conduit ,. These include the socket and bushes needed to make connections , and also bends and inspetion covers , some of which are illustrated in Figure 3,3 , The use of bends and inspection covers is not , however , regarded as good practice , because they provide in adequate room for draing in cable and becuse they look unsightly when the installation is completed . For long lengths of run , inspection sleever are available .
  Conduit is thick enough for the cross-sectional area of the metal to provide a good earth continuity path . The conduit can , therefore , be used as the earth continuity conductor and no separate cable or wire need be used for this purpose . It is essential that the conduit , with all its fittins and screwed joints , should form a continuous conducting path of low impedance and the safety of the installation depends on good eletrical contact at all the joints . Even though it ma be decided not to use the steel conduit as the circuit protective conductor , in preference for a separate protective conductor , usually copper , the conduit must be erected properly with tight joints . Since it is classed as an exposed conductive parts of the installation and therefore could becomes live in the event of a fault , it requires earthing properly .
  Conduit is made in two standard finishes ; black enamel and galvanized . it is almost universal practice to use galvanized conduit where it is exposed or where it may be subject to damp .
  The final connection to mchines and mechanicalequipment such as pumps , boilers , fans , fan heaters , workshop equipment and so on is usually  made in flexible conduit . The fixed wiring terminals on the machine is not known and so the outlet box can olny be placed  to within a foot or so of its exact position . Solid conduit from this to the machine could involve a large number of bends in a short distance which would be difficult to make and impossible to pull cable through . Flexible conduit can take up a gentle curve and also serves to isolate the fixed wiring from any mechanical vibrations on the connected machine and allows for belt tension adjustment of the motor .
  There are several types of flexible conduit . metallic flexible conduit is shown in figure 3.4 . it is made from a stepped strip which is wound in a continuous spiral so as to produce a long cylinder with spiral corrugations . The material used is normaly galvanized steel . Flexible conduit is also made in anumber of plastic nmaterials . In some of these the flexibility is conferred by a corrugated structure , as in the case of metallic flexible conduit , and in others by the flexible properties of the material itself .
  Flexible conduit cannot be used as a protective conductor . This is obvious in the case of plastic flexible conduit which is made of non -conductng material , but it is so even in the case of metallic flexible conduit . The flexibility  here is conferred by the corrugated structure , and as the conduit bends , the corrugations open out . They ramins sufficiently overlapping to keep out dirt and moisture but are not in hard enough contact with each other to be relied upon to give an adequate electrical path . To make up for this , a separate circuit protective conductor must be run wherever flexible conduit is used . The circuit protective conductor is either put inside the conduit with the other cables or it can be placed outside the conduit . In either position , it must be bonded to the rigid conduit at both ends of its run . A clamp for connecting an external circuit protective conductor with solid conduit is shwon in FIgure 3.5
  There are other application for flexible conduit . it requires with certain system of industrialized building in which sections of floors and walls are precast in factories away from the building site . In order that electrical wiring can be put into these slabs after they have been erected , conduit is cast in them and exposed ends are left at the edges where the slabs will be joined together on site . The slabs are lifted into position on the building and joined to each other by in situ concrete , grout , or some other suitable structural method . At the same time as this is done , the exposed conduit ends  , in adjacent slabs , are linked together by short lengths of flexible conduit . the flexible conduit can take up a gentle 'S' shape and this make up for some lack of alignment between opposite ends of the fixed conduit . Small errors in casting need no

Conduit

In a conduit system the cables are drawn into tubing called conduit . The conduit can be steel or plastic . Steel conduit is made in both light gauge and heavy gauge , of which heavy gauge is much more frequently used . In both cases , it can be made either by extrusion or by rolling sheet and welding it along the longitudinal joint . The latter is specified as welded conduit and the former as seamless . Seamless conduit is generally regarded as the better quality . The different sizes of conduit are identified by their nominal bore and in the case of electrical conduit the nominal bore is always the same as the outside diameter of the tue . Thus 20mm light and heavy gauge conduits both have the same outsid diameter and consequently must have slightly different inside diameter . This is the opposite of the convention used for pipes of mechanical engineering in whichthe nominal bore usually corresponds more closely to the inside than the outside diameter . Electrical conduit is specially annealed so that it may be readily bent or set without breaking , splitting or kinking .
  Heavy gauge conduit is normally joined together by screwed fittings : there is a standard elecrical thead which is different from other treads of the same nominal diameter . A screwed connection between two lengths of conduit is shown in Figure 3.1  . A male electrical thread is cut on the ends of both lengths of conduit to be joined and a standard coupler with a female electrical thread is screwed over them . A lock nut , which has been previously threaded well up out of the way on one of the male threads , is then wound down and tightened against the coupler . The screwed connection is relied on for continuity of the earth path and the lock nut is essential to prevent the socket working its way along the threads until it engages nmore on one conduit than on the other . The reason for wanting an earth path is discussed in chapter 9 . method  of jointing conduit to boxes of the kind described  in Chapter 1 are shown in figure 3.2 A bush of some sort must always be used to provide a smooth entry into the box , to avoid sharp corners which could damage the cable insulation , and in certain cases to maintaine earth continuity .

Wiring ( Introduction )

To the average user the olny important part of the elecricity service is the outlets at which he received electricity . To the engineer concerned with deigning or installing the service , the system of cables which links these outlets to each other and to the supply coming into the building is just as important and perhaps even more so . In practice , the electrical service is a complete interdependent system and the practical engineer thinks of it as a whole , but , as with the teaching of any sucject , one has to break it down into parts in order to explain it in an orderly fashion which will make sense to a student with no previous knowledge of the subject .
  In this chaptr , we shall considr different ways in which cables can be installed in a building . The calculation of the size of particular cables shall leave to chapter 4 and the selection and grouping of outlets to be served by one cable we shall leave to chapter 5 . For this chapter , we assume  that we know where cables are to run and discuss olny how to get them into the bulding.This aspect of the electical service can for convenience be called "method of installation'
  A method of installation consists of talking a suitable type of cable , giving it adequate protection and putting it into the building in some way . The subject can , therefore , be fairly logically considered by considering types of cables , methods of protection and methods of installation . The types of cable available and in general use have been described in Chapter 2 . The protection against mechanical damage given to cale is sometimes part of the cable itself , as with PVC insulated PVC sheathed cables , and sometimes part of the method of installation , as with conduit systems . it can be more confusing than helpful to take a logical scheme if things too rigidly and , rahter than deal with protection in a chapter of its own , we are dealing with it partly in the previous chapter and partly in this , according to whether it is associated with the cable or with the method of wiring .
  It is probably true to say that one of the commonest methods of installing cables is still to push them into conduit and we shall devote most of out attention to this .

Standard relevan to this chapter are ( chapter 2 )

BS 2316 Radio frequncy cables
BS EN 13602 Copper wire for the manufacture of electric cables
BS 6004 Electric cables , PVC insulated , non -armoured cables for voltages up to and including 450/750v . for electric power , lighting and internal wiring .
BS 6007  Electric cables , single core unsheathed heat resisting
BS 6195  Insulated flexivle cables
BS  6207-3  Mineral insulated cables
BS 6346 PVC insulated cables
BS 6480 paper insulated cables
BS 6500 Electric cables , flexibles cords
BS EN 50214 Flexible cabes for lifts

IEE Wiring Regulations particularly applicable to this chapter are
Regulation                412-2
Chapter                    52

Co-axial cables

Radio and TV system are now frequently built as part of the engineering services of new buildings . One aerial is made to serve a number of outlets at each of whicha receiving set can be plugged in . The single aerial is usually at the highest point of the buildings or group of buildings in the system and is connected by cable to each outlet .The cable used for this does not carrry large current and is not subjected to large voltages but it does carry a weak signal at high freqencies . The signal must not be lost and to avoid loss of the signal , the cable must have a low impedance at the frequency being used . It must also be constructed so that it does not pick up unwanted high  frequency signal , for example by capacitance or inductance between itself and nearby main cables .
  To satisfy these requirements , radio and tV distribution system are cabled with radio frequency cables . These normally have a single insulated conductor . A metal cover is placed over the insulation to screen the conductor from unwanted signals , and this cover is in its turn protected by an overall sheath of non-conducting material . Thus the single conductor is surrounded by a circular cover and a circular sheatrh which are concertrica and have the conductor on their axis . hence the cables comes to be knwon as coaxial cable . Such a cable is shown in Figure 2.10 ; it has its single inner conductor cased in polythene insulation with a cable braid outer conductor and a final sheath of PVC .
  The inner conductor can be either solid or stranded , while the screen can take several forms . It can be a one -piece sheath in either aluminium or copper , or it can be of cable braid , which in turn can be either single or double , or it can be of steel tape or of lead , for the insulation , the commonest materials used ae polythene  ,PTFE and polypropylene . The last part of the construction  is the outer sheathing and this may be of metal tape , cable , metal braid  PTFE , lead alloy  , PVC , nylon or polythene . Evidently , there is a large variety of coaxial cables and properties differ somewhat . The choice of what to use in any particular case is determined by the electrixal characteristic required for that particular application and this depends on what equipment the cable is to be used for . SOund broadcasting operates on lower frequencies than televisyen and cables for sound olny do not need to meet quite such stringent condition as those for televisyen transmission . Audio-frequency cables suitable for microphones and loudspeaker connections for public address systems and for broadcast relay systems are similar to high-frequency cables but are not made to such exacting specifications . Examples of audio-frequency cables are shown in Figure 2.11

Enhanced fire performance

The commonest insulation used today is PVC .Its ignition temperature is sufficiently high for it not to be regarded as a flammable material , that is to say , a fault in the cable is unlikely to ignite the insulation . But if a dire from some toehr cause engulfs the cable , PVC will burn and give off dense smoke and acid fumes which can create a hazard greater than the original fire . To avoid misunderstanding here , it should be added that a cable fault , particularly if the insulation is damage , can ignite adjacent material such as loose paprt or timber , even if it does not ignite its own insulation  .
In order to give better performance in fires , alternative materials have been developed . One type of cable for enhanced fire performance has XLPE insulation with cable armouring bedded in a compound which has low smoke and fume properties it is designated LSF ; it consist of inorganic fillers in polymers such as ethly vinyl acetate and ethylene propylene rubber . THe construction is illustrated
  Other cables have EPR ( ethylene propylene rubber ) insulation with a sheath of elastomer which is heat and oil resistance and flame retardannt , and is therefore designated HOFR . These cables are available with and without a cable armouring between the insulation and the sheath .
  It will not come as a surprise that the better performance of the cable , the more expensive it is . Consequently , it is important to consider carefully the application and precise requirements . It has olny been possible here to give an indication of special purpose cables available . In practice , final selection cannot be made without discussion with manufracturers and suppliers .

Flexible cord

flexible cord is the name given to a particular types of cable . It is one which is flexible and in which the cross-sectional area of each conductor does not exceed 4mm square . Flexible cords are used for suspending luminaires and for connections to portable domestic appliances having lowe power consumption. THey are , therefore  , usually left exposed in rooms and , to be suitable for such use , are made with a variety of coverigs .
  Ordinary lighting flex consists of a stranded copper conductor with PVC insulation , covered with an outer layer of PVC , as shown in FIgure 2.7 . Circular flex is made for the connection to household appliances such as irons , kettles , and so on , and consists of rubber-insulated cable inside an external cotton braiding . Either two or three rubber-insulated cables may be used and to obtain the circular cross-section  , cotton padding is wound around them inside the outer braiding . This is illustrated in figure 2.8. There are also flexible cords made with silicone rubber insulation with a covering of braided varnished glass fibre . Flexibles hich have glass-fibre insulation and an outer glass-fibre braid have already been mentioned .

Lead
Cables insulated with paper and sheathed with lead hae already been described .

Metal
The metal sheathing of MIMS cable is an integral part of the cable and has already been described

Bare risers
These have already been described . They sometimes have a covering of thin PVC which is put over them for extra protection for maintenance personnel who may be working on a system with some of the conductors live . The covering does not perform the protective function of the other kinds of sheathing discussed .

PVC

Cable insulated with PVC often has a thicker PVC sheath over the insulation and is then described as PVC insulated PVC sheathred cable or , simply , PVC/PVC . More than one insulated conductor can be embedded in the same sheath , so that one can have single , twin  , three , or four-core PVC/PVC cables , if one of the conductors is intended as a circuit protective conductor , it requires no insulation , although it must be sleeved at terminals and connections , up to a cross-sectional area of 6mm square , and may be enclosed directly in the sheath . Figure 2.6 showns a cable known as twin and earth PVC/PVC . It has two PVC insulated conductors and one unisulated conductor all embedded in the same PVC sheath .

Rubber
Butyl rubber and silicone rubber cables are usually sheathed with thick butyl and silicone rubber respectively .

Sheathing

We have described how a cable is made from conductor with insulation around it . Electrically , this is all that is needed to make a device to carry electricity from one place to another , but if the cable is to survive in use , it must also withstand mechanical damage , and the installation , which is enough to achieve electrical protection , is seldom strong enough to give adequte mechanical protection . Something further , therefore , has to be provided over the insulation , and it can either be made an integral part of the cable or provided by entirely separate means .
  For example , the steel armour of a thermoplastic PVC cable , which has been described above , gives mechanical protection and is part of the structure of the cable . Similarly ,the metal sheathing of MIMS cable gives the cable all the mechanical protection that is needed . Neither of these cables could be used without its built-in mechanical protection , and MIMS could certailny not even be made without it . In these cases , although one may try to dra a logical distinction between the function of insulation and that of sheathing , in practice the two must be done together . PVC insulated cable , on the other hand , is strong enough to be handled as it is during erection , but is too liable to mechanical damage to be left unprotected for long . THere are two practicable ways of giving it additional protection . One way is to install it in either doncuit or trunking and the other way is to put a sheath around the outside of the insulation . THe irst way gives protection by the use of particular method of wiring and the second way does it by makiing the cable asheathed cable , MEthod of wiring are discussed in the next chapter and the rest of this one is devoted to types of sheated cable .

Mineral insulated copper sheathed cable

MIMNS cable is extemely robust and , when properly installed , has an indefinite life . It can be used outdoors and for usch use is usually supplied with an overall covering of pVC . It is then known as MIMS PVC sheathed . Since PVC is embrittled with ultraviolet light , this PVC covered cable should not be installed where it will exposed to direct sunlight . THis is not as drastic a restriction on its use as may appear since it is probably unwise to run any cable where it is so exposed that direct sunlight can reach it . In any such situattion , it would also be too vulnerable to damage by vandalism and from animals . because of its robustness , MIMS requires no further protection , but will not withstand being struck by sharp objects. It is , therefore , more easily built into the structure of a building than other cables , nearly all of which must have some form of enclosure around them . Because it has an idenfinite life . There is no need for facilities to make it possible to reqire the installation . For both these reasons , it can often be used where no other cable would be entirely satisfactry . MIMS cable can carry a higher current than other cables with same size conductors because the insulation can withstand a higher conductor operating temperature . However , its current carrying capacity depends on wheather it is bare , PVC-covered , exposed to touch , or in contact with a combustible materials .IT follows that for a given current the cable can be smaller if MIMS is used than if another type of cable is used . This is very useful property which makes it possible to conceal MIMS cable in corners which are not large enough to hide the larger cable that would have to be used with another system .
  The magnesium oxide insulation is hygroscopic and will lose its insulating properties if left unprotected against the ingress of moisture from the atmosphere . To prevent this happening , MIMS cable must be terminated in special seals and glands , which are supplied for the purpose by the cable manufacturers . IF the cable is cut and the ends left unsealed for any length of time , as can happen in the course of work on building sites , moisture can penetrate  the insulation and render the cable useless . In most cases , however , moisture will penetrate unsealed ends for olny a short distance of not more than 50mm . It is then sufficient to cut off the damaged end of the cable , fter which the remainder can be used in the normal way . If the cable is carrying full rated current , it will operate at about 90 celcius ; care must be taken that the accessory or luminaire to which the cable is connected is designed to withstand 90 celcius .

Rising mains

Where they run horizontally at high level along the walls of workshops . plain connectors can be fixed at short intervals and short cables run from each set of connectors to a switch fuse fixed on the wall immediately below or above the trunking . The switch fuse can then be connected to serve a machine near it on the floor of the workshop .

MICC
These letters stand for mineral insulated copper covered ; this types of cable is also known as mineral insulated copper sheathed , which is abbreviated to MICS and as mineral insulated metall sheathed ,. abbreviated to MIMS . This last description may refer to aluminium sheathing , as well as to copper sheathing ; aluminium was used as a sheathing some time ago . it may be encountered during refurbishments . All versions of this type of cable consist of single-strand cables embedded in tightly compressed magnesium oxide  , which is enclosed in a seamless metal sheathed . The construction is illustrated in Figure 2.5 .

Air itself is a good insulator . Whilst it cannot of course be wrapped around cables in the ordinary sense of wrapping insulation around , it does form the insulation when bare conductors are used

Bare conductors are used principally on rising mains i high buildings . These are the main distributing electric power from the main intake of a building to distrivution boards at different levels . The scheme of distribution is discussed in Chapter 5 and here we are concerned olny with the construction of the rising-main bars . Bare conductors used for rising mains must be correctly spaced from each other to give the necessary air gap for adequate insulation , be not subject to flexing under fault conditions , and should have a protective casing . They must be made inaccessible to unauthorized persons and must have feedom to expand and contract . There are several proprietary system of bare rising mains , and a typical one is illustrated in Figure 2.4 . it is frequently used for the vertical distribution in blocks of flats .
  The conductors are held in porcelain or sometimes plastic cleats . Apart from supporting the conductors , the cleats keep them the correct distance awy from each other for air gap to have sufficient insulation for the working voltage of the system . The cleats are fixed to the back of a metal trunking which completely encloses the conductors . The dimensions are , of course , such that the air gaps between the conductors and the trunking gives enough insulation . The front of the casing is hinged and can be opened for maintenance . it is possible to put a solid insulating plate acrpss the inside  of the casing atevery floor level to form a barrier to air and smoke moving up and down the casing . In many places this has to be done to satisfy fire prevention regulations , and also BS 7671 .Banks of fuses can be fixed within the casing to form a distribution boards as part of the vertical distributor .
  A similar system can be used for horizontal distribution  . here again there are seeral proprietary system consisting of horizontal conductors supported on insulators inside metal trunking . THy are used particularly in factories .

Paper

paper-insulated cable was used for power distribution for nearly a century . it is too bulky to be used for the small cables of final circuits within buildings , or for most of the submains . THe smallest particable rating is 100A , and its cheif use is for the Electricity Supply Company's underground low-voltage and medium -voltagedistribution .
  The conductor is either stranded copper or stranded aluminium , the latter becoming increasingly popular as its price adcantage increases . Whichever is used , it is heabily stranded to give good flexibility , which is important in a cable of such comparatively large size . Paper specially made for the purpose is used as an insulator . It is essential that it should have good mechanical properties to be suitable for this application . Paper itself is a hygroscopic , fibrous material , and has to be impregnated with an oily compound to make it fit for use in cables . The compound used is a heavy mineral oil mixed with resin . On its own impregnated paper , insulation would be too fragile to be used unprotected , and  lead sheath is therefore applied over the insulation . Further strengthening and protection can be applied according to the intended use of the cable and the physical wear to which it may be exposed . A very good strong protection is afforded by steel cable or tape .
  Figure 2.1 shows a single-core PVC - insulated steel wire armoured PVC-covered cable . This is conventionally referred to as a PVC /SWA?PVC cable . Figure 2.2 shows a three-core PVC-insulated steel wire armoured cable with a PVC covering . The abbreviationf for this is PVC/S/SWA/PVC cable . A considerable number of variations on this basic design is possible and , for any given application , a cable can olny be chosen with help of a cable manufacturer's catalogue .
  PVC is now used for the larger power and sub-main cables and has superseded paper insulated cables for these application . This construction of such cables is similar to that of paper insulated cables , and another example is shown in Figure 2.3 . THis particular cable would be describned as three-phase straight concentric  , which would be abbreviated system , where the armouring forms the CPC and the neutral conductor .

Silicone rubber

This is completely resistant to moisture and is suitable for temperatures from - 60 celcius to 150 celcius . it is undamaged after repeated subjection to boiling water and low pressure steam .  and is therefore used on hospital equipment which ahs to be sterilized .
  Although it is destroyed by fire , the ash is non-conductive and will continue to serve as insulation if it can be held in place . A braid or tape of glass-silicone rubber will hold it , and cable made with this construction is very useful for fire alarms . Thermosetting rubber of 180 celcius is used in hot -air saunas .

Glass
Glass fibre has good heat-resisting properties and is therefore used for cables which are employed in high-temperature surroundings . One example is the internal wiring of electric ovens . Another application which may not at first sight seem to require heat-resisting cable lies in flexible cords for liminaires . Although the object of an incandescenet lamp is to convert electrical energy into light , most of the energy is in fact dissipated as heat . many luminaires restrict the paths available for the removal of heat and in consequence produce high local temperatues . The high temperatures is transmitted to the flexible ord both by direct conduction through the lamp socket to the conductors and by an increase in the local ambient temperature . if the flexible cord is to last any length of time , it must be capable to withstanding the temperature it is subjected to .
  One type of flexible cord is made from tinned copper conductors insulated with two layers of glass fibre , which is impregnated with varnish . A glass fibre braid , also impregnated with varnish , is applied over the primary insulation . This type of cord can be used at temperatures up to 155 celcius . If it is made with nickel-plated conductors and a silicone-based varnish , it then becomes suitable for temperatures up to 200 celcius .

Thermosetting insulation

There are plastics available as alternatives to PVC which have the advantage of being able to operate at higher temperature . The most usual is XLPE , which is a cross linked polyethylene compound . Another alternative is hard ethylene propylene rubber compound , which is designated HEPR . These materials are normally used olny in cables which have cable armouring over the insualtion and an outer sheath over the armouring . The outer sheath is generally of PVC . The construction is then similar to that of the PVC wire armoured cable shown in Figure 2.3 , the olny difference being that the inner insulation is XLPE or HEPR of PVC .

Butyl rubber
This insulation is used for cables which are to be subjected to high temperatures . It is , for example , used for the final connections to immersion heaters , for the control wiring of gas-fired warm-air heaters and within airing cupboards . it can safety be used for ambient temperatures up to 85 celcius . Buty rubber also has greater resistance to moisture than natural rubber .

Inuslation

Every conductor must be insulated to keep them apart , keep the flow of current within the conductor and prevent its leaving or leakng from the conductor at random along its length   . The following types of insulation are in use .

Thermoplastic PVC
polyvinyl chloride is one of the commonest materials used by man today . It is a man-made thermo-plastic which is tough , incombustible and chemically unreactive . Its cheif drawback is that it softens at temperatures above about 70 celcius  . it does not deteriorate with age and wiring carried out in PVC insulated cable should not need to be renewed in the way that wiring insulated with most of the older materials had to be . PVC  insulated  cable consists of cables of the types described  above with a continuous layer or sleeve of PVC around them . The olny restriciton on this type of cable is that is should not be used in ambient temperatures higher than 70 celcius.

Conductors

The commonest conductor used in cables is copper . The olny other conductor used is aluminium . Copper was the earlier one to be used , although aluminium has the disadvantage of being much weaker than copper . Consequently BS 7671 states that the minimum permissible cross-sectional area is 16mm square . Aluminium 's greatest assets are that it is cheaper than copper , lighter , and that its price is less liable to fluctuations .
  COnductors have usually been made exept for the smallest sizes , by twisting together a number of small cables , called strands , to make one larger cable . A cable made in this way is more flexible than a single cable of the same size and is consequently  easierm to handle . Each layer is spiralled on the cable in the direction opposite to that of the previous layer ; this reduces the possibility that the strands will open under the influence of bending forces when the cable is being installed . 1mm square has a solid core , 1.5 mm square and 2.5 mm square is available as solid or stranded core ; sizes above these are available as stranded core olny .

cable ( introduction )

Electricity is conveyed in metal conductors , which have to be insulated and which also have to be protected against mechanical damage . When the conductor is insulated to make a usable piece of equipment for carrying electricity , it becomes a cable . This nomenclature makes a convenient and logical distinction between a bare conductor and insulated cable , but in practice the terms ' conductor ' and 'cable' are in fact used interchangeably and it is olny the context which makes clarified what is being referred to . We shall try to avoid confusion and shall discuss conductor first and the insulation appllied to them afterwards .

Standard relevant to this chapter are :

BS 67   ceiling roses
Bs 196  protected types non-resersile plugs , socket outlets , cable couplers and appliance couplers
BS 546  Two ploe and earthing pin plugs , socket outlets and adaptors
BS 1363  13 A plugs , socket outlets and boxes
BS 3535-2  isolating transformets , and safety isolating transformets .Specification for trnasformers for reduced system voltage .
BS EN 60742 Isolating transformets , and safety isolating transformers .
BS 3676/BS EN 60669-1  - Switches for domestic and similar purposes .
BS 4177 cooker control units
BS EN 60309 Industrial plugs , socket outlets and couplers
BS 4573 Two -pin reversible plugs and shaver socket outlets .
BS 4683   Electrical apparatus for potentially explosive atmosphere . Type of protection 'n'
BS 5125  50 A flameproof plugs and sockets
BS EN 60079-14  Electrical apparatus for explosive gas atmospheres . Electricall installations in hazardous areas ( other than mines ) .
BS 5419 Air-break switches up to and including 1000V a.c.
BS EN 60947-3  Specification for low-voltages switchgear and controlgear
BS EN 60529 Specification for degree of protection provided by enclosures (Ip code )
BS EN 50014  Electrical apparatus for potentially explosive atmosphere
BS 5733 : 1995 General requirement for electrical accessories
BS 6220 : 1983  Junction boxes
BS EN 50281 -1-1,1-2   Protection of apparatus for use in presence of combustible dusts IEE Wiring Regulations BS 7671 particularly to this chapter are :
Regulations  412-03
Regulations  471-05
Section  476
Section  511
Section 512
Section 537
Section 553

Protection of equipment against ingress of liquid

Second characteristic numberal and degree of protection

0-- No protection
1-- Protection against drop of condensed water : drops of condensed water falling on the enclosure shall have no harmful effect .
2-- protection against drops of liquid : drops of falling liquid shall have no harmful effect when the enclosure is tilted at any angle up to 15 degree from vertical .
3-- Protection against rain : water falling in rain at an angle up to 60 degree with respect to the vertical shall have no harmful effect .
4-- protection against splashing : liquid splashed from any direction shall have no harmful effect .
5-- Protection against water-jets : water projected by a nozzle from any direction under stated conditions shall have no harmful effect .
6-- Protection against conditions on ships' decks ( deck watertight equipment ) : Water from heavy seas shall not enter the enclosure under prescribed conditions .
7-- protection against immersion in water : it must not be possible for water to enter the enclosure under stated conditios of pressure and time
8-- protection against indefinite immersion in water under specified pressure : it must not be possible for water to enter the enclosure.

Protection of persons against contact with live or moving parts inside the enclosure and protection of equipment against ingress of solid goreign bodies ( protection against contact with moving parts inside the enclosure is limited to conteact with moving parts inside te enclosure which might cause danger to persons )

The first characteristic nummeral and degree of protection

0---- No protection of persons against contact ingress of solid foreign bodies . No protection of person against contact with live or moving parts inside the enclosure .
1----Protection against accidental or inadvertent contact with live or moving parts inside the enclosure by a large surface of the human body , for example , a hand , but not protection against deliberate access to such parts . protection against ingress of large solid foreign bodies .

2---- Protection against contect with live or moving parts inside the enclosure by fingers .
         Protection against ingress of medium-size solid foreign bodies .
3--- Protection againt contact with live or moving parts inside the enclosure by tools , cables or such objects of thickness greater than 2.5mm
Protection against ingress of small solid foreign bodies .

4--- Protection against contact with live or moving parts inside the enclosure by tools , cables or such objects of thickness greater than 1mm .protection against ingress of small solid foreign bodies .

5-- Complete protection against contact with lve or moving parts inside the enclosure . Protection against harmful depostis of dust . The ingress of dust is not totally prevented , but dust cannot enter in an amount sufficient to interfere with satisfactory operation of the equipment enclosed .

6--- Complete protection against contact with live or moving parts inside the enclosure . Protection against ingress of dust .

Enclosure

The enclosure of any equipment serves to keep out dirt , dust , moisture and prying finges . This is a separate matter from protetion against explosion ; a piece of electrical equipment may have to be mounted outdoors and be protected against the weather where there is no risk of explosion  , or it may be indoors in a particularly dusty but non-flammabe atmosphere .
  An internationnaly agreed system has been developed to designate the degree of protection afforded by an enclosure , it consist of the letters Ip followed by two digits. The letters stand for International protection and the digits indicate the degree of protection . The first digit , which may be from 0-6 , describes the protection against ingress of liquids . In both cases , the higher the numeral the greater the degree of protection . The definitions of the levels of protection are given in tables 1.3a and 1.3 b which are based on BS En 60529 . Specification for degrees of protection provided by enclosures ( IP code ) .
  This method of classification can be aplied to all the equipment described in this chapter and the distribution equipment and luminaires discussed

Hydrogen

The surface ignition temperature of hydrogen is T1 that is 450 celcius . Each gas data must be cheked througly before specifying equipment .
  Electrical equipnment sutable for use in hazardous areas has to be marked with the protection it affords . The mark commences with the letters EX , which have been internationally aggred to indicate explosion protection , continues with the type , and where relevant , the group for which the equipment has been certified , and concludes with the temperature classification . An example of marking CE 0000 II 2 G ; CE suitable for use in the European Community , 0000 the certifying test house registration number , II explosion proof , 2 category , and G suitable for gas .

  Nearly all the accessories described in this chapter , including switches , socket outlets and boxes , are available in versions with various classes of hazaed protection . Distribution equipment and luminaires , which are discussed in a variety of types of protection .

The risk of flammable material

Group I for which the typical or representative gas is methane , is reserved for mining application olny and is therefore of interest olny to mining electrical engineers .
Group 11A is for gases with properties similar to propane and require more than 200 mili joules of energy to ignite .
Group IIB is for gases with properties similar to ethylene ( >60 mili joules to ignite ).
Group IIC (> 20 mili joules to ignite is for the most haardous gases , of which the typical one is hydrogen . These categories relate to the minimum ignition energy in milijoules required to cause ignition , at the most volatile gas air mixture .
  It will be noted that the zones are numbered 0-2 in decreasing order of risk wherea the groups are numbere I-IIC in increasing order of risk . These classificication deal with the magnitude and type of risk .
  Equipment designed for use in hazardous areas is itself classified according to the method used for achieving protection  . Each types of protection is referred to by a letter .
  Types d refers to equipment with a flameproof enclosure . The principle adopted with this types of protection is that a spark inside equipment should not cause fire outside it , It is not practicable to design equipment so that no air or vapour can get inside it . It is , however , possible to design it so that the air gaps between inside and outside are so narrow and so long that any flame starting inside will be extinguished before it has travelled to the outside . This is the method used for type d equipment .
  Type e is a method of protection which applies olny to non-sparking equipment . The design of the equipment is such as to keep temperatures low and givee increased protection against mechanical damage which could cause an electrical fault . ZTHis is achieved by such features as non-sparking cable terminations , additional insulation , increased creepage and clearance distances , and , in the case of liminaires , special lampholdrs . The requirement of low internal temperature makes it inapplicable to heavily rated machines .
  Type N is similar to type e but has a reduced level of protection . Consequently , whereas type e equipment can be used in Zone 1 , type N equipment can be used olny in zone 2 .
  With type p protection , the enclosure  of the equipment contains air or an inert gas at a pressure sufficient to prevent the surrounding vapour entering the enclosure . Since no enclosure is completely vapour -tight , there must be some eleakge out of the enclosure . Equipment suitable for this method of operation must be capable of withstanding the necessary internal pressure  , and must be connected to a network of compresed air or gas which contains a low-pressure switch to disconnect the electrical supply in the event of  loss of pressure .
  Intrinsically safe equipment limits the energy available for causing ignition . The maximum current which may flow depends on the voltage but , in general , most intrinsically safe equipment is designed for operation on extra low voltages . It is permissible to limit the current by means of a barrier diode . This type of protection is designaed type i ; it may be designated type ia or type ib according to the number of faults it can sustain during testing .
  Table 1.1 is based on BS EN 60079-14:1997 : part 1 , Electrical apparatus for explosive gas atmospheres . Electrical instalations in hazardous areas ( other than mines ) show which type of equipment may be used in which zone . Within a zone in which it is permitted , type e , N , or p equipment may be used with gas of any group . EQUIPMENT WITH TYPE d or i protection is further subdivided according to the group for which it is safe .
  These provisions deal with the risks of ignition arising from operation of equipment under normal or fault conditions . It may also be necessary to limit surface temperatures . The safe temperature in an area does not necessarily depends on the magnitude or type of risk , and an additional classification is usual . Table 1.2 is based on BS 4683 : Part 1/ BS EN 50021 Electrical apparatus for potential explosive atmospheres . Types of protection 'n' , show the classes which are used to designate the maximum permitted surface temperature . It is possible for equipment having any type of protection to have any temperature classification , although one would not expect intrinsically safe equipment to have a lower class than T5 or T4 while it is difficult for other types of equipment to achieve classes T5or T6 . It should be noted that there is no real relationship between minimum ignition temperature and maximum surface temperature permitted . Hydrogen is a class IIC gas but its .

For dusty atmospheres the following definitions apply

1 Zona 20 ( ATEX category 1 D {DUST} ) A place in which an explosive atmosphere in the form of a cloud of combustible dust is present continuosly or for long period or frequently .
2 Zone 21 ( ATEX category 2D {Dust} ) . A place in which an explosive atmosphere in the form of a cloud of combustible dust in air likely to occur in normal operation occasionally .
3. Zone 22 ( ATEX category 3D {Dust} ) . A place in which an explosive atmosphere in the form of a cloud of combustible dust in air not likely to occur in normal operation but , if it does occur , will persist or a short period olny .
If there is no likelihood at all of flammable atmosphere the area is a safe one .
  THe magnitude of the risk which determines which zone an area is in depends on such things as the process producing the flammable gas or vapour or dust cloud , the rate of production in relation to room size , the risk of leakage and the distance of the area from the source of the hazardous material . These factors are assessed by a safety specialist who designjates the zone classification of areas on a site and it is not usual for the electrical designer to hae to do this him/herself .
  The type of risk depends on the properties of the gas , vapour , or dust concerned . For gases , dangerous , substances are , accordingly , classified into four groups , dependings on the minimum ignition energy of the gas and on the ability of a flame emerging from a narrow joint to ignite it :

Harzardous area classified

1) Zone ) ( ATEX category 1G {Gas} ) A place in which an explosive atmosphere consisting of a mixture with air of flammable substance in the form of gas , vapour or mist is present continuously or for long periods or frequently .

2) Zone 1 ( ATEX category 2 G {Gas}) A place in which an explosive atmosphere consisting of a mixture with air of flammable substances in the form of a gas , vapour or mist is likely to occur in normal operation occasionally .

3) Zone 2 ( ATEX category 3G {Gas}) A place in which an explosive atmosphere consisitng of a mixture with air of flammable substance in the form of gas vapour or mist is not likely to occur in normal operation but , if it does occur , will persist for a short period olny .

hazardous Areas

There are industrial processes which involve a risk of fire or explosion . Generally , the risk arises because flammable vapour or dusts are present in the atmosphere  . For example , in coal mines there is always the possibility of methane appearing in sufficient concentration to ignite or burn . In such cases any electrical equipment in the area subject to risk must be specially designed to reduce that risk .
  The mere flow of electricity will not ignite a vapour unless the temperature becomes to high . The temperature can be kept low by adequate sizing of the cables so that this is not a problem as far as the installation is concerned . The surface temperature of motors , luminaires and other electrical equipment must , however , be considered . Vapour  can also be ignited by spark at a terminal or switch or as a result of mechanical damage causing a spark or local hot spot . THere are various ways of designing equipment to reduce the risk in harzardous with European standard if the national standard do not exactly match . If the national standard are identical , then they will be designated as a Euro-Norm EN .
  Under the European ATEX directive 1992/92/EC . on the /minimum requirement for Improving the Safety and Health Protection of Workers at Risk from Explosive Atmospheres ' , it is necessary to consider both the type and magnitude of the risk . The magnitude of the risk is the probability of a dangerous concentration of flammable vapour , and hazardous areas are classified into three zones according to the likelihood of such a concentration :

Connectors

It is often necessary to join cables together . In the wiring of buildings this is rarely done by soldering . Good soldered joints can be made in factory conditions , but thew conditions existing on a building site , and the quality of work that can be done under such conditions , are such that joints may not be sufficiently reliable . Also , the time takien to make them would put up the cost of the electrical service considerably . Crimping the cables is a more cost-effective method of jointing cables ; this is achieved by squeezing special lugs onto the cable conductor by means of a special tool . It is also common practice to join cables by means of connector blocks , which require olny mechanical terminations to the cables . A connector nlocks is illustrated in Figure 1.21 . It consists of two screw-down-type terminals solidly connected to each other , mounted in an insulated casing . The end of each cable is pushed into one of the terminals , with the insualtion taken up to the connector , with no bare conductor visible , and the screw is tightened on to it . The screw grips the conductor , holding it firmly in place and at the same time making a good electrical contact . As the two terminals are solidily connected within the insulated case , the result is that there is a good electrical path between the two cables . Joints and terminals made in this way must be available for inspection . In addition to this , all joints and terminations must be enclosed within a non-combustible material . Therefore , any accessory without an appropriate back plate must not be fixed to a combustible surface such as wooden partition , without the use of a pattress . A joints or termination made by welding , brazing , soldering , crimping , or encapsulating need not be available for inspection .
  With such connector blocks , it is possible to join cables neatly within the boxes which have already been described . In general , joints should be avoided and single length of cable run from one piece of equipment to another , but when an occasion arises when this cannot be done , connector blocks may be used .
  The author has tried in this chapter to give a survey of the more important accessories and to give an idea of the wide range available . It is not possible to descrie every accessory made ; a full knowledfer can be obtained olny by a study of many accessory made ; a full knowledge can be obtained olny by a study of many manufacturers's catalogues  and , preferably , by the use of the accessories on actual sites

Laboratories

Laboratories in schools , universities and industrial establishments often need special services which are not required in other areas. The most common electrical service of this kind is an extra low voltage supply . This is usually obtained from a stabilised supply  unit which is plugged into the bench mains sockets , the socket being installed on angled bench trunking . For higher current applications , a transformer from the mains , which can either provide a fixed secondary voltage or be of the tap-changing type to give a choice of voltages , or even a variable voltage transformer , may provide the supply . A rectifier is often incorporated so that an extra low voltage a.c supply . Laboratory benches supplied through angled bench trunking , as shwon in Figure 1.20 , are provided with special terminals to connect equipment to the laboratory supplies which must be shrounded so that there are no live conductors exposed to touch as the laboratory equipment is being connected .

Mounting Box

Figure 1.18 shows a pattress for use with batten lampholders . Figure 1.19 shows a different types of pattres .This is useful with some modern building methods in which the wiring is insalled in a special skirting . The skirting is at floor level , but this is too low for socket outlets and the latter are , therefore , a little above the skirting so that at each outlet , cables have to rise a small vertical distance . The pattress shown provides a neat and convenient way of doing this . It has also been known to happen that in the course of erecting a new building , an electrical outlet is wrongly placed . For example , a heating pipe to a radiator may run right in front of the box left to take a socket outlet . The type of pattress shwon in Figure 1.19 is a neat way of extending the wiring to an adjacent position , where the alternative might be to demolish large parts of a wall alread built in order to give acces to conduit buried in it , as the olny means of extending that conduit .

Pattresses

It can be happen that an outlets , such as a socket outlets or ceiling rose , has to be placed a small distance in front of the structure available to support it . This can happen , for example , when a wiring system is installed on the surface of walls and ceilings and there is a step in the surface which the wiringt cannot follow , so that it has to be supported off the surface . It is then necessary for some sort of distance piece to take up the gap between the fitting and the surface behind it . Standard components are available for this and are known as pattresses . A box for use with a circular socket outlet is shown in Figure 1,17 , which also gives a sketch showing the use of the box with surface conduit . The inclusion of the box makes it possible for the cables to enter the socket outlet from the back , whereas without it , there would be an untidy junction of the conduit with the bottom of the socket outlet  . It is now fairly standard practice in commercial situations to install trunking which incorporates the socket outlets .

Ceiling roses

In some situations , it is undersirable to have the lampholer hanging on the end of a flexible cable while there is no objection to having the lamp at ceiling height . In such cases , one make use of a batten lampholer ; which is illustrated in Figure 1.15 . It combines the terminal block of the ceiling rose with the lampholder in one fitting , and it can be screwed directly to a standard circular box on the ceiling . A batten lampholer could also be used o fix a light to a wall , but the lamp would project perpendicularly from the wall . The angled batten-lampholder shown in Figure 1.16 has the lampholder at an angle to the rose so that when the whole fiiting is put on the wall , the lamp is at a downward angle . Such angled battenholders can be obtained either with the lampholder at a fixed angle or with the angle adjustable .
  Lampholders frequently have protective shield which are intended to prevent accidental contact with either metal parts or with the lampholder pins themselves . Lampholders with such shields are shown in Figure 1.13 and 1.15 . These shields are often referred to as Home Office Skirts .

Lampholders and ceiling roses

In public buildings the luminaires are fixed as part of the electrical installation . In housing , the choice of the lampshade or luminaire is usually left to the owner or tenant and is made once the dwelling is occupied . plain lampholders are , therefore , provided which will accept ordinary 100w and 150 w tungsten bulbs , and which usually have a ring or skirt to which a normal lampshade or similar liminaire can be attached . The top of the lampholder screws down to grip the flexible cable cord on which it is suspended from the ceiling . Typical lampholers are shown in Figure 1.13.
  The flexible cord on which the lampholder is suspended performs two functions . It carries the electric current to the lamp , and it supports the weight of the holders , lamp and shade . Its physical strength , therefore , just as important as its current carrying capacity and it has to be selected with this in mind . At the ceiling itself , the wiring in ( or on ) the ceiling must be connected to the flexible cord . The connection is made by means of a ceiling rose , which is illustrated in Figure 1.14 . it consists of a circular plastic housing with a terminal block inside and a bushed opening on the underside where the flexible cord to the lampholder can come out of the rose . In installations which have the main wiring inside ceiling , this wiring enters the rose through the back or top of the rose ; when the main wiring runs exposed on the surface of the ceiling ,it enters the rose through a cut-out in the inside of the rose .
  Ceiling roses are made with three line terminals in addition to an earth terminal . The reason for the third terminal is used , it temains live even when the light atttached to the ceiling rose is switched off  . It must , therefore , be shrouded so that it cannot be touched by accident if ever the flexible cord is being replaced ; complete circuit isolation for this task is strongly recommended . Ceiling roses are available which incorporate a plug and without disruption to other parts of the same circuit .

Clock Connector

Special outlets are made to which electric clocks can be easily connected . A typical one is shown in Figure 1.12 and can perhaps be considered as a special -purpose fused connection outlet .
  It contains a 2A fuse and terminals to which the cable from the clock can be connected . The fuse is needed because a clock outlets is usually connected to the nearest available lighting circuit . The fuse protecting the whole circuit will never be rated at less than 5 A , and may be as much as 15 A . The clock wiring is not suitable for such a large current and must , therefore , have its own protection at the point at which the supply to it branches from the main circuit . The necessary protection is provided by the fuse in the connector . The front of the connector has an opening through which the clock cable can be taken out to the clock . In most cases , the clock connector is made flush with the wall and the clock is subsequently fixed over it . However , surface connector are available , and in this case the clock would be fixed next to the connector with a short length of cable run on the surface of the wall between the clock and connector . With the developemtn of quatz battery clocks , clocks connectors are seldom used .

Telephones outlets

To avoid the need for alot of surface cable fixed after a building is occupied , it is quite common to put in wiring for telephones as part of the services built into the structure as the building is erected . This wiring must , of course , be brought to suitable terminals in the positions at which the telepohoens are to be connected later . The olny essential requirement is an opening through which standard telephone cable can be brought out neatly . A plate with a suitable outlet which fits into a standard box , is shown in Figure 1.11 .
  THe more modern practice is to connect each telephone set to the permanent installation via a telecom socket and plug ; in the UK , a BT pattern in used ,  which is slightly different to the US pattern . The socket forms part of a lid which scres onto a standard conduit box at the agreed outlet positions . An outlet of this kind is shown in Figure 1.11 .

Tv outlets

Housing design today has to accept that every flat , maisonette or house will have a televisyen which may require connection to an outdoor aerial . It was common to provide a communal aerial system which serves all dwellings on an estate from single aerial . The chief reason for doing this is that it avoids the ugliness of a large number of aerials , all different pattern , put up close to each other by different people . It has the further advantage that one powerful aerial erected in a carefully chosen position can give better reception than the aerial which indiidual occupiers install . This signal may be fanned out to individual  dwellings through a mains supplied booster . Terrestrial channels may be accessed through satellite aerials .
  If a communal aerial system is installed , it becomes necessary to run a televisyen aerial cable from the aerial to an outlet in each dwelling , or hotel bedroom . There has to be a suitable terminal in the room , and this takes the form of a socket capable of accepting the coaxial plugs used on the end of aerial cable . An outlets of this kind is shown in Figure 1.10 . Since a televisyen set also needs a power supplt . It is usual to provide a mains socket outlets near the aerial outlet . One manufacturer makes a combined unit having an aerial socket and 13 A socket outlets within one housing .
  For radios which require both an aerial and an earth connection , special two-pin outlets are available . These can also be combined in a single unit containing the mains socket outlet as well as the two-pin outlet .

Circular boxes

In addition to rectangular boxs of the sort illustrated , circular boxes are also made . These are useful for general conduit work and terminating wiring at points which are to take fittings .
  When boxes are used for connecting lengths of conduit rather than for housing other accessories , they must have the open side covered with a blank plate . A typical plate is shown in Figure 1,9 , Circular plates are also made for circular boxes . It should perhaps not need saying that when a box is recessed in a wall , the cover must be left flush with the surface of the wall so that it can be removed to give access to the cables inside the box . This is particularly important if the system is installed with the intention that it should be possible to rewire or add cables later .

  Boxes have a wiring terminal which enables a cable to be connected to the metal of the box. This is used for connecting a circuit protective conductor ., The purpose and use of earthing is discussed in Chapter 9 . The important of the earth terminal on the box arises when the accessory which is housed in the box has to be earthed through the box . AThis is particularly important when a plastic conduit system is used which necessitates the use of a separate circuit protective conductor . These must then be some means of connecting the circuit protective conductor to the accessory and this can become difficult i there is no suiotable terminal in the box for making the connection . it is also recommended that an accessory requiring earthing , installed on a metallic conduit installation , is provided with a lead between the earth terminal on the box and the accessory , if the box has adjustable tags such as on some knockout boxes .