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
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