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
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