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 .
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