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 .