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