to the 16th Edition IEE Regulations
   
   
   
 
 

chapter 5
Earthing

chapter 6
Circuits

Cables, conduits and trunking
  4.1 - Cable insulation materials 4.4 - Cable supports, joints and terminations
  4.2 - Cables 4.5 - Cable enclosures
  4.3 - Cable choice 4.6 - Conductor and cable identification


4.5.1 - Plastic and metal conduits

A system of conduits into which unsheathed cables can be drawn has long been a standard method for electrical installations. The Regulations applying to conduit systems may be summarised as follows:

1. - All conduits and fittings must comply with the relevant British Standards.

2. - Plastic conduits must not be used where the ambient temperature or the temperature of the enclosed cables will exceed 60C. Cables with thermo-setting insulation are permitted to run very hot, and must be suitably down-rated when installed in plastic conduit. To prevent the spread of fire, plastic conduits (and plastic trunking) must comply with ignitability characteristic 'P' of ES 476 Part 5.

3. - Conduit systems must be designed and erected so as to exclude moisture, dust and dirt. This means that they must be completely closed, with box lids fitted. To ensure that condensed moisture does not accumulate, small drainage holes must be provided at the lowest parts of the system.

4. - Proper precautions must be taken against the effects of corrosion (see {4.2.5}), as well as against the effects of flora (plant growths) and fauna (animals). Protection from rusting of steel conduit involves the use of galvanised (zinc coated) tubing, and against electrolytic corrosion the prevention of contact between dissimilar metals eg steel and aluminium. Any additional protective conductor must be run inside the conduit or its reactance is likely to be so high that it becomes useless if intended to reduce fault loop impedance.

5. - A conduit system must be completely erected before cables are drawn in. It must be free of burrs or other defects which could damage cables whilst being inserted.

6. - The bends in the system must be such that the cables drawn in will comply with the minimum bending radius requirements {4.4.2}.

8. - Allowance must be made, in the form of expansion loops, for the thermal expansion of long runs of metal or plastic conduit. Remember that plastic expands and contracts more than steel.

9. - Use flexible joints when crossing building expansion joints

Table 4.12 - Maximum spacing of supports for conduits
Conduit diameter
Rigid metal (m)
Rigid insulating (m)
(mm)
Horizontal
Vertical
Horizontal
Vertical
Up to 16
0.75
1.0
0.75
1.0
16 to 25
1.75
2.0
1.5
1.75
25 to 40
2.0
2.25
1.75
2.0
Over 40
2.25
2.5
2.0
2.0

 

 

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Extracted from The Electricians Guide Fifth Edition
by John Whitfield
Published by EPA Press Click Here to order your Copy

Click here for list of abbreviations