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 60°C. 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}.
7. - The
conduit must be installed so that fire cannot spread through
it, or through holes cut in floors or walls to allow it
to pass. This subject of fire spread will be considered
in greater detail in {4.5.2}
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
|