4.3.6 - Thermal insulation correction
factors
The use of thermal insulation in buildings,
in the forms of cavity wall filling, roof space blanketing,
and so on. is now standard. Since the purpose of such materials
is to limit the transfer of heat, they will clearly affect
the ability of a cable to dissipate the heat build up within
it when in contact with them,
The cable rating tables of [Appendix 4]
allow for the reduced heat loss for a cable which is enclosed
in an insulating wall and is assumed to be in contact with
the insulation on one side. In all other cases, the cable
should be fixed in a position where it is unlikely to be
completely covered by the insulation. Where this is not
possible and a cable is buried in thermal insulation for
0.5 m (500 mm) or more, a rating factor (the symbol for
the thermal insulation factor is Ci) of 0.5 is applied,
which means that the current rating is halved.
|
Table 4.5 - Derating factors (CI)
for cables up to 10mm² in cross-sectional
---------------- area
buried in thermal insulation.
|
|
Length in insulation (mm)
|
Derating factor (CI)
|
|
50
|
0.89
|
|
100
|
0.81
|
|
200
|
0.68
|
|
400
|
0.55
|
|
500 or more
|
0.50
|
If a cable is totally surrounded
by thermal insulation for only a short length (for example,
where a cable passes through an insulated wall), the heating
effect on the insulation will not be so great because heat
will be conducted from the short high-temperature length
through the cable conductor. Clearly, the longer the length
of cable enclosed in the insulation the greater will be
the derating effect. {Table 4.5} shows the derating factors
for lengths in insulation of up to 400 mm and applies to
cables having cross-sectional area up to 10 mm².
Commonly-used cavity wall fillings, such
as polystyrene sheets or granules, will have an adverse
effect on p.v.c. sheathing, leeching out some of the plasticiser
so that the p.v.c. becomes brittle. In such cases, an inert
barrier must be provided to separate the cable from the
thermal insulation. PVC cable in contact with bitumen may
have some of its plasticiser removed: whilst this is unlikely
to damage the cable, the bitumen will become fluid and may
run.