4.3.2 - Current carrying capacity of conductors
All cables have electrical resistance,
so there must be an energy loss when they carry current.
This loss appears as heat and the temperature of the cable
rises. As it does so, the heat it loses to its surroundings
by conduction, convection and radiation also increases.
The rate of heat loss is a function of the difference in
temperature between the conductor and the surroundings,
so as the conductor temperature rises, so does its rate
of beat loss.
A cable carrying a steady current, which
produces a fixed heating effect, will get hotter until it
reaches the balance temperature where heat input is equal
to heat loss {Fig 4.8}. The final temperature achieved by
the cable will thus depend on the current carried, how easily
heat is dissipated from the cable and the temperature of
the cable surroundings.
PVC. is probably the most usual form of
insulation, and is very susceptible to damage by high temperatures.
It is very important that p.v.c. insulation should not be
allowed normally to exceed 70°C, so the current ratings
of cables are designed to ensure that this will not happen.
Some special types of p.v.c. may be used up to 85°C. A conductor
temperature as high as 160°C
is permissible under very short time fault conditions,
on the assumption that when the the fault is cleared the
p.v.c. insulation
will dissipate the heat without itself reaching a dangerous
temperature.
Fig 4.8
Heat balance graph for a cable
A different set of cable ratings will become
necessary if the ability of a cable to shed its beat changes.
Thus, [Appendix 4] has different Tables and columns for
different types of cables, with differing conditions of
installation, degrees of grouping and so on. For example,
mineral insulation does not deteriorate, even at very high
temperatures. The insulation is also an excellent heat conductor,
so the rating of such a cable depends on how hot its sheath
can become rather than the temperature of its insulation.
For example, if a mineral insulated cable
has an overall sheath of LSF or p.v.c., the copper sheath
temperature must not exceed 70°C, whilst if the copper sheath
is bare and cannot be touched and is not in contact with
materials which are combustible its temperature can be allowed
to reach 150°C. Thus, a 1mm² light duty twin mineral insulated
cable has a current rating of 18.5 A when it has an LSF
or p.v.c. sheath, or 22 A if bare and not exposed to touch.
It should be noticed that the cable volt drop will be higher
if more current is carried (see{4.3.11}).
[Appendix 4] includes a large number of Tables relating
to the current rating of cables installed in various ways.
The use of the Tables will be considered in more detail
in
{4.3.4 to 4.3.11}.
