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
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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 70C, 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 85C. A conductor temperature as high as 160C 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.

Fig4.8 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 70C, 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 150C. Thus, a 1mm 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}.

 

 

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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