Temperature rises in mineral insulated metal sheathed (MIMS) cables can be high enough to cause burns and deterioration of the insulation of other equipment such as accessories or pvc cables with which they may be in contact
A bare mineral insulated cable, under full load conditions, is permitted to have a sheath temperature of up to 105'C (Table 4G2A of BS 7671 refers).
Use of the higher ratings given in Table 4G2A for a bare MIMS cable could result in the risk of burns, fires, other injurious effects and damage to the equipment such as an accessory that is not suitably rated.
For example, where a MIMS cable is installed adjacent to an ordinary PVC cable, care must be taken that the permitted temperature rise in the MIMS cable is not so high as to cause deterioration in the insulation of the PVC cable. This applies to MIMS cables bunched in air with PVC-insulated and sheathed cables as well as to MIMS and PVC-insulated cables installed together in trunking.
Solution
If the MIMS cables are loaded at currents no greater than those shown in Tables 4G1A (sheath operating temperature 70'C), and the appropriate rating factors for ambient temperature and grouping are taken into account, no problems are likely to arise
Alternatively, bare MIMS cables which are likely to operate at temperatures greater than 70'C should be installed in such a manner that they:
- Cannot be touched, and
- Are segregated from other cables, and
- Are terminated into equipment suitably rated for such elevated temperatures.
Regulation 131.3.2 refers
Regulation 131.3.2
Persons, fixed equipment and fixed materials adjacent to electrical equipment shall be protected against harmful effects of heat or thermal radiation emitted by electrical equipment, particularly the following consequences:
- Combustion, ignition, or degradation of materials
- Risk of burns
- Impairment of the safe function of installed equipment
Electrical equipment shall not present a fire hazard to adjacent materials