Normal current rating of BAJAJ CABLES as given in the tables takes into account temporary overloading which often occurs in use. However to safeguard against excessive thermal damage, it is essential that protective switchgear of a proper rating is used in circuit.
Cables laid in air or in ducts can safely be overloaded to 1.5 times the amount given in the current rating tables for four hours provided the protective system uses a class P fuse designed to operate at a 50% overload.
In case the protective circuit has a different fuse rating, select a cable by multiplying the tripping current by 0.67 to arrive at a suitable size of cable. For example for carrying 200 amps current in the air 70 sq. mm. single core cable is suitable (current rating is 230 amps.). This cable can therefore be overload to the extent of 230 x 1.5 = 345 amp. provided that protective fuse rating is 230 x 1.5 = 345. However, if the protective fuse rating is 400 apm, normal rating 400 x .67 = 268 amps. 95 sq. mm. cable should be used. Similarly current rating for cables in the ground can safely be used for an overload factor of 1.3 for four hours. It is essential that the protection is designed to operate at a 30% overload. In case this is different, select a cable to carry 0.77 times the current at which protection is designed to operate.
SHORT CIRCUIT RATING FOR ALUMINIUM CONDUCTOR CABLES
With a high increase in KV A capacity of the power distribution system, cables are expected to carry short circuit currents of high magitude. Normally rated at 70°C our insulating material permit a short circuit temperature of 160°C. With the high interrupting capacity expected of a cable under short circuit, it is essential that protective of fuses in the system are designed to minimise the duration as far as possibe. Short circuit rating of a cable can be calculated as under:
Where Ish = 75.8 X A/root t Ish : Short circuit current in r.m.s. amps. t : Duration of short circuit in seconds. A : Area of conductor in sq. mm.
Constants are tabulated below for different duration of short circuit:
