Selection of Fuse Wire

Selection of Fuse Wire Points : selection of fuse wire, Cable and Fuse Ratings for Motors The fuse rating for steady load circuits such as heating loads should be equal to or next greater than the rating of smallest cable in the circuit. For fluctuating load circuits like those of motor, capacitor and transformer loads fuse should have such current-time characteristic as to allow the short time over current to flow without blowing. For this it is sometimes necessary to select fuses of rated current greater than that of the cable of the circuit, In motor circuit fuse is to be rated for short circuit and not for over current protection which is looked after by over current protection provided in starter. The over current protection, on the other hand, is not suitable for clearing heavy short circuit current. As very heavy starting current over long period and such these are rated at twice the full load current of motor. If this were not so fuses would operate every time the motor is started up. With three phase motors it is always desirable to be liberal in selecting the size of the fuse rather than allowing one of the fuses to operate causing motor to run on single phase and get damaged for good for motors which have protracted starting period or are driving heavy inertia loads it may be necessary to select fuses of required current-time characteristics.

For other type of fluctuating load circuits such as capacitor circuit, fluorescent lighting circuit, battery charger circuit and transformer circuit, the fuses should, as a general guide, have 50% are rated current. Note 1. If one fuse feeding a three phase motor has operated, it is a good practice to replace all the unblown fuses also otherwise there may be single phasing trouble. For the same very reason all the three fuses should always be of same rating.

2. Fuses are inserted in live wires only and not in neutral wire. Cable and Fuse Ratings for Motors These are given in Table below: