Industrial and Utility Application Guide

A fuse may rated for a.c. usage or d.c. usage or both. If the circuit is a.c. then the fuse must have appropriate a.c. ratings, or if the circuit is d.c. then the fuse must have appropriate d.c. ratings.

The voltage rating of the fuse must equal or exceed the rated voltage of the circuit.

The breaking capacity of the fuse must equal or exceed the fault level at the point of installation.

The current rating of the fuse must exceed the circuit load. The current rating of the fuse must be less than the continuous current rating of the cable. For overload protection the conventional fusing current of the fuse must protect the cable against its maximum current carrying capacity.

For protection against earth faults at 5.0 seconds and 0.4 seconds the earth fault loop impedance of the circuit must be less than the maximum earth fault loop impedance (Zs) specified for the fuse-link rating in Tables 41.2 and 41.4 of BS 7671 (formerly IEE Wiring Regulations).

“gG” fuse-links discriminate in a ratio of 1.6:1. Therefore the upstream major fuse-link must have a current rating of at least 1.6 times the current rating of the downstream minor fuse-link.

For protection of capacitor circuits a general rule is that the current rating of the fuse should be at least 1.5 times the rated current of the capacitor.

The current rating of the fuse-link should be at least twice the full load current of the maximum number of lights to be switched simultaneously.

For thermal derating of fuses at temperatures above 40ºC a general rule is that the current rating is decreased by 0.5% for each 1ºC above 40ºC. Where the fuse is mounted in an enclosure use the internal enclosure temperature if known. Where the internal temperature is unknown it should be taken as the external ambient temperature +15ºC.

For installations situated at altitudes of over 2000 metres a general rule is that the current rating of the fuse is decreased by 0.5% for every 100m above 2000m.

Motor starter manufacturers recommend the current ratings of “gG” fuse-links which can be used in conjunction with motor starters. This simple and effective means of co-ordination to withstand inrush currents will normally give adequate short circuit protection to the motor starter.