Protection by fuses

It is very common to employ the radial system shown and to use a major or upstream fuse in the supply connection (PD4) and minor or down-stream fuses in the individual load circuits (PD1, 2 and 3).

Clearly, each minor fuse must have the time/current characteristic needed to protect its load circuit and a fault on a particular load should only cause its associated minor fuse to operate. The major or upstream fuse (PD4) will also carry the fault current but it must not operate or be impaired.

For faults which cause relatively small currents to flow, the arcing times, as proportions of the pre-arcing times, are small and consequently discrimination can be predicted by comparing the time current curves of the major and minor fuses. Provided that the curves for the minor fuses are to the left of that for the major fuse, i.e. the minor fuses operate more quickly, then discrimination should be obtained.

At higher fault current levels which will result in melting of the minor fuse in less than 100 ms, the arcing time of the minor fuse must be taken into account. This is done not by considering the actual values of time, but by using the I2t values. The requirement is that the pre-arcing I2t of the major fuse shall exceed the total operating I2t of the minor fuse by a reasonable margin (say 40%).

An integrated system protected by fuselinks excel in this application, giving minimum disruption in the system. The standardisation of gG fuselink characteristics ensures that discrimination between fuselinks can be achieved on a 1.6:1 ratio of current ratings for most practical situations. The 1.6:1 ratio represents two steps in the R10 series of ratings, i.e. a 100A (downstream) fuselink will discriminate with a 160A (upstream) fuselink.

The particular case which arises when discrimination has to be achieved between fuses on the two sides of a transformer, the effective transformation ratio needs to be taken into account.