Handbook of Electrical Engineering

156 HANDBOOK OF ELECTRICAL ENGINEERING

High voltage contactors use similar protection devices to those used with high voltage circuit
breakers, except that high voltage fast-acting fuses are also connected in series with the contactor. The
protection devices tend to be more sophisticated and are usually mounted away from the contactor and
fuse assembly, in a relay compartment. Single-phase protection is usually required for high voltage
motors. Earth fault protection also tends to be more sophisticated and special current transformers
and relays are necessary. See Chapter 12 for details of protection relays and their coordination with
each other and with their associated equipment.

7.4 Fuses for Motor Control Centre Outgoing Circuits

Fuses are chosen to match the normal current of the load. The fuse current rating is always chosen
to be higher than the load current by an amount called the ‘fusing factor’, which is given by:-

Fusing factor=

Fuse rating, in amps

Normal load current, in amps

For low voltage motors the fusing factor is larger for the small motors (less than 15 kW) than it
is for the large motors (up to 250 kW). Figure 7.4 shows how the typical fusing factor varies for low
voltage motors. The slope and bias of the lines in the figure will be different for each type or ‘model’
of fuse. For high voltage motors the fusing factor tends to be between 1.5 and 2.5. The characteristics
of the fuses vary according to the type of load, e.g. continuous motor load, very intermittent motor
load, feeder transformers, static heaters, thyristor controlled loads, power rectifier loads.

Figure 7.4 Low voltage fusing factor for induction motor circuits.