Page 84 - Offshore Electrical Engineering Manual

Types of Interrupter 71

MAKING CURRENT
The most onerous fault for a circuit breaker to handle is where a short circuit
fault exists on a part of the system which is isolated from the generator(s), and
the intervening circuit breaker is closed. This would mean that the full system
voltage is applied with no decrement. If the fault is close to the circuit breaker,
then the intervening impedance is small, giving the highest instantaneous current
flow.

TYPES OF INTERRUPTER
HRC CARTRIDGE FUSES
1. A fuse is basically a device with a central conductor that is designed to melt
under fault conditions. Interruption is achieved by spacing the two ends suf-
ficiently far apart for the arc to be naturally extinguished. However, to obtain
a consistent performance characteristic, fuse elements are carefully designed
for particular voltage and current ratings. The ceramic cartridge tube is filled
with powdered quartz and sealed. The silver element is not a continuous strip of
silver, but is necked in short sections to reduce pre-arcing time. It may also have
sections of low melting point, ‘M’-effect material, to improve performance at
low fault levels (see Fig. 2.5.4).
2. The physical design of the element such as the length and shape of the necks
will depend on the application of the fuse and its operating voltage. If the
maximum arc voltage is exceeded, the pre-arcing time will be reduced and
the fuse will not operate according to its standard characteristic. Since a fuse
is purely a means of protection against overload and fault currents, isolators
or contactors are installed in series to carry out normal switching
operations.
3. The advantage in using fuses is that the resulting switch-fuse device is often
less expensive, smaller and lighter than the equivalent circuit breaker, particu-
larly in the higher power ranges. Exceptions to this for low voltage distribution
equipment are some of the higher performance current limiting miniature circuit
breakers. These will be discussed later in the chapter.
4. The disadvantages with fusegear are:-

a. HRC fuses must be replaced, once operated and no matter how reliable the
circuit is, some holding of spare fuses will be necessary and replacement
of the larger bolted fuses is time-consuming. Permanent power fuses are
available, which use the thermodynamic characteristics of liquid sodium to
interrupt fault currents for a few milliseconds during which time a suitable
switching device is operated to isolate the faulty circuit. This type of fuse is
not (to the author’s knowledge) used offshore.
b. The standard ranges of fuses available are limited in current and fault capac-

ity below the maximum ratings available in switchgear ranges.