as some fuse link particles. There also may be a very
loud report. Accordingly, when one is working in the
vicinity of a fuse cutout, care should be taken to stay
clear of the exhaust path. In addition, when closing a
cutout, it is good practice to look down, away from the
discharge path, since there is always the possibility of
closing the cutout into a short circuit.
RECOMMENDED SIZE OF PRIMARY
FUSE.Table 4-3 gives the recommended size of
primary fuses to use with different transformer voltage and
kilovoltampere ratings. The table also gives the normal
full-load primary current rating of the transformer.
It is general practice not to protect distribution
transformers against small overloads. To do so would
cause unnecessary blowing of the fuses and frequent
interruption of the service, both of which are
undesirable. It is customary, therefore, to provide fuses
which have a higher current rating than the current
rating of the transformer.
AIR SWITCHES.As their names imply, air
switches are switches whose contacts are opened in air.
Air switches are further classified as air circuit
breakers, air-break switches, and disconnects.
Circuit Breaker.A device used to complete,
maintain, and interrupt currents flowing in a circuit
under normal or faulted conditions is called a circuit
breaker. The circuit breaker has a mechanism that
mechanically, hydraulically, or pneumatically
operates the circuit breaker contacts. Insulating oil, air,
compressed air, vacuum, or sulfur hexafluoride gas is
used as an arc-interrupting medium and a dielectric to
insulate the contacts after the arc is interrupted. If the
circuit is to open automatically during overload or
short circuit conditions, the circuit breaker is equipped
with a tripping mechanism to accomplish this. Thus,
circuit breakers normally are used where control of the
circuit, as well as protection from overload, short
circuit, and so forth is desired, such as at generating
stations and substations.
Air-Break Switch.The air-break switch can
have both blade and stationary contacts equipped with
arcing horns. These horns are pieces of metal between
which the arc forms when a current-carrying circuit is
opened. As the switch opens, these horns are spread
Table 4-3.Fuse Size for Transformer Installations