The trend toward larger unit substation transformers and larger connected
kVA loads on such substations has given way to power circuit breakers in
tested combination with current-limiting fuses. This is routinely done in
order to increase the short-circuit interrupting rating of the switchgear. This
combination can be used for all frame sizes. The fuses cause the same problems
with single phasing as fuses in the switchboards; however, there are
numerous features that compensate for this problem. First, most fuse assemblies
are attached directly to the breakers themselves so fuses cannot be
removed or installed unless the breaker is out of service. Most manufacturers
solve the single-phasing problem by either an electrical or a mechanical
means of blown fuse detection, which in turn causes the breaker to trip
immediately after the fuse has cleared. On the largest frame sizes, where the
fuses must be mounted apart from the breaker cubicle, the fuse assembly is
on a truck or roll-out which is mechanically interlocked with the breaker it
serves. It should be noted that the overcurrent protection for overloads is still
handled by the breaker’s overcurrent trip devices, and that the fuse is not
expected to clear except for the most severe short circuits