Some light industrial systems or commercial buildings will use switchgear
or switchboards with a high pressure or bolted pressure, three-pole switch
acting as incoming service main disconnects. The principal feature of these
switches is their continuous current capacities of up to 3000 or 4000 A. At these
currents, very high contact pressure is required on the conducting surfaces in
order to hold temperature rises to reasonable levels. The switches themselves
carry an interrupting rating similar to those for three-pole interrupter
switches but not as high as that for power circuit breakers. Interrupting capability
for short circuits is almost always handled by current-limiting fuses,
which are an integral part of the switch. Most manufacturers provide single
phase, blown fuse, and ground fault accessories so that the switches can be
used on low-voltage service entrances. Unlike trip devices applied to circuit
breakers, these protective devices are not self- powered. Rather, they take operating
voltage from a small control power transformer on the source side of the
switch. Generally, the mechanical design of these switches is based on a minimum
of operations. The number of operations is expected to be less than for
insulated-case breakers; usually limited to isolation during maintenance or
for serious ground faults not cleared successfully by other means.