There are two basic types of low-voltage switchgear structures, they are
indoor and outdoor types. Indoor switchgear consists of a front section containing
circuit breakers, meters, relays and controls, bus section, and cable
entrance section. The outdoor section is similar to the indoor switchgear
except a structure that is provided around it for weatherproofi ng. Bus bars
are available either in copper or aluminum. When aluminum bus is s pecifi ed,
bolted joints should be made with Belleville washers to minimize cold fl ow
characteristics and maintain tight connections. Generally, bare bus bars are
used. However, insulation can be specifi ed on special orders. The normal
clearance between phase to phase and phase to ground is 2 in. to minimize
creepage for 600 V rated equipment. The standard high-voltage withstand is
2200 V AC for phase to phase and phase to ground for a period of 1 min.
Totally enclosed low-voltage switchgear, in its present form, began to gain
acceptance through the 1950s. The manufacturers of the time marketed threepole
circuit breakers and their enclosures as equipment that was safer for the
user’s personnel, more reliable, and as having advantages over fuses; namely,
prevention of single phasing on three-phase AC systems. Today, low-voltage
switchgear takes on many specialized forms and functions that combine metering,
monitoring, control, protection, and distribution. Major manufacturers