Low-voltage AC switchgear designs are still widely applied to low-voltage
direct current (DC) distribution centers up to 250 V. Previously, manufacturers
provided two-pole, draw-out circuit breakers for DC switchgear. Today, the
same three-pole design, and three-phase bus arrangement, is provided for
both DC and AC applications; with the extra pole either unused or placed in
series with one of the others according to the particular manufacturer’s application
preferences. As of this writing, direct-acting overcurrent trip devices
are not offered for the new low voltage power circuit breakers. The directacting
and electromechanical trip devices have been replaced by microprocessor
based (electronic) trip devices for overcurrent protection. However,
in the molded and insulated case low voltage circuit breakers both electronic
and thermal-magnetic overcurrent trip devices are offered. The electromechanical
and dicect-acting trip devices are still available in the secondary
market as replacement for the older low voltage power circuit breakers.
Low-voltage generator paralleling switchgear continues to become more commonplace
as utilities and consumers strike agreements for cogeneration or load
curtailment contracts. Although similar in form to unit substation type switchgear,
it is vastly more sophisticated in the areas of protection and control. It is common
today to see low-voltage switchgear with protective relaying that used to be
found only on medium-voltage switchgear in a utility’s generating station.