IV. MCCB APPLICATION MYTHS VS FACTS
Over the course of the past 30 years, the authors have
been challenged to refute a number of claims regarding the
application of MCCBs. This seems to be especially prevalent
when the discussion is focused on their merits versus other
overcurrent protective device technologies. Without digressing
to a “mine is better than yours” debate, it is important at this
point to at least correct some common misconceptions
regarding application of MCCBs in today’s power distribution
systems.
No! Many users have elected to derate the nameplate
interrupting rating of MCCBs, in part because the battery of
tests MCCB designs are subjected to as a part of the UL489
test standard are typically not well known or understood. This
decision is driven by the notion that the device may be
capable of interrupting the maximum nameplate rating one
time, but it would then be unusable and incapable of
interrupting the next fault. In actual practice, these devices are
subjected to tests to prove them safely capable of interrupting
multiple fault currents. The UL489 test standard requires
MCCBs to interrupt the maximum nameplate fault current
twice. Industry standard ampere interrupting capacity (AIC)
ratings have been established to simplify breaker selection
and application among the multiple manufacturers. It should
be noted that derating of MCCB interrupting ratings should be
considered on the rare occasion that these devices are
applied on a corner grounded delta connected system.