MCCBs are available in a wide range of ratings and are generally used for
low-current, low-energy power circuits. The breakers have self-contained
overcurrent trip elements. Conventional thermal-magnetic circuit breakers
employ a thermal bimetallic element that has inverse time–current characteristics
for overload protection and a magnetic trip element for short-circuit protection.
Conventional MCCBs with thermal-magnetic trip elements depend on the total
thermal mass for their proper tripping characteristics. This means that the proper
sized wire and lug assemblies, which correspond to the rating of the trip element,
must be used on the load terminals of such breakers. Many manufacturers are
now switching over from bimetallic elements to power sensor (electronic) type
trip elements. Magnetic-trip-only breakers have no thermal element. Such breakers
are principally used only for short-circuit protection. Molded-case breakers
with magnetic only trips fi nd their application in motor circuit protection. This
arrangement is desirable for smaller motors where their inrush current can ruin
a delicate thermal element but where protection for winding failure is still needed.
The breaker provides the instantaneous (INST) protection and fault interruption,
and other overload devices in the starter handle the long-time overload protection.
Nonautomatic circuit breakers have no overload or short-circuit protection.
They are primarily used for manual switching and isolation.