Quantcast Figure 510. Cutaway view of typical molded case circuit breaker

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TM 5-683/NAVFAC MO-116/AFJMAN 32-1083
Molded Case --
-Arc Chute
-Movable Contact
Trip Indication
Trip Free
Trip Bar
Figure 510. Cutaway view of typical molded case circuit breaker.
field, there are many variables that must be recog-
of the circuit breaker to this overload is indicative of
nized and taken into account. Underwriters Labora-
its reaction throughout its entire overcurrent trip
tories, Inc. (UL) "Standard for Branch Circuit and
range. The 300 percent load is chosen as the test
Service Circuit Breakers" (#489) is the basis for
point because it is relatively easy to generate the
performance standards for all molded case circuit
required current in the field. Also, the wattage per
breakers bearing the UL label. Anyone testing
pole from line to load is small enough so the dissi-
molded case circuit breaker performance character-
pation of heat in the non-active pole spaces is minor
istics should study these standards and be familiar
and does not appreciably affect the testing results.
with the conditions specified for the qualifying
Various test equipment and test procedures are
tests. The principal purpose of field testing is not to
available for molded-case circuit breaker testing (re-
determine if the breakers exactly meet the manu-
fer to the circuit breaker manufacturer for recom-
facturer's published curves but rather to determine
mended testing equipment and procedures). Test
if the device is furnishing the protection for which it
equipment generate high currents at low voltages
was installed; namely, the protection of that part of
and are safe and convenient to use for field testing.
the electrical system to which it is applied. For
For specific minimum and maximum tripping times
instance, a circuit breaker that trips in less than the
given 300 percent current flow, refer to the manu-
minimum time shown by the manufacturer's trip
facturer's document for the breaker being tested (fig
time curve may furnish more protection than ex-
511). If the breaker does not trip within the speci-
fied bandwidth, then the breaker should be re-
pected. When field testing circuit breakers, it is
placed. The instantaneous magnetic trip character-
recommended that the overcurrent trip test be per-
istics of the breaker can be influenced by stray
formed at 300 percent of rated current. The reaction


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