For more information:
800-TEC-FUSE
www.littelfuse.com
8. Avoid Hazards of Improperly Selected or
Maintained Overcurrent Protective Devices.
Whether in the design or maintenance of
an electrical system, hazards exist if the
proper overcurrent device is not selected and
applied. Circuit breakers and other electrical
equipment must be maintained and serviced
regularly to ensure that they will operate
properly when needed. Unfortunately, in many
industries and especially during economic
turndowns, the tendency is to limit or eliminate
regularly scheduled maintenance on circuit
breakers and other electrical equipment.
However, the potential costs associated with
OSHA violations, liability lawsuits, workers
compensation, equipment replacement, and
lost production far exceeds the costs of
regular testing and maintenance of circuit
breakers and other electrical equipment.
OSHA 29 CFR 1910.334(b)(2)
“Reclosing circuits after protective
device operation. After a circuit is
deenergized by a circuit protective
device, the circuit may NOT be
manually reenergized until it has been
determined that the equipment and
circuit can be safely reenergized. The
repetitive manual reclosing of circuit
breakers or reenergizing circuits
through replaced fuses is prohibited.
NOTE:
When it can be determined from the
design of the circuit and the overcurrent
devices involved that the automatic
operation of a device was caused
by an overload rather than a fault
condition, no examination of the circuit
or connected equipment is needed
before the circuit is reenergized.”
In this section of the regulations, OSHA
recognizes the importance of knowing why
the overcurrent protective device has opened.
If the fuse or circuit breaker opened due to
an overload, no examination of the circuit or
connected equipment is necessary. However, if
the overcurrent protective device opened due
to a short circuit fault, catastrophic results can
occur if the fuse or circuit breaker is replaced or
closed on the short circuit before it is corrected.
This is especially important for circuit breakers
and switches because short circuit currents can
permanently damage the equipment to the point
that it will not operate safely when reenergized.
Circuit Breakers
Circuit breakers, like fuses, are rated to safely
interrupt their maximum interrupting current
only once. Molded Case Circuit Breakers
(commonly referred to as MCCB’s) must meet
the requirements of UL489, “Standard for
Safety,” Molded-Case Circuit Breakers, Molded
Case Switches and Circuit Breaker Enclosures.
This standard allows manufacturers to list their
circuit breakers at varying degrees of available
fault currents, current-limiting ability and other
characteristics. They must be applied within
the maximum limitations of their ratings.
Circuit breaker manufacturers typically
recommend that their circuit breakers be cycled
ON and OFF at least once each year to keep
the tripping mechanism from seizing under
certain environmental conditions. Cycling circuit
breakers ON and OFF manually may help keep the
switching mechanism from seizing, but may not
guarantee that the tripping mechanism will operate
properly. Some manufacturers also recommend
that their circuit breakers be periodically tested and
recalibrated under carefully controlled conditions.
When testing time-current characteristics,
recommendations state the circuit breaker
being tested must be at room temperature. This
practice would increase equipment downtime
while the circuit breaker to be tested cools
down after it is removed from service.
The National Electrical Manufacturers Association
(NEMA) has published standard AB 4-2003
entitled, “Guidelines for Inspection and Preventive
Maintenance of Molded Case Circuit Breakers
Used in Commercial and Industrial Applications.”
It deals exclusively with the maintenance and
care of Molded Case Circuit Breakers to provide
reliable protection. The expected lifetime of a
circuit breaker, however, depends on circuit
conditions and its’ environment. Standard AB
4-2003 emphasizes that safe work practices
MINIMIZING ARC-FLASH AND OTHER ELECTRICAL HAZARDS
Circuit breakers trip-
ping mechanisms
could seize up and
not operate properly
if not maintained
to manufacturer’s
specifications.
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