near energized equipment or conductors. The following are suggestions for the operation of
fuses, which will aid in obtaining satisfactory performance:
(a) Verify that fuses are disconnected from all power sources before servicing
(b) A blown fuse indicates an overload or a short circuit. Do not replace fuses until
faults are located and corrected.
(c) Always replace a fuse with a fuse of the same type and rating. Never replace a
K-type fuse with an H-type fuse. Similarly, if a sign calls for a certain manufacturer's fuse, do
not substitute this for another. Although it may be the same class, the interrupting capacity may
(d) Special care should be taken to see that the fuses are securely locked or latched in
the closed position.
(e) Replace all fuses of a group when one or more have blown, such as both fuses on a
single-phase transformer or all three fuses on a three-phase transformer bank. Although a fuse
did not blow, it may have been damaged by the fault.
(f) Spare fuse units should be stored in such a manner that they will not be damaged
and will be readily available when needed.
(g) Fuses used on static capacitors should not be removed or replaced without first
discharging capacitors. Capacitors used in power applications have a discharge resistor to reduce
the voltage to a specified value in a specified time after being disconnected. Sole reliance on this
feature for safety is not advisable.
4.6 LOW-VOLTAGE CIRCUIT BREAKERS. Low-voltage circuit breakers fall into two basic
classifications and are defined in the following paragraphs.
4.6.1 Molded-Case Circuit Breakers. A molded-case circuit breaker is assembled as an
integral unit in a supporting and enclosed housing of insulating material.
4.6.2 Low-Voltage Power Circuit Breakers. A low-voltage power circuit breaker has a metal
frame and is used on circuits rated at 600 VAC and below.
4.6.3 Air Circuit Breaker. The term air circuit breaker is often used when describing
low-voltage power circuit breakers. Since the arc interruption takes place in the air in both
molded-case circuit breakers and low-voltage power circuit breakers, this term really applies to