Part
12470
Rev. 2 3/25/99
15
MAINTENANCE OF MOLDED CASE CIRCUIT BREAKERS
The molded case circuit breaker essentially consists of two separate elements. One element is a set
of contacts and suitable mechanical linkage for manual operation of the breaker as a switch in an
electrical circuit. The other element is a device that senses and reacts to an overload or short circuit.
Normally, the time delay overload device is thermal and the instantaneous overload device, when
supplied, is magnetic. The thermal element usually uses a bimetallic strip; two pieces of dissimilar
material bonded together. An overload causes an increase in heat which will result in moving the
bimetallic unit and eventually trip the circuit breaker. The magnetic element operates with no
intentional time delay to provide instantaneous protection against high magnitude faults.
PLANNED MAINTENANCE PROGRAM
A scheduled program for maintenance of molded case circuit breakers consists primarily of "good
housekeeping" in conjunction with visual inspections and electrical tests. A brief outline is given
below:
1.
Clean
All types of molded case circuit breakers should be externally cleaned so that the heat
produced in normal operation can be dissipated properly. It is possible for dirt or dust
caused by normal plant conditions to accumulate and prevent proper dissipation of heat,
resulting in a nuisance operation of the breaker.
2.
Tighten Connections
This is particularly important because loose electrical connections can cause extra heat
which may result in an unnecessary operation of the breaker.
3.
Test
The molded case circuit breaker should be subjected to a simulated overload and the tripping
time measured. This is important because after a period of inactivity, the overload device
may become stiff or inoperable. The only way to determine this condition and eliminate the
stiffness is to electrically operate the breaker on a periodic basis. Manually opening and
closing the main contacts of the breaker does not move any of the mechanical linkage
associated with the overload device. Testing may be as often as every 6 months or as long as
every three or four years, depending upon conditions where the breaker is installed.
THERMAL ELEMENT
