4. MAINTENANCE OF MOLDED CASE CIRCUIT BREAKERS
DESCRIPTION:
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 electric circuit. The other element is a device to sense
and react to an overload
or
short circuit. Normally, the time delay overload device
is
thermal and the instantaneous overload device, when supplied, is magnetic. Some
newer styles include solid-state trip elements and operate very similar to low voltage
power circuit breakers.
The thermal element 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 due to the difference in heat expansion characteristics, releasing a
latching spring which trips the circuit breaker.
A small percentage
of
molded case circuit breakers achieve their time delay through the
use
of
an electromagnet, whose operation is opposed by a fluid filled dash pot. The
magnetic element operates with no intentional time delay to provide instantaneous
protection against high magnitude faults.
In small molded case circuit breakers, the instantaneous element is not adjustable
as
it
is factory set and sealed.
In
larger molded case breakers, the instantaneous pickup of
the trip may be adjustable and is set with an adjustment screw. This type breaker may
be shipped from the factory with the instantaneous element set at maximum
if
the
setting is not specified by the purchaser; therefore, it is necessary to check these
adjustable instantaneous settings before putting the breaker
in
service.
PLANNED
MAINTENANCE PROGRAM:
A scheduled program for maintenance
of
molded case circuit breakers consists
primarily
of
"good housekeeping" in conjunction with visual inspections, tightening
of
connections and electrical testing. A brief outline is given below:
1.
CLEAN - All types
of
molded case circuit breakers should be externally cleaned
periodically 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 will cause deterioration
of
the breaker terminals and an eventual phase
to phase
or
phase to ground fault.
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 3
or
4
29
To Next Page
Loading...
Need help?
General
