Field maintenance of MCCBs is normally limited to a visual
inspection, cleaning, and tightening of connections. Any MCCB found to have a
cracked case when inspected should be replaced because of possible interrupting
capability loss.
One other observable phenomenon that requires attention is
tracking. Tracking is an electrical discharge phenomenon indicated by a leakage
path that is directionally erratic (similar to the pattern of a lightning
stroke).
This phenomenon forms from electrical stress over a long
period of time, especially in unclean environments, and will eventually lead to
a flashover. MCCBs are designed and manufactured to require no internal
maintenance throughout their lifetime.
MCCBs are factory calibrated and sealed, and the breaker
should not be tampered with. However, some MCCBs have a removable cover that is
not intended only for installing trip units.
This does not mean that maintenance can be performed on the
MCCB with the cover off. The manufacturer did not intend this to be the case.
The following items should be checked on an MCCB during periodic maintenance
schedule:
a) Breaker overheating. Check the breaker for overheating
when it is operating under normal load at a normal operating temperature. Check
the face of the breaker using the palm of the hand.
If hand contact cannot be maintained for 3 s, the breaker
should be removed from service and checked further to determine the cause of
the overheating. Thermal imaging equipment is also available to determine
overheating conditions.
b) Connection check. Visually check the breaker and
bus/cable connections for evidence of overheating. Overheated copper
connections can usually be cleaned and dressed satisfactorily for reuse.
Overheated aluminum connectors must be replaced. All connections must be
tightened to proper torque levels specified after cleaning or replacement.
c) Mechanical operation. All MCCBs should be exercised
(opened and closed) several times to ensure proper mechanical operation.
Exercising an aged breaker is particularly important, considering many breakers
are never called upon to operate by an overcurrent condition. Exercising the
breaker can free binding mechanisms.
d) Breaker testing. Many breakers are Þtted with
thermal-magnetic trip units or magnetic (instantaneous) trip units only. Some
modern MCCBs and insulated-case circuit breakers (ICCBs) on the market today
contain electronic trip devices (ETDs).
1) Thermal unit test. The MCCB is tested by passing 300% of
the MCCBs ampere rating through each pole of the MCCB. The test results should
be compared to the manufacturer's time-current characteristics (curve sheet).
For multipole MCCBs, these curve sheets are based upon
current in all poles of the breaker and are utilized for coordination purposes;
thus, the curve sheet should be examined for maximum single-pole trip time. Not
all curve sheets specify a maximum single-pole trip time, but when one is
available it should be noted.
2)Magnetic unit (instantaneous) test. The MCCB is tested by
passing the magnetic rated trip amperes through each pole of the circuit
breaker. The circuit breaker should trip within the following parameters
according to the applicable frame size:
250 A or less: +40% to Ð25% of the instantaneous current
setting.
400 A or greater: +25% to Ð25% of the instantaneous current
setting.
The testing of instantaneous trip units usually requires a
high current test set for all but the smallest frame sizes of MCCBs. Care must
be taken when checking the instantaneous unit with high current to insure that
the thermal-trip unit is not the cause of the MCCB tripping. The high current
should be placed on the MCCB for very short periods of time with adequate cool
down time allowed between applications.
3) Shunt trip. Shunt-trip devices are utilized to trip an
MCCB via some external device operation (e.g., ground-fault relay). If an MCCB
is equipped with a shunt-trip coil (solenoid), the unit can be verifed by
applying the rated voltage across the coil, with the MCCB closed. The
shunt-trip device trips a mechanical latch (or trip mechanism) that trips the
MCCB.
4) Insulation resistance. All poles should be tested in
accordance with standard insulation resistance testing guidelines at 1000 V dc.
Resistance values of less than 1 MW should be considered inadequate and the
cause should be investigated. Insulation tests should be performed between the
line and load terminals with the MCCB open, between adjacent poles, and from
each pole to the grounded parts of the MCCB.
5) Contact resistance. Contact resistance should be measured
using an ohmmeter capable of measurements into the micro-ohm range. Contact
resistance measurements on some of the smaller sized MCCBs are not practical
where the test lead clip is larger than the MCCBs terminal.
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