FIELD MAINTENANCE OF MOLDED CASE CIRCUIT BREAKER (MCCB) TUTORIALS

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.

6) Rated load test. A rated load hold-in test can be run if there is some doubt of the MCCBs ability to carry rated load. With the MCCB in free air, all three poles are connected in series with jumpers of a short length and adequate capacity. Applying rated load current for a minimum of 30 min should not cause the breaker to trip.

Related post



No comments:

PREVIOUS ARTICLES