Maintenance of Molded Case Circuit Breakers (MCCBs)
Maintaining circuit breakers deserves special attention because of its importance for routine switching and protection of other equipment.
A circuit breaker may not work due to a lack of preventive maintenance.
The need for maintenance of circuit breakers is often not obvious, as they can remain inactive, whether open or closed, for long periods of time.
Circuit Breakers That Will Not Be Used For 6 Months Or More It should be opened and closed several times in a row to verify proper operation and to remove any accumulation of dust or foreign matter on moving parts and contacts.
Molded case circuit breakers are designed to require little or no routine maintenance throughout their normal life. Therefore, the need for preventive maintenance will vary depending on the operating conditions. Since a build-up of dust on the locking surfaces can affect the operation of the circuit breaker, molded case circuit breakers should be exercised at least once a year. Maintenance of Molded Case Circuit Breakers (MCCBs).
Routine testing should be done every 3 to 5 years.
Maintenance of Molded Case Circuit Breakers
Routine maintenance tests
Routine maintenance tests allow personnel to determine if circuit breakers can perform their basic circuit protective functions . The following tests can be performed during routine maintenance and are intended to ensure proper operation of circuit breakers.
The following tests should only be performed on circuit breakers and de-energized equipment.
Insulation resistance test
A megohmmeter can be used to test between opposite polarity phases and live parts of the circuit breaker to ground. A test must also be carried out between the line and the load terminals with the circuit breaker in the open position. The load and line conductors should be disconnected from the circuit breaker during insulation resistance tests to prevent the test measurements from also showing the resistance of the connected circuit.
Resistance values less than 1 megohm are considered dangerous and the circuit breaker should be inspected for possible contamination on its surfaces.
Milivolt Drop Test
A millivolt drop test can reveal several anomalies Poor conditions inside a circuit breaker, such as eroded contacts, contaminated contacts, or loose internal connections. The millivolt drop test must be performed at a DC voltage rating of 50 Amps or 100 Amps for large circuit breakers, and at or below the rating for smaller ones.
The millivolt drop is compared to the manufacturer’s data for the circuit breaker under test.
The circuit breaker connections should be inspected to determine that a good gasket is present and that there is no overheating. If overheating is indicated by discoloration or signs of electric arcing, the connections should be removed and the connection surfaces cleaned.
Overload trip test
Correct action of the overload trip Circuit breaker components can be verified by applying 300% of the circuit breaker rated direct current to each pole. The essence of this test lies in the automatic opening of the circuit breaker and not in the tripping times, these being able to be greatly affected by the ambient conditions and the test conditions.
The mechanical operation of the circuit breaker should be checked by turning it on and off several times.
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