MAINTENANCE OF MOLDED CASE CIRCUIT BREAKERS

Frequency of Maintenance.-Molded case circuit breakers are designed to require little or no routine maintenance throughout their normal life­time. Therefore, the need for preventive maintenance will vary depending on operating conditions. As an accumulation of dust on the latch surfaces may affect the operation of the breaker, molded case circuit breakers should be exercised at least once per year. Routine trip testing should be performed every 3 to 5 years.

Routine Maintenance test.-Routine maintenance tests enable personnel to determine if breakers are able to perform their basic circuit protective functions. The following tests may be performed during routine maintenance and are aimed at assuring that the breakers are functionally Operable. The following tests are to be made only on breakers and equipment that are deenergized.

Insulation resistance test.- A megohmmeter may be used to make tests between phases of opposite polarity and from current-carrying parts of the circuit breaker to ground.A test should also be made between the line and load terminals with the breaker in the open position. Load and line conductors should be dis­connected from the breaker under insulation resistance tests to prevent test measurements from also showing resistance of the attached circuit. Résistance values below 1 megohm are considered unsafe and the breaker should be inspected for possible contamination on its surfaces.

Millivolt drop test.- A millivolt drop test can disclose several abnormal conditions inside a breaker such as eroded contacts, contaminated contacts, or loose internal connections. The millivolt drop test should be made at a nominal direct-current voltage at 50 amperes or 100 amperes for large breakers, and at or below rating for smaller breakers. The millivolt drop is compared against manufacturer's data for the breaker being tested.

Connections test.- The connections to the circuit breaker should be inspected to determine that a good joint is present and that overheating is not occurring. If overheating is indicated by discoloration or signs of arcing, the connections should be removed and the connecting surfaces cleaned.

Overload tripping test.- The proper action of the overload tripping components of the circuit breaker can be verified by applying 300 percent of the breaker rated continuous current to each pole. The significant part of this test is the automatic opening of the circuit breaker and not tripping times as these can be greatly affected by ambient conditions and test conditions.

Mechanical operation.- The mechanical operation of the breaker should be checked by turning the breaker on and off several times.