Circuit Breaker Testing
Preventative Maintenance, Inspection, and Testing
The need for preventative maintenance of molded case circuit breakers will vary depending on operating conditions. Where heavy dust conditions exist, for instance, and the circuit breakers are not called upon to operate, an accumulation of dust on the latch surfaces may affect the operation of the breaker slightly. Dust accumulation can usually be cleared from the latch by occasionally manually turning the breaker “OFF” and “ON,” thus exercising the circuit breaker. When normal maintenance inspection and cleaning of bus connections, relays, lug connections, and other parts of the distribution system is being made, it is advisable to operate and check the circuit breaker operation. Recommended procedures are of two types:
General Procedures for Routine Field TestingRoutine field testing of molded case circuit breakers is intended to enable personnel, such as a plant maintenance manager, to determine without laboratory conditions or complicated equipment that a particular breaker is able to perform its basic circuit protective functions. The following constitutes a guide and recommendations for the type of tests which Industrial Tests, Inc. performs during routine maintenance. The tests described below are based on proven standard maintenance practices and are aimed at assuring that the breaker is functionally operable. This is in contrast to those tests which are performed in a factory or shop environment to specifically check the circuit breaker performance with manufacturers’ published calibration performance curves which must be done under closely controlled conditions of ambient temperature as well as stable power system conditions.
CAUTION: It should be understood that all test are made only on breakers and equipment that are completely de-energized.
1.) Insulation Resistance Test: Extreme atmospheres and conditions may reduce the dielectric withstand ability of any insulating material including those of which molded case breakers are made; therefore, the first routine check recommended to be made on installed breakers is a resistance measurement test. To make this test, an instrument commonly known as a “megger: is used. The voltage recommended for this test should be as least 50% greater than the breaker rating. 1000V is the standard test voltage; however, a minimum of 500 volts is permissible. Tests should be made between phases of opposite polarity as well as from current carrying parts of the circuit breaker to ground. Also, a test should be made between the line and load terminals with the breaker in the “OFF” position. Resistance values below 100 megohm are considered unsafe and should be investigated for possible contamination on the surfaces of the molded case of the circuit breaker.
NOTE: For individual breaker resistance readings, load and line conductors should be preferably disconnected. If not disconnected the test measurements will also include the characteristics of the attached circuit.
2.) Connections Test: Connections to the circuit breaker should be inspected to determine that a proper electrical joint is present. If overheating at these connections is evident by discoloration or signs of arcing, the connections should be removed and the connecting surfaces cleaned before breaker is re-installed. It is essential that electrical connections be made properly to prevent and to reduce overheating. Aluminum connectors (lugs) are plated and should not be abrasively cleaned. If damage is evident, the lugs should be replaced. In making connections with aluminum conductors, use a joint compound made for the purpose.
3.) Contact Resistance Test: Extensive operating of the circuit breaker under load conditions beyond that for which the circuit breaker was intended may cause deterioration of the contacts. A simple way to test for such deterioration is by measuring resistance across each pole of a breaker. This may be done by use of a resistance bridge or measuring the voltage drop across the circuit breaker while a current is flowing through it. A millivoltmeter will be required. Any convenient current value can be used as long as it is sufficient to obtain a voltage reading but below the rating of the breaker. A comparison between the poles of the breaker or similar breakers can be made. A difference of as much as two to one may indicate that the breaker’s contacts should be cleaned. Excessive millivolt drops across a complete breaker can be an indication of several abnormal conditions within the circuit breaker such as eroded or contaminated contacts or loose connections. This test is an important indicator of the acceptability for continued use of the circuit breaker.
4.) Overload Tripping Test: A general indication of the proper action of the overload tripping components of the circuit breaker can be verified by selecting certain percentages of the breaker rating, such as 300%, and applying this separately to each pole of the circuit breaker to determine if it will open automatically. The significant part of this test is that the circuit breaker will operate and since conditions of ambient and types of connections used for the tests greatly affect the results of tripping times of the circuit breakers, they become of little significance in these tests. Refer to NETA Standards for trip times which are acceptable in this test. When verification of tripping characteristics, other than to determine if the circuit breaker is functional is required, refer to the Manufacturers trip curves and instruction books.
5.) Instantaneous Magnetic Tripping: In routine tests, it is more important to determine that the magnetic feature is operating and will trip the breaker, rather than the exact value at which the instantaneous magnetic feature operates. Again, exact determination of magnetic trip values can be obtained , but precise control of test conditions must be adhered to if results are expected that will represent the published data.
6.) Mechanical Operation: During routine tests, mechanical operation of the breaker should be checked by turning the breaker ON and OFF several times. If the circuit breaker is equipped with a “reset”, the circuit breaker should be tripped, reset and turned ON several times. This will remove any dust accumulation on the mechanism and latch surfaces.
It is again emphasized that common maintenance practices for electrical equipment should be adhered to in field testing both new and installed molded case breakers. Usually the standard routine operating checks listed will be sufficient to assure proper functioning protecting devices. Where molded case breakers are factory calibrated and sealed, the seal should not be broken and the breaker itself should not be tampered with. Circuit breakers with removable covers may be checked for contact cleanliness, connections, and latch cleanliness by making careful visual inspection.
