How to Test Resistance of a Circuit Breaker Contact

Measure the resistance of electrical connections, such as terminations, joints, connectors, busbar sections, cable connections, and so on. Contact resistance testing is sometimes referred to as Ductor testing. These might be busbar segments or cable connectors, which link any two conductors. Ohmmeters are the tools needed to conduct the ductor test, and because this is one of its main uses, they are sometimes referred to as ductor testers.

Dielectric withstand test

A circuit breaker’s dielectric withstands test determines whether the circuit breaker contacts can withstand over-voltages. The test involves applying a high voltage (typically between one and ten thousand volts) to two electrically isolated conductors. When the test results show an excessive current flow, the insulation has failed, and electrical arcing has occurred. It can occur within the insulation material, over its surface, or through the air.

A HIPOT test set is recommended to conduct a dielectric withstand test. The manufacturer provides the test voltages used for this purpose. The breaker’s contact should be connected to the line side of each phase to perform a dielectric withstand test. All other phases should be tied together. Vacuum circuit breakers should be checked using manufacturer-provided test voltages.

Direct current (DC) overpotential test

Performing a DC overpotential test of circuit breaker contact is an important procedure that verifies the resistance of the breaker’s contact to electrical stress. It is a very important test for higher voltage circuit breakers, as failure to pass it could cause an explosion, fire, or even death. The test is optional, but it is recommended for older circuit breakers. The test can also detect gross defects in the primary insulation, such as tracking, and it is used to test the performance of control wiring and potential transformers. 

The DC overpotential test of circuit breaker contact can be done by injecting a direct current into the breaker. The test measures the voltage drop across the breaker contacts when a direct current is introduced into the circuit. Visual checks can be critical as they help you identify faulty circuit breakers. It is also important to look for paint and metallic surfaces. During the test, you should be able to identify the faults and make the necessary repairs.

Dynamic resistance measurements

Regular dynamic and static resistance measurements of circuit breaker contacts are useful for evaluating the contact system and detecting potential problems early. The static resistance measurement of the circuit breaker contact is a common method used to determine the resistance of the contact system when the interrupter unit is closed. However, these measurements do not provide a full picture of the condition of the contacts. In addition, internal inspection of the circuit breaker is a time-consuming and labor-intensive process. Therefore, a dynamic resistance measurement can simplify the task.