Breaker Contact Resistance Test

The contacts in the circuit breaker need to checked periodically to ensure that the breaker is healthy and functinal. Poorly maintained or damaged contacts can cause arcing, loosing phase, and even fire. The two common checks conducted on the contacts of a circuit breaker are the visual inspection check and the contact resistance check. The Visual inspection check involves examining the contacts of the circuit breaker for any pitting marks due to arcing and worn or deformed contacts. The second check is the contact resistance measurement. This involves injecting a fixed current, usually around 100A, 200A and 300 A through the contacts and measuring the voltage drop across it.  This test is done with a special contact resistance measuring instrument. Then, using Ohm's law, the resistance value is calculated. The resistance value needs to be compared with the value given by the manufacturer. The value should also be compared with previous records. Both these tests need to be done together. As there are cases of contacts having good contact resistance yet being in a damaged conditions. Thus, for a contact to be certified healthy, it needs to have a good contact resistance and should clear the visual inspection test.

Test procedure according to IEC 62271-200

  • The measured resistance across the complete main circuit of an assembly of metal-enclosed switchgear and controlgear is indicative of the proper condition of the current path.
  • A measurement of the resistance of the main circuit shall be made for comparison between the switchgear and controlgear type tested for temperature rise and all other switchgear and controlgear of the same type subjected to routine tests.
  • The measurement shall be made with D.C by measuring the voltage drop or resistance across the terminals of each pole.
  • The current during the test shall have any convenient value between 50A and the rated normal current.
  • The measurement of D.C voltage drop or the resistance shall be made before and after the temperature rise test with the switchgear and controlgear at the ambient temperature.
  • The measured resistances in these two tests shall not differ by more than 20%.
  • The measured value of the D.C voltage drop or the resistance shall be given in the type test report.

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