The value of a primary circuit contact resistance (the resistance between two terminals of any pole of a breaker) is typically measured before a circuit breaker is first put into service and during maintenance inspections. Contact resistance is a value that is specified as a reference for all kinds of circuit breakers. According to IEC 56 (now IEC62271-100) this resistance value should be measured using a test current between 50A and the breaker's nominal current value. According to ANSI C 37.09 the lowest test current is 100A. Other national and international standards specify similar regulations.

The value of the test current is usually defined by manufacturer. Most of manufacturers (ABB, Alstom, etc.) specify test current at 200A. Some electrical utilities have their own regulations (Vattenfall-Sweden measures contact resistance for breakers and disconnect switches at 300A). Others use 500A or 600A test current, while some do not exceed 100A.

Our opinion is that the current value of 300A (often even 200A) is sufficient for correct and reliable measurement.

If there is a question about the accuracy of measurement or condition of contacts, the measurement can be repeated using the higher current value. Our experience shows better result using the CONT mode - continuous current measurement at 200A to 300A in the duration of 50sec.

Higher test currents increase the accuracy, but the test current above 300A provide insignificant improvement in the accuracy. Higher currents require bigger cables, that are heavy and make the procedure of resistance measurement troublesome and complex, without any significant result improvements.

We recommend the test current of 300A. For most situations 200A test current is sufficient. In case of doubt, repeat the test using the Cont mode, longer duration measurement at currents of 300A or 200A.

It is recommended to use 400A or 500A for situations where the contact resistance measured is expected to be below 20µΩ.

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