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Contact Resistance; if one pole resistance deviates by more than 50% from the lowest pole, is the breaker bad?

Published date: 06 August 2019

Is a breaker bad if there is a greater than 50% variation between poles?

Product Line:
Circuit Breakers

PowerPact, Masterpact, Miniature

The short answer is, not necessarily.  Per industry standard NEMA AB-4:  "The millivolt drop of a circuit breaker pole can vary significantly due to inherent variability in the extremely low resistance of the electrical contacts and connectors.  Such variations do not necessarily predict unacceptable performance and should not be used as the sole criteria for determination of acceptability."

NETA (International Electrical Testing Association, Inc.) has published a specification (not an industry standard) named "NETA Acceptance Testing Specifications" which is widely used by independent test companies.  It suggests that pole resistance that deviates by more than 50% from the lowest pole should be investigated.  It does not indicate that such a result constitutes a "failure," though it is generally mis-interpreted as such.  Their suggested test procedure simply states that it should be investigated.

It should further be noted that the 50% specification is neither scientific nor a reliable predictor of breaker performance in service.  One evidence of this is that it will pass a circuit breaker with high but consistent pole resistance values, but "fail" a similar breaker with much lower resistance values if the deviation from the lowest resistance is more than 50%.  A second evidence is the weak correlation between pole resistance testing and temperature in service.

NETA does not identify their specification as pass/fail criteria, and neither does Schneider. Of course a customer may return any breaker they wish for analysis, but warranty credit will be based on the breakers actual performance and conformity to recognized standards and internal quality assurance requirements.

A better test is to put the breaker on a load bank or to place into service and monitor for temperature abnormalities per the UL standard guidelines.  See attachment.


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