Industry Standards and Practice

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.”

The International Electrical Testing Association, Inc.® (NETA®) 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 commonly misinterpreted as such. Their suggested test procedure simply states that results outside of this arbitrary target should be investigated.

It should be noted that the 50% specification is neither scientific nor a reliable predictor of circuit breaker performance in service. This becomes clear when one considers that a circuit breaker with high but consistent pole resistance values will meet this specification, but a similar circuit breaker with much lower resistance values will not meet the specification if the deviation from the lowest resistance is more than 50%. For example, a circuit breaker with pole resistance readings of 50, 60, 76 milli-ohms would “fail” the test but a circuit breaker with 130, 180, and 150 milli-ohms resistance would pass. A second evidence is the observable weak correlation between new-circuit breaker pole resistance testing and temperature in service.

NETA does not identify their specification as pass/fail criteria, and neither does NEMA or Schneider Electric. While not a perfect test, circuit breakers meeting the NETA specification are not likely to have thermal issues in service; however, circuit breakers not meeting the NETA specification may also be good and variant test results merely indicate that further investigation is in order.