Static micro-ohm resistance readings

Author: Keith Flowers

There is a common misconception regarding resistance across the main contacts in the primary current path of power circuit breakers. The belief is that this resistance is an indicator of the general health of the circuit breaker, specifically its ability to reliably carry current without overheating or nuisance tripping. While in some cases, high contact resistance may be a symptom of a circuit breaker that has worn or damaged main contacts, the value is virtually meaningless in new or nearly new circuit breakers.


By design, WL power circuit breakers have internal main contacts that are self-cleaning when the circuit breaker operates under load. Particularly for new circuit breakers, but applicable for any circuit breaker that has been left open for a period of time, any contaminants are removed during loaded operations. In order to maintain this functionality, it is important that the users of such circuit breakers do not attempt to adjust, clean, dress, or polish the main contacts.


Siemens Industry, Inc. (as well as other manufacturers of low-voltage power circuit breakers) does not recommend performing milli-ohm or milli-volt drop tests made across circuit breaker contacts, because it has been shown that such tests are not a reliable or accurate method of determining the suitability of a given circuit breaker for its intended application.  The resistance of the main current path (including the contacts) in a low-voltage power circuit breaker is variable. Contact resistance and the measurement of contact resistance is affected by:

  • Climate
  • Number and magnitude of past interruptions
  • Foreign material between the contacts
  • Oxidation of contact surfaces
  • Assembly tolerances
  • Where and how the readings are taken.

Static micro-ohm or milli-volt drop readings are highly sensitive to all of the previously mentioned criteria, a known and acceptable condition to Siemens and other manufacturers of power circuit breakers.


Contact resistance in the order of magnitude that is measured in the field (20-200 micro-ohms), aside from being variable, is negligible both in theory and in practice, as are the phase-to-phase differences occasionally measured. The NETA document “Acceptance Testing Specifications” makes an arbitrary statement as to the allowable tolerance of this measurement when no tolerances are stated by the manufacturer. As applied to the WL circuit breaker, this arbitrary pass / fail criterion has no basis in the practical operational environment of a power circuit breaker.


For the reasons above, Siemens Industry, Inc. neither publishes any tolerances for nor recommends the testing of static micro-ohm or milli-volt drop across the main contacts of the WL circuit breaker.  Siemens WL circuit breakers are qualified to UL 1066 and UL 489 as well as conforming to relevant recognized industry performance standards such as ANSI / IEEE C37.13, C37.16, C37.17, C37.20.1, and IEC 60947-2. The WL circuit breakers also meet all recognized industry testing and qualification requirements such as ANSI C37.50, UL 489, and IEC 60947-2.


Siemens Industry, Inc. does not accept NETA’s “Acceptance Testing Specifications” as an accurate means of validating the quality or operation performance of its type WL low-voltage power circuit breaker. Any test report or claim stating that
a WL circuit breaker is not suitable for its intended application or service on the basis of contact resistance or milli-volt drop, will not be considered acceptable evidence of nonconformity for warranty action by Siemens. Any other conformity issues or conditions will be considered under the Siemens Industry, Inc. Standard Terms and Conditions of Sale.

Mailing address

Siemens Industry, Inc.

7000 Siemens Road

Wendell, North Carolina 27591

United States