Eaton Logo

Determining molded case circuit breaker integrity after clearing a fault

Find Eaton molded case circuit breaker MTBF and reliability data, as well as the Eaton application note about determining circuit breaker integrity after a fault, attached (there is good info in the app note about UL testing).

Additionally, sound recommendations from product engineering state:

Regarding breaker tripping on overcurrent, if it is known the fault level the breaker was subjected to was at, or near, its interrupting rating, the breaker should be replaced after one such event.

Regarding breaker tripping on overcurrent, if it is not known what level of fault current the breaker was subjected to but it was suspected to be a very high current fault event, the breaker should be replaced after no more than two such events.

To expound on a reference found in the attached app note about breaker integrity, the guideline for field inspection and field testing of molded case circuit breakers is titled NEMA AB 4.  Found in AB 4 are a number of inspection recommendations, one mechanical test, and five electrical tests.  In inspecting and testing a breaker on site, AB 4 can act as a blueprint to help develop a concise and accurate picture of that breaker’s suitability for continued use in the field.  A free copy of NEMA AB 4 may be obtained from NEMA’s web site at www.nema.org; type “ab4” in the Search field at the top of the web page, hit the keyboard Enter key, and follow the instructions that appear for downloading a copy of the document.  Email registration is required.

  1. MTBF_MCCB Reliability Estimate_7-6-2011.pdf
  1. Determining Breaker Integrity After a Fault_AP01200006E_11-2007.pdf