I am distressed by the discussion of circuit breakers in this thread.
First of all, because the links posted clearly -once you plow through them- do NOT apply to the sundry breakers we have in our household panels. For example, the 'trip test' method of applying 3x rated current is completely at odds with inverste time breaker operation and UL tests. NEMA, UL, and the breaker manufacturers themselved are united on this point: there's no way to 'test' a breaker's ability to trip at the desired level. '
Fo residential breakers yes, but for commercial and industrial applications ANSI/NEMA AB 4 deals with ?Guidelines for Inspection and Preventive Maintenance of Molded Case Circuit Breakers used in Commercial and Industrial Applications.? It includes both overcurrent, hold in, and INSt trip testing procedures.
The introduction of this standard has some good info regarding the original post.
Good practice includes periodic circuit breaker maintenance during plant shutdown or during a regular maintenance period as specified, for example, in NFPA 70B. When a circuit breaker operates automatically, good practice dictates that the source of the overcurrent should be located, and if it is suspected that the operation was at or near the
interrupting rating, the circuit breaker?s condition should be checked prior to circuit re-energization. When appropriately maintained, molded case circuit breakers provide reliable protection for many years. The exact lifetime of the breaker, however, is determined by the circuit breaker?s operational duty and by its environment.
With respect to operational duty, for some circuits there will be occasional overload conditions or lowcurrent fault conditions. Here, the operating life will be tens of years. In other circuits, there may be high short-circuit-current faults but it should be noted that bolted faults at the breaker interrupting rating are rarely encountered. These will reduce the circuit breaker?s operating life and may necessitate circuit breaker replacement. Molded case circuit breakers are evaluated to the UL 489 (NEMA AB 1) standard Molded-Case Circuit Breakers, Molded-Case Switches and Circuit-Breaker Enclosures. They are subjected to thousands of endurance test operations (UL 489 Table 7.1.5.1); 50 overload test operations for circuit breakers rated up to 1600A and 25 operations for circuit breakers rated 2000A and higher (UL 489 Table 7.1.3.1); 3 to 7 interrupting tests, depending on breaker type, at limited fault current (UL 489 Table 7.1.7.1, 7.1.7.2, and 7.1.7.3); and two interrupting tests at maximum short-circuit-current rating.Thus circuit breakers have an extensive but finite interrupting capability, and breakers that experience multiple high short-circuit-current faults should receive a thorough inspection with replacement if necessary.
With respect to environmental effects, circuit breakers are sometimes exposed to high ambient temperatures, to high humidity, and to other ambient conditions that are hostile to long term performance. For example, industries may have corrosive environments or could be associated with dusty environments that could affect operating parts. It is not intended that molded case circuit breakers be disassembled for inspection. Rather, NEMA AB 4 should be referenced during periodic maintenance or during specific inspection following a high shortcircuit current fault. This document is intended to ensure that molded case circuit breakers are well maintained, and provides guidelines for circuit breaker replacement.
