Volume 44, Issue 2 (March 1999)
Heat Transfer Testing of Thermal-Magnetic Circuit Breakers
The forensic engineering investigation of a fire will include the post-fire examination of electrical panelboards and circuit breakers if such equipment is available on the premises and survives the fire. Post-fire circuit breakers may be found in the on, off, tripped, or unknown position. It is sometimes assumed by investigators that a tripped circuit breaker is the result of an overcurrent condition either before or during the fire. Sometimes the tripped (or untripped) circuit breaker is used to prove or disprove theories about the cause of the fire. Since thermal-magnetic circuit breakers employ thermal sensors, it should be possible for the heat from a fire to be transferred by convection, conduction, and radiation into a circuit breaker and cause it to trip in the absence of an overcurrent condition. It should also be possible for the heat from a fire to shift the circuit breaker time-current characteristic curve and cause the circuit breaker to trip at reduced currents. The question then becomes, at what ambient temperature will the circuit breaker be expected to trip? Also, can the heat from a nearby fire be conducted by the metallic circuit conductors into a circuit breaker and cause, or assist it to trip? If so, what fire conditions are required to cause the circuit breaker to trip? This paper reviews the operating principle of thermal-magnetic circuit breakers, describes a series of heat transfer tests conducted on nine circuit breakers of various ratings and from various manufacturers, presents the results of the tests in graphical form, and discusses the potential impact of conduction and convection heat transfer on circuit breaker performance during fires.