Thermal magnetic breakers use a set of bimetallic breakaway contacts to trip the breaker which are susceptible to high heat environments in which the current carrying capacity must be de-rated.

A de-rating chart can be found in the SpeedFax Catalog in the Unusual Operating Conditions section for VL breakers.

Solid state electronic trip units do not utilize bimetallic contacts and are not susceptible to high heat environments. However, the VL electronic trip units will self protect the solid state circuitry by initiating a trip condition if temperatures reach an unacceptable lever for the circuit breaker.