Fixed thermal Trip vs fixed magnetic trip current ratings


I am looking into a series of 2 pole circuit breakers (Sprecher + schuh 's series KTU7 UL489 molded case circuit breakers) and they seems to have 2 ratings, a thermal trip rating and a magnetic trip rating. What is the difference between these two ratings? Is the magnetic trip rating the same as the instantaneous trip rating?

Best Answer

The thermal setting is a thermal overload protection. It has a time-delayed response. The heavier the overload, the faster it trips. It may be implemented by something like a bimetallic strip.

The magnetic setting is instantaneous short-circuit protection. It has no intentional delay. It may be implemented by something like a solenoid plunger, restrained by a spring.

Here is an illustration of a typical time-current curve for a miniature circuit breaker (Clipsal 4 series) showing the thermal and magnetic trip characteristics. Note the thermal trip takes a long time - minutes or seconds - while the magnetic trip responds in 10ms or less.

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