Correction factors with direct current for the thermal-magnetic trip units of 3VA1 molded case circuit breakers

With DC systems, a correction factor must be applied to the magnetic trip unit.


Where ambient temperatures differ from the calibration temperature, temperature compensation as described in chapter Derating of 3VA1 molded case circuit breakers must be performed.


Starting point:

A 3VA1 molded case circuit breaker 160 A with a TM240 trip unit is used in a DC system. The molded case circuit breaker is required to trip instantaneously in response to an overcurrent of 1200 A.

The setting value for the instantaneous short-circuit protection (Ii) is:

Ii = 1200 A · 0.7 = 840 A