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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.

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

Example:

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