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How does the Thermal Memory work on the C440 Overload Relay

A thermal memory circuit models the heating and cooling effects of motor on and off operations. Thermal memory prevents immediate restart after trip to allow the motor to cool down. The overload after it trips will have a thermal memory value of 100% and if you attempt a start within 10s for example, a trip may occur quickly if the motor goes a bit above full load. The decay down to “running capable” is in the 2minute range generally speaking. The thermal memory value will decay while the motor is not running. The thermal memory decay curve is similar to other simple overloads.

Upon a thermal overload (or even a power shutdown) the C440 goes into a sleep mode. During this sleep mode the thermal memory is slowly decreased. The sleep mode can last up to 20minutes. If the motor starts before that ~20minutes then the thermal memory will be decreased but not zero. Zero would take a while longer.

The overload Cold Definition is defined as no stored thermal memory (0 TM).

The thermal memory will be at 0 if the overload has not been energized for 1 hour or longer.

The overload Hot Definition is based on the thermal memory at 75%.

This 75% is based on 75% of the 120% trip requirement which corresponds to a continuous current at 90% of the overload dial setting.


When the overload is initially started from cold (0 TM) the thermal memory will increase to its steady state thermal memory within 1 hour.

When the motor is turned off or trips the thermal memory starts to decay immediately.

If the overload did trip the thermal memory had reached 100%.


If the motor is started immediately after a trip or after the motor was turned off the thermal memory will typically be above 75%.

Therefore, the trip times may be faster than the hot trip curve values.

It takes a minimum of 2 minutes for the thermal memory to decay from 100% to 75%.


C440 / XTOE Product Overview