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Danfoss MCD500 softstarter: thermistor connection

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    Danfoss MCD500 softstarter: thermistor connection

    Hello all!
    During commissioning of a switch panel, thermistor input on one of the MCD500 softstarters was short-circuited. Now it displays "thermistor trip" permanently. Is there a way to reset it or the unit is damaged permanently?

    #2
    Connect a wire between the thermistor input terminals on the starter. Thermistor circuits work by looking for the resistance to increase, which can also mean an open circuit. So by putting a jumper wire between the terminals, you are emulating a low resistance circuit, which should be interpreted by the trip unit as a cold thermistor; good to go If it still displays “Tripped”, the circuit is damaged internally and is unlikely repairable without replacing whatever control board it is in.
    __________________________________________________ ____________________________
    Many people are shocked when they discover I am not a good electrician...

    I'm in California, ergo I am still stuck on the 2014 NEC... We'll get around to the 2017 code in around 2021.

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      #3
      Originally posted by Jraef View Post
      Connect a wire between the thermistor input terminals on the starter. Thermistor circuits work by looking for the resistance to increase, which can also mean an open circuit. So by putting a jumper wire between the terminals, you are emulating a low resistance circuit, which should be interpreted by the trip unit as a cold thermistor; good to go If it still displays “Tripped”, the circuit is damaged internally and is unlikely repairable without replacing whatever control board it is in.
      Thanks for the reply.
      I connected a 100ohm resistor to thermistor input and then I was able to reset "thermistor trip" error. So probably the measuring circuit is intact. But when I disconnect thermistor input, I still get the same error. The other softstarters in this panel have exact same settings, but they don't trip when thermistor is diconnected.

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