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I have been searching a while now and haven't found the answer. On an AB 3 phase motor starter with a heater type overload. Can the nc contacts on the heater overload be used as a plc input? Is there any code that says this should not be done? Thanks for your time. Ryan
And got the following answer:
certainly you can bring NC to PLC input but this is not what NC contact on overload relay is for... usually overload has two contacts, one is NO and other is NC. NC contact should be connected in series with coil of the contactor. reason is that if there is condition that caused overload trip, it is imperative to shut down that contactor (using the NC contact) regardless if contactor is still powered from some circuit or not. this is meant to be fail safe, something you don't want to do in PLCs (because it is just so convenient to force that signal or change code). but if the PLC is safety PLC, it is suitable for safety applications and more than appropriate for this taks too. the NO contact is "auxiliary" and it is ment for announciation (PLC monitoring for example). now whether or not this is required by local code (killing contactor coil by NC contact of the overload relay), you need to verify locally (each country has own standard which may be more or less strict than others). often overloads only have NC contact and they are designed in such a way that this contact can be used only for mentioned purpose (Allen Bradley products for example). also please note that old thermal-only overloads are not recommended since only respond to thermal trip. today one should use overload relay that has both thermal and magnetic trip or better yet - electronic overloads. specially if group of motors has common overcurrent protection (usually on small motors).
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