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Fail Safe Contacts
The logic behind fail safe contacts


Usually with the term fail safe, I take this to mean a no contact being held closed when conditions are ok. And if something goes wrong with the sensor, it reverts to no and stops the machine. I have recently came across a design showing a nc contact which opens when conditions are not ok and stops the machine. The reasoning I read was that the machine was only to be stopped if a set point was reached by the sensor and was to remain running at all other times even if the sensor was to fail.

Has anyone came across this before because as stated I have usually seen the opposite?


Open circuits are typically used in "fail safe" circuits, closed normally indicates "run"...

n.c on ok condition is common in my field. This protects against broken circuits. Sometimes we see a n.o. held closed contact that protects against a power failure.