Nc contact of proximity sensors

This probably started in the days of relay logic as it might be convenient to have a N/C contact on the sensor so that the logical 1 is when sensor does not detect anything, like all buttons/sensors etc. it is sometimes convenient to have a N/O + N/C contact for different circuits. also this is the equivelent of light on/dark on sensing. so if you need a signal (light on)when no object is present use N/C or N/O when dark on (object present).
 
Something also to keep in mind, if you decide to use a solid state device, is do you need a sinking or sourcing device. This is dependant upon what type of input card you are using, be it a sinking or sourcing.
 
I often see them used as overtravel limits connected to servo drives. That way, if a wire breaks, an overtravel fault stops the drive.

Paul
I have seen NC prox switches fail in the closed direction. For overtravels, I always recommend a real limit switch with force guided contacts. (The customer doesn't always use this but it's what I would recommend).
 

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