Question about Safety Switches

Why are Safety Switches drawn in the literature showing and labeling NO contacts NC. In other words if you put a meter across the contacts marked NC in the un-actuated position it reads open. I assumed I had the wrong switch till I remembered seeing it on other safety switches and checked it. The way I read the mechanical drawing it looks like the contacts are open when actuated as well but apparently I am missing something. Maybe it's obvious but it seems confusing to me. Would someone be kind enough to explain this?

Thanks
Bill

No, I don't think you are missing anything. The way it was explained to me is that with safety gate switches where a tongue is inserted to actuate the switch, the "normal" condition is the "safe" condition. I agree this is opposite of the way we normally think.

Thanks

Emergency Stop inputs are programmed as if they are normally open contacts, (held closed). This is so that they 'fail safe'. Any break in the circuit signals an emergency situation. 'Closed circuit = no problem'. If it were wired as closed circuit = problem, a broken wire would prevent detecting a fault = doesn't 'fail safe'. Limit switches for axes are also usually normally closed, so that loss of continuity indicates a problem vs. presence of a signal indicating a problem.

wdd0422 said:

Why are Safety Switches drawn in the literature showing and labeling NO contacts NC. In other words if you put a meter across the contacts marked NC in the un-actuated position it reads open. I assumed I had the wrong switch till I remembered seeing it on other safety switches and checked it. The way I read the mechanical drawing it looks like the contacts are open when actuated as well but apparently I am missing something. Maybe it's obvious but it seems confusing to me. Would someone be kind enough to explain this?l

Click to expand...

Yes, like Steve said, at first it does appear odd, but if you think of them like an E-stop, it makes sense. E-stops use normally-closed contacts, and are shown on diagrams in their 'un-actuated' state (released, with contacts closed). If you think about it, a guard switch's 'un-actuated' state would really be with the actuator inserted. When you open the door, the switch is actuated, just like an E-stop.



-Eric