N/C switch not safe? Beginner question.

New to PLC programming, experienced in a few other computer languages.
I am having trouble understanding why a normally closed safety switch is inherently unsafe in a ladder program. It was explained to me that if the wire is cut it will never shut off.
I seem to be missing something moving from the world of hard wired devices to program controlled devices. Why wouldn't the program stop if the wire to the input was cut? It shouldn't get a signal from the input and therefore shut the output off!
Would appreciate the help or a redirect to a good site to continue my education.

I am having trouble understanding why a normally closed safety switch is inherently unsafe in a ladder program.

Click to expand...

I think you might have this reversed. A normally closed physical STOP switch of ANY type (safety or otherwise) is traditionally considered to be safer because cutting, breaking, or disconnecting a wire will perform the same action as a NC switch: stop the machine.

However, you have to realize that the instruction in your ladder program does not have a wire to be cut, and is NOT the physical switch contact, but rather a logic instruction LOOKING AT or examining the physical switch contact, therefore it will be go true when the closed physical contact is closed - an "Examine If Closed" instruction, which often appears to beginners to be a Normally Open instruction (thus incorrect). This is a common mis-perception for 99% of the beginners. Trust me, you need to use a "NO" instruction to look at or read a NC physical switch. Remember, you only use a "NC" or "Examine if Open" instruction when you want your PLC rung logic to have the Opposite state of the physical device.

Perhaps what you have been told is that if you can use a NC or "Examine if Open" instruction in your ladder logic, it then means that your physical STOP switch is Opposite, or Open, which would then be less safe than a NC physical STOP switch. Remember there are TWO locations and TWO devices here: The physical switch, and the PLC instruction. They are not, and have never been, the same thing.

Take a look at Ron Beaufort's videos, they will help clear up the confusion. http://www.ronbeaufort.com/main_frame_page.htm

And here is more info to help confuse you.

http://www.patchn.com/index.php?opt...d=58:xioxic-explained&catid=20:plc&Itemid=118

tpace said:

New to PLC programming, experienced in a few other computer languages.
I am having trouble understanding why a normally closed safety switch is inherently unsafe in a ladder program. It was explained to me that if the wire is cut it will never shut off.
I seem to be missing something moving from the world of hard wired devices to program controlled devices. Why wouldn't the program stop if the wire to the input was cut? It shouldn't get a signal from the input and therefore shut the output off!
Would appreciate the help or a redirect to a good site to continue my education.

Click to expand...

I think a bit of information overload is here. All good info but a bit much. Read twice to get good comprehension. Here is du it urself at home to get the idea.

STEP 1
get a battery and lite bulb and NC switch and NO switch.

Wire with NC on one side + or - does not matter for now.
then wire to NO then to lite and other side of line.

Press NO and lite should lite. While pressing NO press NC and lite turns off.

Press NO again then cut wire between NC and NO lite turns off.

STEP 2
Remove lite bulb and wire in a coil of a relay. If it has two contacts great - wire one in parallel with NO only. wire the lite and otehr contact in series across the line. You now have a model of standard industrial 3 wire control circuit.

Dan Bentler

Fail safe

So in my mind fail safe is a alarm condition that is triggered in case of for example sensor failure/wire break. Typically your software should then rely on a closed circuit TRUE for a condition to be safe.

So my sensor would be wired normally closed in the field.

How you interpret this signal will depend on the specifics of your ladder design. For example if the input is used for a motor start it should be a OPEN contact.

If it is used to trigger the alarm/event it should be a closed contact.

RheinhardtP said:

So in my mind fail safe is a alarm condition that is triggered in case of for example sensor failure/wire break. Typically your software should then rely on a closed circuit TRUE for a condition to be safe.

So my sensor would be wired normally closed in the field.

How you interpret this signal will depend on the specifics of your ladder design. For example if the input is used for a motor start it should be a OPEN contact.

If it is used to trigger the alarm/event it should be a closed contact.

Click to expand...

Correct, depending on the circumstances. According to OSHA, a PLC should not be the safety disconnect, you should have hard wired e-stops accessable to operators. This even applies to light curtains, which I am using now and lve them!