Safety issue

The correct way to implement a STOP pushbutton is to : (i) hardwire it onto a Normally Closed contact and (ii) Program it as a Normally Open contact.
If it's implemented as you've shown ie. hardwired to a Normally Open contact and programmed as a (NOT) normally Closed, then it's unsafe because if a cable becomes detached from the Stop Pushbutton there will be no indication on the ladder logic.It will only become apparent when the stop pushbutton is pressed (possibly as an emergency) and nothing happens.

 

Indeed, stop buttons must be normally closed, so a break in the wiring close to it will cause the machine to stop, instead of not allowing it to stop when the button is pressed.

 

To what diagram are you refering? If I search on "stop button" I find this image:

Image http://sub.allaboutcircuits.com/images/04062.png
And the next two following images in volume 4, Chapter 6
http://www.allaboutcircuits.com/vol_4/chpt_6/4.html

These (3) images seem to have the "stop button" as a nornally closed contact. as you say it should be. Am I on the the wrong image? Am I missing something? If there is a safety issue in LIEC we would like to correct it.

 

I think the OP is referring to http://www.allaboutcircuits.com/vol_4/chpt_6/6.html - just over half way down.

Dave

 

Now that you have got me on the right page:

My reading is that Tony first illustrates the unsafe method here:
http://sub.allaboutcircuits.com/images/04342.png

And then shows the failsafe way to do it here:
http://sub.allaboutcircuits.com/images/04371.png

In other words there is no problem with the text as it is. My mistake.