which contact (NO or NC) of limit switches are used for safety operation, if their is a failure incase of wire cut.

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It's normal to use the contact that opens at the end of travel i.e. the circuit is maintained by the switch. For critical operation you often see two switches in series.

I have seen systems where they also wire back the normally open contacts and check in logic to make sure when one set of contacts open that the other set closes. If you have 2 switches wired that way you can get a warning that a switch is nearing the end of it's life.

Regards
Roy

 

If it's a safety related application, then you should use safety rated switches together with a safety relay. The manufacturers usually have examples in their literature of how to apply them. There is no single answer to this, because the correct type depends on the safety category that the application requires. That in turn will depend upon the laws and regulations of the country you are in.

It is not possible to give a simple answer to a question like this, because there are so many factors that may come into it. Safety device manufacturers also have special sensors for some applications that might be better suited to what you are doing. I would suggest having an experienced person have a look at your application to advise you if you are not sure of what you are doing.

 

Hello,

I prefer to use the both Contact "NO and NC". When the state of both contacts have been changed this mean failure case. otherwise when the state of only one contact has been changed this mean wires cut or any another mistakes.

my best wishes,
Eng.Hassan

 

Correct me if i'm wrong, but to answer your question, I would say normally open.

 

Does your Digital Input card have "NAMUR" and line monitoring facility?

Then it doesn't matter which way you wire the switch a break or short circuit in the cable before the switch can also raise a fault alarm/ trip depending on your requirement.

Gray

 

I would back up what M Griffin said previously about about

1) There being no single answer because of the laws and regulations to which you are required to design and because of the application requirements.

2) Having an experienced person look at the design if you are not sure.

I would add that to my knowledge there is no specific requirement to use safety certified devices for a particular duty. It just makes justification of your design easier if you can do this when your design is reviewed. The most important thing for any instrument is that it can detect the condition required in the environment it is operating in.

I order to determine what is an appropriate configuration you first need to determine what your performance requirements are with respect to probability of failure, spurious trip rate, environment, industry standards & associated practice, etc. Sounds ominous but these are things I would normally expect to do on a project or have access to on a modification.

Hope this helps
DaveMH