PLC safety input wiring

atatari

Member
Join Date
Sep 2002
Posts
92
Hello,

I have a sensor to indicate the machine guard door is open.I was told to wire it to the relay dry contact and then from the relay to the PLC input modual.
Is there any safety publication out there regarding safety signals of wiring to the PLC ?
why couldn't I wire it strait to the input modual?

any safety links I can go to?
maybe I am confused between hardwire and software programming?

Thanks
 
There have been some posts about this in the past, if you do a search for "safety" or "E-Stop" I would imagine you could dig them up.

The biggest thing to remember about safety is that you don't want it to rely on your PLC. What happens if you plc goes belly up? Or the input for that sensor goes bad and the machine starts with someones hand in it.

Also you don't go into detail about the sensor you are using but you shouldn't just use any old sensor when it comes to safety, it should be something that is made for that purpose. Schmershal, Banner, Sick and I am sure a host of other companys make interlocks and such for doors. You should check those out if you aren't already using something of that sort.
 
On the machines that I design and retrofit I always incorporate a safety relay module like the ones that BANNER sells. The safety relay has been designed to monitor the various gate switches and pressure mats to ensure they are working properly. I use the safety relay to supply power to my output card and also to provide a status indication to the PLC. As glaverty said the PLC should never be trusted as a safety device unless it is specifically designed to do so.
 
This is my understanding on E-stops. Please correct me if I am wrong, or feel free to expound.

I agree with glaverty. I don't know the actual legalities of this issue nor where to read the actual standards. I would assume that there are some.

Common practice has been to classify such safeties as E-stops. They are usually wired in series to a safety relay. This relay will electrically shut down the equipment in the safest way to prevent injury.

Approved safety switches and relays should be used. They are set up so that if you physically cut the wire to any of the safety devices (or smash with a crecent hammer :)), the series circuit to the safety relay will open up, activating an E-stop condition.

There is such a thing as a safety PLC, but the costs of implementation far exceed the cost of actual safety relays, controllers, etc.

A set of auxilary contacts whether off of the safety relay or the actual safety device are used as an echo input to the PLC for alarming purposes and to help quickly identify the location of the problem.

The echo signal back to the PLC does not have to be fail safe. Many times the NO contacts are used for this purpose to make full use of the NO/NC blocks that used to come with most E-Stop switches.

The fuzzy area is usually in deciding how to isolate the EMF once the safety relay drops out.
 
Safety

I am currently working on a system retrofit that has NO E-stop, while at a group sitdown I asked WHY this was so. The plant operators and engineer believed that they they could depend on the disconnects (not in series) for each function were ok. That COULD MAYBE be ok IF, All people knew where ALL disconnects were located. Point being the retro will include an E-STOP until somebody comes out of the woodpile and gives it to me in writing, that an E-Stop is NOT needed. The only one ever zapped by me was ME. BE SAFE
 
If this is a E-stop you should hard wire it, beacause if PLC fales the machine will still stop and protect workers. I think that you should look up the Canadain safety guide on how to install safeties. Most plants have a copy of this guide.
 
atatari said:
Hello,

I have a sensor to indicate the machine guard door is open.I was told to wire it to the relay dry contact and then from the relay to the PLC input modual.
Is there any safety publication out there regarding safety signals of wiring to the PLC ?
why couldn't I wire it strait to the input modual?

any safety links I can go to?
maybe I am confused between hardwire and software programming?

Thanks

what safety you mean here: Safety for the guard door or safety wiring for PLC (what i mean is the PLC card itself)?

Sorry for the question, i just a little confused.

Thanks
 
re: Safety

Also, please be aware that some standards (depending on country) require the sensing device to be able to carry two signals back to your safety relay (ie a Schmersal door latch). This way if there is a short in one wire, your door will not be bypassed. Also note that some engineers are requiring that the door latch itself be mechanically in two part and wired back so that if the key breaks off in the door switch, you still cannot bypass the safety. I notice that you are from Canada so you need to be aware that if you are in Ontario, it is now required that a P.Eng. approve all safety systems prior to them operating in a plant. This is the responsibility of the end user, however it is becoming more prevalent that the automation company building the system will take on that responsibility, for their own coverage.

Regards,

Steve
 
I understand that safety guards have to be assigned a CAT of B, 1, 2, 3, or 4 and this then is used to select the guard switch and relay/s. A system simular to a risk assessment is used which takes into account the damage to body and warning etc. It is by no means simple but it in UK at least has to be in writting so quite easy for me. I request the assessment then select the device from a book which complies with the Cat assessment. A guard which stops a graze to a finger and the danger can be seen well in advance i.e. CAT B may be OK to used a standard PLC but once its above B then a safety rated PLC to the CAT required must be used. At CAT 4 they are self monitoring duel everything i.e. Not only control to contactor is cut but a second contactor is also in circuit so if contacts burn in then second unit will still stop it. They have a timed check of both contacts so if there is a gap between the contacts making or braking it locks out etc. But of course it still does not help as instead of reporting a machine will not reset the operator pushes in another E/stop and resets it to defeat the timed monitor. I think most the guard relay manufactors do some books on the systems and also some retailers like LC automation in UK. But if your told to do it in writting and all instructions on guarding have to be in writting then I would not think you have a problem you can visit your boss if he sends you a visitors request pass.
Not sure about other countrys but the UK rules are strict but many still do not obay the rules but more and more top management are being given castodial sentences by the courts so slowly we are being forced to work to rule. It was a dirty word used by unions but now its becomming a way of life.
 
safe or danger

In every answer is something
I would use a safetyrelais to shut down
emergency stop MUST be on when rotating parts are present.It is part of machine code.
The output of the relais can be used as input for the plc. The PLC does not have to be switched off just the rotating or dangerous parts have to be stopped.
For more demanding a special safety PLC does exist (beckhoff for example)
In europe CE is requiring a code.
 

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