Why NOT to Wire E-Stops in Series

khelza

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I had someone explain a few years back why is was unsafe to wire e-stops (or other safety devices) IN SERIES back to a safety relay/PLC.


I am not referring to the fact that, unless you have auxiliary contacts, you won't know which safety device was tripped; It was something to do with how you could reset one, and it would think that the whole safety circuit was reset. Does anyone have a link or description of what I am describing? đź“š
 
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I had someone explain a few years back why is was unsafe to wire e-stops (or other safety devices) back to a safety relay/PLC.


I am not referring to the fact that, unless you have auxiliary contacts, you won't know which safety device was tripped; It was something to do with how you could reset one, and the circuit would think that the whole safety circuit was reset. Does anyone have a link or description of what I am describing? đź“š

I believe there is an argument goes something like this:
- 3 ESTOPs in series to a Safety relay
- 1 fills with water and is rendered inoperable, so that it cannot shut down the system
- there is no monitoring of each ESTOP circuit, so no one knows that the ESTOP will not work

Sorry - I can`t help with a link to what you are describing. In my experience, resetting the ESTOP that was triggered is SUPPOSED to reset the whole safety system.
 
Isn't there a standard on the protection of e-stops against water ingress?

Nothing stops you having three contacts in your button whereby two contacts are used for the e-stop relay and the third one for indication of which stop is pushed.
 
Isn't there a standard on the protection of e-stops against water ingress?
The issue I am referring to is more so that the safety circuit is able to reset while still tripped, if wired in series, not so much about water or shorting issues, at least from the way it was described to me.

Nothing stops you having three contacts in your button whereby two contacts are used for the e-stop relay and the third one for indication of which stop is pushed.
I had mentioned that this is not the issue I am referring to.

o_O... something about the relay/PLC sees the input change from one state, and back to the other to other and thinks that is has been reset?
 
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something about the relay/PLC sees the input change from one state, and back to the other to other and thinks that is has been reset?[/I]

So long as all of the e-stops are wired NC I don't see how that could happen unless there was a short?

I'm a very novice controls tech so I would not be surprised if I'm completely wrong here!
 
Perhaps someone was referring to an e-stop system made with a single core instead of the usual two cores in which case the fault would have to happen and correct in the same position on two separate contacts?

You can put a single relay onto a switch and series the relay outputs together... but it would be a matter of cost benefit analysis, wouldn't it?
 
Hadn't thought of this before - Just taking a guessing shot at this....

Two buttons in series with redundant contacts on each - wired in series to two safety inputs.

Estop is healthy.

Button#1 is pressed, but the one contact on fails to open due to a welded contact. The PLC detects the failure of the inputs to act as a pair.

Button#1 is reset.

Even though both safety inputs are now true, the PLC safety instruction is still faulted due to the failure. The instruction requires you to cycle the inputs such that it can see a clean transition off.

You choose to cycle button#2 On-Off-On to do this. The PLC sees a clean On->Off transition. Now the PLC instruction can be reset - but Button#1 still has a welded contact.

So there's a scenario. However, it is no different than hard wiring several buttons in series to a safety relay. So I don't see the issue as a specific issue to safety plc's.
 
You choose to cycle button#2 On-Off-On to do this. The PLC sees a clean On->Off transition. Now the PLC instruction can be reset - but Button#1 still has a welded contact.

I think this is the scenario he was referring to!!! Where button 2 can be reset and tricks the safety relay to thinking both are ok.

So there's a scenario. However, it is no different than hard wiring several buttons in series to a safety relay. So I don't see the issue as a specific issue to safety plc's.
My original post had specified relay/PLC, not specifically PLC, but I think you have the right idea!
 
You can wire E-stops in series.
But you have to calculate PFHd (probability of dangerous fault) to get the resulting PL and compare against the required PLr. Adding E-stops in series will increase PFHd, but you may still stay within PL>PLr.

As to identifying which E-stop is activated, then use an aux contact in each E-stop and wire it to the PLC.

@kekrahulik.
When the safety relay indicates that there is a fault, then the operator must test and verify the safety. He cannot just choose to test 1 E-stop. He must test all E-stops. Only when the safety system is verified to be OK may the machine be taken into operation again.
 
Adding E-stops in series will increase PFHd, but you may still stay within PL>PLr.
I have never heard of this or seen this used...

When the safety relay indicates that there is a fault, then the operator must test and verify the safety. He cannot just choose to test 1 E-stop. He must test all E-stops. Only when the safety system is verified to be OK may the machine be taken into operation again.

I have seen many systems with multiple estops, and never seen it required to test every estop when there is a fault. Normally, the safety relay or PLC will require the dual inputs to be reset, and if they are wired in series, it doesn't know which estop is being reset, only that it WAS reset with a change of state.

Maybe I am not understanding what you are trying to say...
 
I think this is the scenario he was referring to!!! Where button 2 can be reset and tricks the safety relay to thinking both are ok.


My original post had specified relay/PLC, not specifically PLC, but I think you have the right idea!

Basically this is what ISO 14119 is all about.

They are talking about "Mechanical Interlock Devices", generally people are talking about guard doors with electro-mechanical switches but it could just as easily be an E-Stop.

The idea is that if you have "intelligent" safety devices which have their own inbuilt short detection, then you can wire as may of them in series as you like. But if you have electro-mechanical devices then depending on the required Performance Level required then you may only connect a certain quantity of them in series.

A quick google search for ISO 14119 will show up lots of info on this.

I hope this helps.

Regards
Ian
 
JesperMP -

Spot on with the PL analysis.

As for the operator "cannot just choose to test 1 E-stop"... Oh yes he can. He's not supposed to, but with the series wiring he most certainly can and I will guarantee that some definitely will.

If the devices are all wired individually, then he can't because the button with a welded contact will never clear the error.
 
I agree completely with JesperMP.

It all entirely depends on what safety standard you're complying with and when you do a risk assessment or analysis on your machine, depending on the require performance level needed for your machine will dictate if you're able to wire the E-Stops in series with each other. But in any case it is always good practice to have 3 contacts on your e-stop buttons. 2 N.C. for the safety channel and 1 N.O. to the PLC for identifying which button was pressed. On my systems which are typically very small machines, we have multiple E-Stop buttons wired in series (no more than 2-3 usually) with the 3rd N.O. contact for identification and they all pass PLe requirements. (ISO 13849-1)
 
The reasoning is similar to interlocking devices in series connections(See ISO TR 24119). However, if the E-Stops are used less frequently in the application, series connection is permitted without the same reduction in DC as with the interlocking devices. Of course you still need PL evaluation like Jesper suggested.
 

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