Safety Relays Input in Parallel

Hi. We are designing a ESD system on a biogas plant. I'm new regarding the safety standards and wanted to ask a question regarding safety relays.

An Emergency Shutdown System will shutdown numerous equipment and multiple safety relays will be needed (i.e. 12 SR). Is it advisable to parallel the Input (E-stop) to the safety relays? I am not sure if this will conflict with safety standards.

If not, the only solution I can think of is using additional E-stop contact blocks (max of 4) and use safety relays to create multiple E-stop signals which is more costly.

Thanks for the help in advance.


Ivan Joseph

My initial thoughts on this would be to use 1 Master SR and the output of that SR would be run as a loop through all of the other SR's inputs, so if you E-Stop the Master ESR, it would take out all of the slave SR's. You would of course use the required category of circuit for the risk level (Whatever performance level required), such as dual contact E-Stops and dual inputs etc. It is always permissible to use the output contacts of an SR as part of the input circuit to a subsequent "slave" SR so that if the main one detects an E-Stop activation, it will trigger the other(s) to also drop out.

Also have to think about the RESET inputs, ideally in the example of Master / slaves - none of the other SR's resets will work if the Master is not cleared, but this will still allow reset of that slave SR if it only has a local E-Stop activated.

If you need that many safety relays in the system, I'd recommend looking at a safety PLC. The wiring will be much simpler in the long run.

IvanJoseph said:

Is it advisable to parallel the Input (E-stop) to the safety relays?

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I'm hoping that it's just a translation thing, but you should NEVER EVER EVER be putting emergency stops in parallel. They should ALWAYS be in series.. If they're in parallel, either you have to press all four of them to drop out the safety inputs, or you're wiring them normally open, which is just as bad.

With that said...

Generally, I personally would consider it fine to wire any number of e/stops into the same inputs. The reason for splitting up safety devices is to prevent fault masking, which can happen if two or more devices in series are operated together. While this regularly happens with guard switches, it seldom happens with emergency stop buttons, simply because while you're quite likely to have multiple guards open for cleaning or clearing a fault, you're not likely to go pressing multiple stop buttons.

With that said...

That is purely my personal, general assessment, that by no means applies universally. Standards differ from place to place and site to site, and everything you do MUST be subject to your own documented risk assessment, as well as any particular local requirements, be they national laws or site specifications.

With all of that said...

If this were my system, and I were using safety relays, I'd be wiring each safety device into it's own automatically resetting safety relay, and then wiring the relays of all safety relays into one master safety relay, which has a manual reset function. Again, depending on my assessment on the day, I'd potentially put all of the emergency stop PB's in series into one of those "slave" safety relays, but the overall architecture would be the same.

And finally, with all of the above said...

I'd be using a safety PLC.

ASF said:

I'm hoping that it's just a translation thing, but you should NEVER EVER EVER be putting emergency stops in parallel. They should ALWAYS be in series..

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I think they mean to parallel, for example a single E-Stop into 4 relays.

I too would be looking at an expandable, programmable safety relay, controller or PLC. Do you have a safety propriety brand? If so, which one?

Oh right, yes, on a more careful reading of the OP's post I think you're right.

I think if that's the aim, I would run the single e/stop into one safety relay with 8 normally open relay outputs (so you'd likely need an expansion module). Then use two of those channels to wire into each of the four system's own safety relays.

But again, if I had the choice, I'd be throwing it all out and going safety PLC, like you suggest.

Thanks for the replies.

We are still in the design stage and confirming the circuit category level required. We are assuming a single E-stop configuration. Most do not have a Reset button. We are planning to use the E-Stop contacts on some (max of 4 safety relays per E-Stop). Is this okay?

I asked yesterday because there were some part of the design that a E-Stop will control 8 safety relays.

We have looked not into safety PLCs but understand that it will be better specially if the site specification will require a high circuit category level.

Thanks again.

Do your risk assessment, and then get back to us. There will be a number of solutions that meets your requirement for PL (Including Category and Stop Category), complexity, and cost.

We are assuming a single E-stop configuration. Most do not have a Reset button. We are planning to use the E-Stop contacts on some (max of 4 safety relays per E-Stop). Is this okay?

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This raises a couple of points with me:
1) E-Stops should be placed according to the ergonomics section of the AS/IEC/TUV/Whatever standard you are designing to. There's no point having 1 E-Stop in a spot where there will never be people, and it's hard to get to in a hurry.

2) No Reset buttons suggest latched E-Stop buttons, and automatic resets on your safety relays/controller. This can be dangerous, as pulling the E-Stop out is re-energisation of the equipment. A reset button, and manual reset mode ensures that there has been some human intervention before the system is energised, or restarted.

We have looked not into safety PLCs but understand that it will be better specially if the site specification will require a high circuit category level.

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The safety controller isn't just for higher PL, Category or SIL levels. It can often allow you more simple wiring, better troubleshooting, more flexibility, and added functionality.

I can't emphasises enough how important it is to have a robust safety system. It would also be prudent to have your system tested, and even checked by a TUV certified agent if your company has little experience with this sort of thing.

Jeev said:

1) E-Stops should be placed according to the ergonomics section of the AS/IEC/TUV/Whatever standard you are designing to. There's no point having 1 E-Stop in a spot where there will never be people, and it's hard to get to in a hurry.

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We raised that the Estops should be located near the area it will controll but they wanted the Estops mounted in a panel or console.

IvanJoseph said:

We raised that the Estops should be located near the area it will controll but they wanted the Estops mounted in a panel or console.

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Well, get the "they" involved to sign off on a risk assessment that assesses doing it that way is safe.

I had that argument with another client recently. They decided they didn't want an e/stop in a certain area, I refused to remove it until they provided me with a risk assessment assessing the removal as safe. They asked if I could do the risk assessment, I told them that if I did it, the result would be that it wasn't safe to remove it. This went back and forth for 18 months, finally they found a safety officer who was willing to sign his name to a risk assessment saying it was safe, and I removed the e/stop.

You have to cover your own backside. Either do your own risk assessment based purely on your best, objective assessment of what the system needs to be safe, regardless of what upper management want; or have them prepare and sign off on their own risk assessment and do it exactly as stipulated by them.