Safety Relay Explained. Basics of Wiring an Emergency Stop

What is the difference between a safety relay and general purpose relay? Why can’t we just connect the estop to a PLC? How do you wire a basic emergency stop?
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[@disun4152]

Hi Tim, Thank you for the video ! I used a lot of safety relays in our machine safety systems, this is particularly true back to 20 years ago at that time the safety PLCs were not popular as they are today. I used my first safety PLC is Pilz from Germany. Today we are using more Allen Bradley. I like Jocab series safety relays the best like RT9, RT6 even RT11 if you like more contacts. Those relay have good status indications and wiring diagrams. Now ABB bought it.

[@TimWilborne]

Yes I'm learning a lot about safety relays, I'm going to try to arrange some on doing risk assessment and determining SIL levels

[@vinmansucc2698]

The guy just explained problem that I have been having thanks 👍

[@TimWilborne]

Glad to help

[@brahmanand3332]

Please sir , making some video on mass flow meter on coriolis principal...

[@amandatoledo4759]

Hi Tim, great video! I have two questions. (1) What kind of contactors are you using? They appear to be modular contactors with 2-pole, 24 VDC, 2 NC contacts. (2) How would you change the wiring to notify a PLC that the e-stop has been pressed? For my application, when the e-stop is pushed, the PLC needs to send an output signal to the servo drive, telling it to decelerate the servo motor to stop.

[@TimWilborne]

For the most part, they need to be a relay with force guided contacts but I would reach out to the safety controller and servo manufacturer. Between the two of them, they should be able to give you a good idea.

[@Jay-tr3px]

Thanks for the great infos on this vid. Just wondering what happens when you try to short a relay output from A1 to S21 and suddenly sparks? Is the safety switch still functional or is it damaged inside?? Does it need replacing?? Thanks again.

[@TimWilborne]

Hard to say but sparks usually let the magic smoke out.

[@Jay-tr3px]

@@TimWilborne Thanks Tim. But what does "sparks usually let the magic smoke out" mean??

[@ShaneWelcher]

Thanks Tim

[@TimWilborne]

You are welcome Shane

[@mattgardner2641]

what is the model of the estop relay used in this video

[@TimWilborne]

Hi Matt. It is a Phoenix Contact 2700570, PSR-MC40-3NO-1DO-24DC-SP. Here is where I bought it. www.airlinehyd.com/product/PHOENIX-2700570/2217065

[@renemubang596]

Wow,this was helpful

[@TimWilborne]

I'm glad!

[@user-pd8bk4ei1h]

To make it even more confusing in the actual field some estops are push in to estop some are pull out to estop !

[@TimWilborne]

Yikes! Haven't ran into pull to stop Estops.

[@user-pd8bk4ei1h]

@@TimWilborne I found the pull to estop mainly in the Alberta Oil fields many which are remote and they didn't want any "bears" to accidently push in the estop !

[@joesamuelson2784]

I have also seen pull out Estops in gas compression stations and refineries here in the U.S.

[@TimWilborne]

Interesting, thanks for sharing Joe

[@dwaynes5983]

@@TimWilborne Same here never heard of it. In a emergency always want to push in. especially when there are six different locations.

[@lulusehnihpi8901]

why would you have two normally closed on an estop isnt that massively less safe than one nc one no?

[@TimWilborne]

No, you would actually increase the time to detect a failure which opens opportunity for a second failure point which could cause your safety system to fail. This is a common misconception. If you have a normally open contact and the wire breaks, you will not be able to detect it until the estop is pressed. It could be months or even years before it is pressed. During that time, you will have a single point of failure. If you have two normally closed contacts, the moment the wire breaks, the system will fautl.

[@lulusehnihpi8901]

@@TimWilborne interesting nearly everything I have at work is NC NO so this is quite surprising to hear thought your logic makes complete sense. I wonder if it's simply because it's all very old that it uses NO and NC.

[@TimWilborne]

@@lulusehnihpi8901 There are two times that I see NO and NC contacts on an estop 1. This one is the no no...someone wires both contacts to a PLC and programmatically attempts to make a safer system. This one is bad for the reason we already discussed plus a PLC does not have the redundancy necessary to be considered safe...unless it is a safety PLC. 2. This one is OK in certain SIL levels. It is a single channel NC estop and the NO contact is wired to an indicator light to let you know the estop is pressed.

[@lulusehnihpi8901]

@@TimWilborne interesting I'm certain we don't have don't have any safety PLC's interestingly though everything is one open one closed Is a thing in European countries that do NC NO because these are the same machines in France and Germany

[@TimWilborne]

@@lulusehnihpi8901 What are the normally open contacts wired to?

[@fergspan5727]

This is a dual channel safety relay

[@TimWilborne]

Correct

[@darrelljourdan9926]

Another school of thought. An Emergency Stop when pushed should immediately shut down the system. The E stop should completely shut down the control circuit and not be part of the safety relay circuit. Main thing is the safety circuit should be inspected and tested to ensure the machine shuts down when the safety circuit is actuated.

[@TimWilborne]

I wonder how often most companies exercise their safety circuit

[@faridonkarimimusic9377]

I hate troubleshooting safety relay

[@TimWilborne]

They can be a challenge

[@coasterbp]

Playing devil's advocate here... what reliability and predictability do I achieve by wiring an ESTOP to a safety relay that I DON'T achieve by wiring my ESTOP to a safety input card with pulse testing that's connected to my safety PLC? You seem to poo poo that method pretty quickly without expanding on it.

[@jackpast]

I’m guessing he was speaking about a normal, not safety, plc?

[@coasterbp]

@@jackpast ah, maybe.

[@disun4152]

Most safety relays have cross input check with dual channel inputs, like saying one channel is 24VDC the other channel is 0VDC, or one is NO the other is NC contact.That is what we call Category 4 or SIL 3. But if you put more than one relays in series using one safety relay, that is another story. I am not an machine safety expert, and I found it is quite confused when talking about machine safety categories, especially category 3 and 4. In the safety PLC, you can wire up each single safety device, like estop or gate switch to a pair of separate inputs, which make them category 4 standard.

[@TimWilborne]

You are correct Jack, I was talking about a regular PLC input compared to a safety relay. I've got some videos coming on a safety PLC and that in my opinion is the best solution but not every application warrants it or has the budget for it.

[@coasterbp]

@@TimWilborne Understood. I'm just so used to my realm of PLCs where everything we do is safety processors and safety cards. Thought maybe I was missing something. Thanks for the clarification. I'll reel my neck back in now. As always, good stuff, Tim.