Safety E-Stop Relays

I would like your opinions on whether you would require a daily, weeekly or monthly "E-Stop button check" on a Safety E-Stop relay circuit prior to running a machine. Would you force the operator to perform this check through code? Do we need to check the buttons if it is connected to a Safety relay? Would your answer be different for a category 2,3 or category 4 e-stop circuit? Are there any standards out there on this subject?

I do not recall any type of E-Stop standards that require them to do tested. That would probably be from Company to Company.

What I would recommend is making sure they are "True E-Stops". This means that they cut power to the relay when the button is pushed. This will be useful if the button itself fails. Then your relay will spring open cutting power to the motor until you fix your E-Stop.

Hope this helps.

I thought an E-stop was either a button on a computer screen or a button wired to a PLC input module. j/k!

Good description! This is something that I've seen done incorrectly.

OP - I've never heard of specific testing standards, but it's a bit out of my normal lane.

bwwestbrook said:

I do not recall any type of E-Stop standards that require them to do tested. That would probably be from Company to Company.

What I would recommend is making sure they are "True E-Stops". This means that they cut power to the relay when the button is pushed. This will be useful if the button itself fails. Then your relay will spring open cutting power to the motor until you fix your E-Stop.

Hope this helps.

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One of the things you can do is wire a NC contact on the relay in series with the reset button before the latch. This way if the relay is bad (welded), you will not be able to reset the circuit.

    |--|PB1|--|\CR1|--|\CR2|-----|ES|--------(CR1)---|

|---| CR1|---| CR2|------|                 |-----(CR2)---|

   

It will somewhat check itself.

gosenbach said:

One of the things you can do is wire a NC contact on the relay in series with the reset button before the latch. This way if the relay is bad (welded), you will not be able to reset the circuit.

|--|PB1|--|\CR1|--|\CR2|-----|ES|--------(CR1)---|

|---| CR1|---| CR2|------| |-----(CR2)---|

It will somewhat check itself.

Click to expand...

Assuming you use redundant relays (CR1 and CR2) and use NO contacts from both of those in every critical power feed. If you are using CR1 and CR2 to provide enough contacts and only use one or the other then you still have the same problem with a welded relay. Half your circuits will still be hot.

Assuming you use redundant relays (CR1 and CR2) and use NO contacts from both of those in every critical power feed. If you are using CR1 and CR2 to provide enough contacts and only use one or the other then you still have the same problem with a welded relay. Half your circuits will still be hot.

Click to expand...

This is using redundent relays with all the critical power feeds going from power, through CR1, then through CR2, then to the device.

gosenbach said:

This is using redundent relays with all the critical power feeds going from power, through CR1, then through CR2, then to the device.

Click to expand...

That's the correct way, but in all honesty, I've seen hundreds of examples where only CR1 or CR2 are used independently. The sole purpose in those cases were because they couldn't cram more than 8 contacts on a single relay I've even seen where CR1 and CR2 in series drive CR3 and they use contacts off of CR3, thinking they've accomplished redundancy in failures of CR1 or CR2, not even considering a failure of CR3.

That's the correct way, but in all honesty, I've seen hundreds of examples where only CR1 or CR2 are used independently. The sole purpose in those cases were because they couldn't cram more than 8 contacts on a single relay

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I have seen that a lot too. Persionally, I put a lot of weight into machine safety on any project I work on. I do not want someone getting hurt because I took a shortcut. In those cases where you need more than 8 contacts, I figure it is time to upgrade from a simple safety circuit to some sort of safety controller or similar more advanced safety system.

I have had conversations in the past about "What is an E-Stop?" A true E-Stop is not just a button on a screen. That would be a stop pushbutton. An E-Stop will cut power to your machine or motor without communications through the PLC.

I would like your opinions on whether you would require a daily, weeekly or monthly "E-Stop button check" on a Safety E-Stop relay circuit prior to running a machine.

Click to expand...

After getting a bit off the topic of your question, I think it is up to whatever the company wants to do. I also do not recall anything written standard to testing intervals. I would say that if the machine could kill someone, a daily check may be approprate. If the risk of injury or death is low, perhaps monthly would be fine.

gosenbach said:

One of the things you can do is wire a NC contact on the relay in series with the reset button before the latch. This way if the relay is bad (welded), you will not be able to reset the circuit.

Click to expand...

The Banner safety relays that we use have terminals available for just this purpose. From the manual:

If you use a Siemens ASi Mon safety relay on an ASi Network then you can program in that one of the estops (Cat4) needs to be checked daily.

mjohnsonc said:

I would like your opinions on whether you would require a daily, weeekly or monthly "E-Stop button check" on a Safety E-Stop relay circuit prior to running a machine. Would you force the operator to perform this check through code? Do we need to check the buttons if it is connected to a Safety relay? Would your answer be different for a category 2,3 or category 4 e-stop circuit? Are there any standards out there on this subject?

Click to expand...

1. Safety circuit requirements differ around the globe but the E-stop seems a fairly standard concept that every standards association can agree on. It is a requirement that all safety circuits are separate & independent from any control system and also will override any control system (an E-stop cannot be a part of a PLC program - with the exception of programmable safety relays & safety PLC's).

2. An E-stop must provide three things, resetting an E-stop is a deliberate manual action (e.g. twist to release), resetting an e-stop must not by itself cause an automatic restart (prevision for unexpected start-up) and the operator cannot restart without re-setting all the E-stop devices.



If your safety circuit complies with the requirements of a category #2 type circuit then there must be a "periodic check" of the operation of the safety circuit. There is no clear definition of the time period however I recommend this check is every shift change.



If your safety circuit complies with a cat #3 or cat #4 its design and operation is such that if a fault is detected in the operation of the safety circuit it shall not permit a re-start. Thus no periodic check is required but is a good idea to perform one as part of routine maintenance.



As a general rule most brands of electronics safety rule are capable being connected to at least a cat #3 if not cat #4 level. For what is worth use a double pole E-stop and spend the little extra on cable and you have a safety machine.

_Woody_ said:

1. Safety circuit requirements differ around the globe but the E-stop seems a fairly standard concept that every standards association can agree on. It is a requirement that all safety circuits are separate & independent from any control system and also will override any control system (an E-stop cannot be a part of a PLC program - with the exception of programmable safety relays & safety PLC's).

2. An E-stop must provide three things, resetting an E-stop is a deliberate manual action (e.g. twist to release), resetting an e-stop must not by itself cause an automatic restart (prevision for unexpected start-up) and the operator cannot restart without re-setting all the E-stop devices.



If your safety circuit complies with the requirements of a category #2 type circuit then there must be a "periodic check" of the operation of the safety circuit. There is no clear definition of the time period however I recommend this check is every shift change.



If your safety circuit complies with a cat #3 or cat #4 its design and operation is such that if a fault is detected in the operation of the safety circuit it shall not permit a re-start. Thus no periodic check is required but is a good idea to perform one as part of routine maintenance.



As a general rule most brands of electronics safety rule are capable being connected to at least a cat #3 if not cat #4 level. For what is worth use a double pole E-stop and spend the little extra on cable and you have a safety machine.

Click to expand...

Agree with what you say and I must say I have been appaled at the E-Stop circuits on the Canadian and American systems I have worked on. Not sure this indictive of North America or just the industry (conveying only) that I'm in.

1. E-Stops are zoned, you can be standing between two conveyor lines and press an E-Stop (or pull a lanyard) along one line, the other keeps running.

2. E-Stops are used for access to temporarily remove something as standard.

3. The line can restart, simply by pulling the E-Stop out.

4. No Redundancy at all.

5. Bog standard off the shelf 24VDC relays are used as E-Stops relays, one for each contactor.

6. No fault checking of E-Stop circuits.


None of the above would be allowed in any of the UK projects I had worked on in the last 10 or more years.


I've also been appaled at the standard of panels, the panels in North America would not be allowed in the UK, I'm seeing big chunky contactors of the type I last saw in a UK panel in the early 80's. You can touch the power contacts on these you can also touch the contacts of panel isolators, bus sections free and open, jeepers, at times I feel like its the third word here.

For some reason I thought all the safety that was driven in Europe was a world wide thing, it appears not.

PeterW said:

Agree with what you say and I must say I have been appaled at the E-Stop circuits on the Canadian and American systems I have worked on. Not sure this indictive of North America or just the industry (conveying only) that I'm in.

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Peter,

I agree, I can only speak for Canada, more and more companies are jumping on the safety bandwagon here. I am on startup where machines bought from a broker in Chicago are being set up in Belleville. Because this is classified as a new install, a PSHR was performed and most machines require Cat 3/4 safety circuits. The existing machines has 'safety' as microswitches that dropped out an MCR. Easily defeated using a jumper.

Non contact magnetic switches defeated by taping slugs to the sensors!

One machine has a PLC output used to engage the MCR. This machine received a light curtain relay contact as a PLC input. For commissioning, I simply forced the input on, and the machine was happy.

We are currently implementing about $40,000 worth of safety hardware not to mention our labour. I see the safety wave as a booming business for automation companies. Automotive has always been big on safety here, but other industries are not, yet.