E-stop monitoring relay reset.

[killerkaz]

I've been poking around on the net to find an answer to this question but haven't been able to locate a definitive one.

When using a e-stop monitoring safety relay with hardwired maintained e-stop push buttons is it permissible to use a PLC relay output to ultimately supply the momentary reset signal to reset the monitoring relay once all the e-stop buttons have been reset (manually pulled out)?

The thought is that pressing a button on the touchscreen and using a PLC contact to reset the monitoring relay ensures functionality of the touchscreen and PLC program to allow the resetting of the monitoring relay after an e-stop.

[James Mcquade]

I would say no, plc outputs can break, stick together, or be forced in the on position. touchscreens can have the pushbutton interface to stick. if any of these conditions happed when you reset the e-stop pb, the machine can take off.

I would look at NFPA 79 - electrical controls for industrial machinery. Don't have it with me right now.

regards,
james

[killerkaz]

I appreciate the reply and see your point. Following the logic you suggested I would note that a reset push button could be "tied down" or fail closed as well effectively causing the same fault you mention. I guess the risk assessment stage would look at each scenario to determine if the chosen reset method is acceptable, the standards I have read so far do not specifically disallow the method I originally posted about.

[Tharon]

It is permissible to use the PLC output to reset a safety relay, as long as the reset power also goes through a normally closed set of contacts on all external devices. Most true safety relays I have seen have to see the reset input go true, then false, before the relay will reset. This protects against stuck contacts.

In the case of a PLC output controlling the reset on a safety relay, it is no different than a single set of normally open contacts on a momentary pushbutton. Pushbutton contacts can fail as well. The safety relay should protect against a failed reset input.

The added benefit of having a PLC output reset a safety relay is it ensured the PLC is running before a safety relay can be reset. Not entirely required, but sometimes useful.

So whether the reset input power comes from a PLC contact or a pushbutton contact, the power still has to travel through the normally closed contacts on all external devices and the safety inputs for the safety relay have to be true, and the reset input has to turn on and off. Then the safety relay output can energize.

[killerkaz]

Tharon, when you say external devices a you referring to all e-stop operators and power contactors/relays?

Thanks for the input!

[rdrast]

I wouldn't permit it in any system I designed, and it does absolutely nothing towards "Proving" the touchscreen and PLC program are working.

Safety relays aren't level sensitive, they are edge trigerred, which means they will not reset on a hung contact. Even so, I wouldn't want a flaky output or bad programming to make my ESTOP loop auto reset "by accident".

[kolyur]

I wouldn't permit it in any system I designed, and it does absolutely nothing towards "Proving" the touchscreen and PLC program are working.+1


So whether the reset input power comes from a PLC contact or a pushbutton contact, the power still has to travel through the normally closed contacts on all external devices and the safety inputs for the safety relay have to be true, and the reset input has to turn on and off. Then the safety relay output can energize.I have never heard of doing this. My reset PBs are typically wired directly to the safety relay. Can you provide a code or standards reference?

[JesperMP]

When using a e-stop monitoring safety relay with hardwired maintained e-stop push buttons is it permissible to use a PLC relay output to ultimately supply the momentary reset signal to reset the monitoring relay once all the e-stop buttons have been reset (manually pulled out)?Absolutely not. To reset an E-stop relay, there must be a manual operator input (not the same as pulling the button back.)
Automatic reset means that a movement can initiate the moment the E-stop button is pulled out. It is irrelevant that there are software interlocks that prevent an immediate start.

[Tharon]

By external devices I mean any relays and other safety relays which are powered by the safety relay being reset.

I'm pretty sure they throw the "per manufacturer's operating instructions" or the like into the NEC/OSHA codes.

If you look at many Light curtain and Emergency Stop relays that have a reset circuit, in the suggested wiring they show the reset circuit going through a NC contact on all external devices. A "Supervised" reset.

Light curtains with onboard Cat4 Safety call it EDM (At least, Leuze, Banner, and Triad/Pinnacle do). 24VDC Power through the NC contacts of the relays powered by the Safety Outputs is brought back to the Light Curtain. It will not reset from an off condition if this signal is not present. Safety relays used for Cat3/4 Emergency stop circuits from Schmersal, Pilz, Jokab, Allen Bradley, and a couple others I have used all have this supervised reset as an option on their safety relays.

As far as the OSHA/NFPA 79 is concerned, a single NO contact from a momentary pushbutton is not any safer than a single NO contact from a PLC. If it is not safe for a safety relay to be reset, then the reset request signal, regardless of where it comes from, shouldn't be able to reset it.

Personally, I haven't built any machines this way, because I never had a reason to. But I have multiple machines in shop that have come from very large OEMs that are built this way.

If you are afraid that the PLC output will suddenly turn on and reset the relay when you don't want to... what is going to stop a person from walking by and pressing your reset pushbutton which will do the same thing? During a routine OSHA inspection a few years back they caught us on just this. That anyone could walk by and press our reset buttons for our robotic safety enclosures. We had to place a secondary button inside the robot cage tied to a safety rated timer. You had to press the inside button, then within a permanent (non adjustable) set amount of time, close the gates and press the second button located outside.

Whether you like the idea or not, or want to do it, is up to you. But it doesn't violate any rules and is not unsafe.

[JesperMP]

A light curtain is not an E-stop.
A light curtain may be reset automatically, an E-stop never.

[muusic_man]

By external devices I mean any relays and other safety relays which are powered by the safety relay being reset.

I'm pretty sure they throw the "per manufacturer's operating instructions" or the like into the NEC/OSHA codes.

If you look at many Light curtain and Emergency Stop relays that have a reset circuit, in the suggested wiring they show the reset circuit going through a NC contact on all external devices. A "Supervised" reset.

Light curtains with onboard Cat4 Safety call it EDM (At least, Leuze, Banner, and Triad/Pinnacle do). 24VDC Power through the NC contacts of the relays powered by the Safety Outputs is brought back to the Light Curtain. It will not reset from an off condition if this signal is not present. Safety relays used for Cat3/4 Emergency stop circuits from Schmersal, Pilz, Jokab, Allen Bradley, and a couple others I have used all have this supervised reset as an option on their safety relays.

As far as the OSHA/NFPA 79 is concerned, a single NO contact from a momentary pushbutton is not any safer than a single NO contact from a PLC. If it is not safe for a safety relay to be reset, then the reset request signal, regardless of where it comes from, shouldn't be able to reset it.

Personally, I haven't built any machines this way, because I never had a reason to. But I have multiple machines in shop that have come from very large OEMs that are built this way.

If you are afraid that the PLC output will suddenly turn on and reset the relay when you don't want to... what is going to stop a person from walking by and pressing your reset pushbutton which will do the same thing? During a routine OSHA inspection a few years back they caught us on just this. That anyone could walk by and press our reset buttons for our robotic safety enclosures. We had to place a secondary button inside the robot cage tied to a safety rated timer. You had to press the inside button, then within a permanent (non adjustable) set amount of time, close the gates and press the second button located outside.

Whether you like the idea or not, or want to do it, is up to you. But it doesn't violate any rules and is not unsafe.


I agre with Tharon on this one. I have designed many pieces of equipment with E stop circuits. If the estop button is still pressed, the plc can't reset it no matter what. Again understanding that any relays controlled by the estop safety relay have a set of their NC contacts in series with the plc output. In my design, the relays connected to an estop safety relay are always positive guide contact relays for more reliability.

[muusic_man]

A light curtain is not an E-stop.
A light curtain may be reset automatically, an E-stop never.

The OP didn't say he wanted it to reset automatically. He said that once the e-stop PB was pulled out that the operator could push a "PB" on the touch screen and toggle a plc output that, properly wired through relays controlled by the estop relay, would reset the estop monitor.

BTW, can anyone tell me why estop relay manufacturers suggest a wiring method that allows the estop relay to reset automatically when the estop pb is pulled out? Typically it is just a jumper wire on the estop relay.

[iant]

it is a matter of what category level of safety you need.
or Risk Analysis.
remember these are safety relay's
they will not reset if the reset signal is 'Stuck' ON

the bridgable type is predetermined so they will work

there are many options available
.

[JesperMP]

muusic_man, I see that I failed to notice what he wrote at the bottom of his first post.

In that way (via a HMI software button, and a PLC output), I believe it will be OK.
Like already mentioned, the safety relay will monitor the reset input, and fault if the reset hangs for a longer time.

[iant]

the only thing to be careful with is remote reset of light curtains
if you reset these out of site is is possible to liven up a machine with someone inside the machine area

[JesperMP]

the only thing to be careful with is remote reset of light curtains
if you reset these out of site is is possible to liven up a machine with someone inside the machine areaThat does not sound as an application suitable for a light-curtain.
More suited to a trapped-key lock.

[OkiePC]

I have one system that uses a PanelView button to reset a safety relay via a PLC relay output. It has the advantage of allowing the PLC to inhibit "short cyclling" of the big safety contactors that supply power to a bunch of drives. This way, once the e-stop occurs, the PLC can dictate that the circuit cannot be reset for a minimum of 30 seconds, which is enough time for these particular drives to power down to a safe level, and be able to clear away the low DC bus faults they all will generate. It also gives incentive to the operators not to abuse the e-stop when they need the regular stop. They know if they use the e-stop, they will be down at least 30 seconds and have to wait.

I did not design it this way, and I have not done this in the past, although I have thought about doing it. My preference would be to wire a PLC contact in series with the hardwired reset button to gain the same advantage.

[killerkaz]

Thanks for the discussion everyone. I appriciate all the input!

[iant]

That does not sound as an application suitable for a light-curtain.
More suited to a trapped-key lock.
Probably not
But
prdouction minds may think so..
like I said earliey - risk analisys