E-Stop / MCR Training?

I am constantly looking for ways to improve our program. One of the things missing is E-Stop/MCR circuits. Our trainers, nice as they are, did not have E-Stop capability. The output voltage was fed directly from the voltage that fed the PLC's power supply. I modified the trainers and can now output voltage comes from a separate source.



Textbooks totally miss the point here. I have never worked on a real life PLC controlled system that didn't have some sort of E-Stop, yet most training material totally ignores this part of the system.



My experience is limited to the wood products world, and to a high speed assembly plant. The assembly plant was Austrian owned, and there we used European standards. I have forgotten these standards. The saw mills and plywood mills that I worked in didn't seem to have any real expectations for these systems.



I always used series wired mushroom head switches with NC contacts. The relays were nothing special, but I always put at least two sets of contacts in series, just in case a set failed.



Some of the machines had total output power shut down in a E-Stop condition, others allowed power to panel lights and switches. Sometimes all input devices remained hot, while others controlled all machine local input power. Again panel devices to remain energized.



My question is this, what do you guys want a fresh tech to know about E-Stop and MCR(not the instruction?) I can't think of a better source to find out what industry wants, so speak up. Thanks

May want to start with NFPA 79 as a basic guideline, should be able to preview it here: http://www.nfpa.org/freecodes/free_access_document.asp

There are a few more things here: http://www.sti.com/safety/index.htm

Just a thought.
I would suggest a relay with "force guided" or "positive opening" contacts instead of a standard relay.
It will cover you in case the contacts were to weld.
On the trainer? Probably not necessary, as long as the techs understand the difference.

Hope this helps.

I assume (Oh, how I hate that word...) that you know - and are teaching - that EStops by code must be hardwired. They cannot be done through the PLC. We can monitor a second contact on the EStop button with the PLC so we can indicate which button was activated.

Also, the new NFPA79 requires that they be manually reset - i.e., not automatically reset when the EStop button is pulled out. So your guys need to learn to design/troubleshoot that manual-latch circuit.

We design our systems to drop control power on an EStop. The PLC, HMI's, etc., will remain powered, but there will be no live control voltage to be able to "do" anything.

OZEE said:

The PLC, HMI's, etc., will remain powered, but there will be no live control voltage to be able to "do" anything.

Click to expand...

I have seen and have done myself...dropping the HMI and PLC along with the control circuit, but yes hard wire and N/C e-stop's

also there is a control off and control on PB

The above link for NFPA 79 does not appear to work properly, go to this link then select NFPA 79 from the list then agree to the disclaimer, when the NFPA 79 page appears scroll down to where it says preview. http://www.nfpa.org/aboutthecodes/list_of_codes_and_standards.asp

Chapter 9 is about control functions.

Because of time constraints on training programs, sometimes the key concepts get left behind. If you can find a way to convey the "fail safe" concept in your training, you will have given your student a large head start on understanding safety in controls design. The question being, "What happens if I cut the wire in the estop loop?"

Not only are PLCs not trusted for estops at my facility, we don't use the fault/estop inputs to VSDs. We just power them down with a contactor.

OZEE said:

I assume (Oh, how I hate that word...) that you know - and are teaching - that EStops by code must be hardwired. They cannot be done through the PLC. We can monitor a second contact on the EStop button with the PLC so we can indicate which button was activated.

Also, the new NFPA79 requires that they be manually reset - i.e., not automatically reset when the EStop button is pulled out. So your guys need to learn to design/troubleshoot that manual-latch circuit.

We design our systems to drop control power on an EStop. The PLC, HMI's, etc., will remain powered, but there will be no live control voltage to be able to "do" anything.

Click to expand...

Hard wiring is the major point. What you are describing is the track I am on so far. Thanks.

rsdoran said:

The above link for NFPA 79 does not appear to work properly, go to this link then select NFPA 79 from the list then agree to the disclaimer, when the NFPA 79 page appears scroll down to where it says preview. http://www.nfpa.org/aboutthecodes/list_of_codes_and_standards.asp

Chapter 9 is about control functions.

Click to expand...

Thanks. I'll make this available in some form.

DJM said:

Because of time constraints on training programs, sometimes the key concepts get left behind. If you can find a way to convey the "fail safe" concept in your training, you will have given your student a large head start on understanding safety in controls design. The question being, "What happens if I cut the wire in the estop loop?"

Not only are PLCs not trusted for estops at my facility, we don't use the fault/estop inputs to VSDs. We just power them down with a contactor.

Click to expand...

Out of the eight classes I teach, six have "fail safe" concepts. The VSD idea is interesting. I've been in mills that used starters to supply the VSD, but not contactors. Great idea.

I have a little demo, (bad output card) that proves the "Why we don't use PLC's for safety circuits," point. After drilling them on don't pull a disconnect under load, they are faced with a motor that will not stop. Fun ain't it.

plchacker said:

Out of the eight classes I teach, six have "fail safe" concepts. The VSD idea is interesting. I've been in mills that used starters to supply the VSD, but not contactors. Great idea.

Click to expand...

If you're doing this, be absolutely certain to have the starter/contactor on the line side of the VFD. You'll let some of the magic smoke out if you power up a VFD without a connected load.

Though it wasn't related to an estop, I battled a system a while back that had the contactor on the load side of the VFD and they wondered why the VFDs were blowing their output transistors.

Originally posted by DJM:

Not only are PLCs not trusted for estops at my facility, we don't use the fault/estop inputs to VSDs. We just power them down with a contactor.

Click to expand...

Why is it that people feel simple hardware devices are infallable and more complicated hardware devices are inherently a safety issue waiting to happen? I have personally seen more welded or stuck contactors in my life than I have drives that failed in such a way that they would keep running.

The big points are redundancy and cross-checking. If you are really serious, use two contactors, not one. Check the states of the contactors against each other as well as against the commanded state. Redundancy and cross-checking is what makes drives with 'Safe Off' or any of the other 10 or so names out there today safe. I would personmally feel better about a drive using on-board safe off that a design using a single supply side contactor.

Originally posted by OZEE:

Though it wasn't related to an estop, I battled a system a while back that had the contactor on the load side of the VFD and they wondered why the VFDs were blowing their output transistors.

Click to expand...

This was a big issue even as recently as a couple of years ago. Drive manufacturers have gotten wise to this recently and started designing drives that can take this. The AB Powerflex family as well as the Rexroth Indradrive will both tolerate load side contactors. You kind of have to pick your poison with drive contactors. Line side contactors can cook your precharge circuit.

Keith

OZEE said:

If you're doing this, be absolutely certain to have the starter/contactor on the line side of the VFD. You'll let some of the magic smoke out if you power up a VFD without a connected load.

Click to expand...

Yeah, I forgot to mention the part about the contactor being on the line side of the VSD. We just want the motor to stop, we're not trying to release smoke from the drives!

kamenges said:

Why is it that people feel simple hardware devices are infallable and more complicated hardware devices are inherently a safety issue waiting to happen?

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Your question is valid. I don't think anyone here with any time in industry believes that any hardware device is infallible.

I also think that the more devices in use, the better. Our VSD safety / fault circuits are wired, but they get powered down. The issue inevitably comes down to costs. Doesn't everything?

OZEE said:

If you're doing this, be absolutely certain to have the starter/contactor on the line side of the VFD. You'll let some of the magic smoke out if you power up a VFD without a connected load.

Click to expand...

I never would would have thought about load side interuption. So far, I have not used contactors at all.

And too, I'm not building many systems any more. Just our labs, and they need some updates for safety.