E-Stop question.

OK before you get your nickers in a knot, please read my reason for this post

I work in a Community College training the next batch of I/E techs, in our area. Prior to that I worked in the field for some twenty years.

Recently I was asked to take a battery of tests that a local manufacturer gives to select employees. As it happens they are hurting for good people.

While reviewing the hands on protion of this test I ran across an E-Stop circuit.

As best as I could tell from a brief look all was fine except that the PB was a momentary, rather than a detent PB. I asked the guy who designed it and he pointed to the MCR. Fine and well, but odd to me.

Is this acceptable? Again, I did not have much time to look at the details. In all of my experiance, in Europe, and here in the US, I have never seen a momentary switch used for E-Stop. I'm just wondering If I missed something, or if maybe the desinger of the test did.

The plant its self was the most impressive I have ever seen.

Though NFPA79 is not mandatory it is widely followed. Its reference number 10.7.2.2 states "Pushbutton-type devices for emergency stop shall be of the self-latching type and shall have positive (direct) opening operation."

I would use a maintained PB just in case some one else hit the reset button before the person that initiated the e-stop was ready.

Latching and hard wired - end of line. Definately not in the PLC. Would always suggest use of an IEC type 4 safety relay also with 2 contactors and 2 input lines to the relay.

My ignorance is going to show..

Show me the diagram difference between a latching and not latching PB. Please.

I use a symbol like this to denote a latching, turn-to-release E-Stop.



I don't like push-pull E-stops. I've had operators hand me the head that broke off from too much pulling...



-Eric

Latched, with key-reset, hehe

before you get your nickers in a knot

Click to expand...

If you are going to use quaint English phrases (as in England English)

Then at least spell them correctly - its knickers with a 'K'

and the usual terminology is don’t get your knickers in a twist

meaning 'don’t get unnecessarily cross or angry'

I am still to find an E-stop button that would satisfy all my needs: twist-lock, illuminated, sturdy enough to withstand the operator abuse. Usually it's any two out of three. Not that this is all I ever need in this life, but...

Which makes me ask a related question: does anyone ever does keyswitch guards bypassing? I know a lot of people do: some types of machines would be virtually unservicable without it.

However, there seems to be no mentioning of the safest way to accomplish this in any of the safety standards (unless I did miss something big time). Yes, you can duplicate all your safety loops, use safty relays with external contact monitoring, et cetera, et cetera. But then, at some point, you introduce a plain old keyswitch into the circuit which, like all the plain old switches, is prone to fail. One turns the key into "guards enabled" position and the fused contact block still bypasses the part of the safety loop...

Does anyone aware of existence of a 22.5 mm keyswitch with force-guided contacts?

You may want to check www.sti.com

LadderLogic said:

I am still to find an E-stop button that would satisfy all my needs: twist-lock, illuminated, sturdy enough to withstand the operator abuse.

Click to expand...

I've seen several references to this lately. How can an E-Stop be used too much? It seems that if they are using the E-Stop to routinely stop the machine, then either they need more training, or the controls engineers need to give them more convenient alternatives to gracefully stop the machine. A latched E-Stop should be used in emergencies, and a momentary PB should be used for a cycle stop.

I guess it depends on the field, type of machines involved, what may happen but it has been my experience that E-Stops will be used as much (probably more) than a cycle stop. There can be numerous reasons for this and the bad part is people usually hit an E-Stop very very hard.

I have seen situations where one person had to operated multiple machines so when a jam occured, which in the paper industry you can expect, he stopped it using the E-Stop.

Technically many machines today may run 24 hours a day and the only reason to stop them is when a problem occurs.

Eric Nelson said:

I use a symbol like this to denote a latching, turn-to-release E-Stop.

I don't like push-pull E-stops. I've had operators hand me the head that broke off from too much pulling...



-Eric

Click to expand...

Thats funny, they always tell me that the button just fell off. Too much pulling huh? Maybe your onto something here.

rsdoran said:

You may want to check www.sti.com

Click to expand...

Thank you, but just two days ago I met their sales rep with a catalog this (watch the fingers) thick. There is a keyswitch in it, but it is intended for light curtains reset and such, so it has regular contacts on it.

How can anyone use an E-stop too much? Easy: they bang the hell out of it. I like IEC type pushbuttons but am really missing an E-stop with metal parts in the actuating mechanism.

Thats why I am not fond of the 22.5mm IEC style pushbuttons, I prefer the nema rated 30mm and this is one area I believe AB is strong in.

I asked about the diagram for a latching ESTOP because the only symbol I have ever seen is a NC pushbutton with a mushroom head. I thought an ESTOP was always symbolized using the mushroom, if not can someone provide more information on this.

I have no idea if positive (force) guided are available for pushbuttons or not but I think positive break are. To show the difference here are a couple of pictures.