Emergency Stop implementation

I'm planning a scheme for emergency stop buttons in the workshop and am considering using the existing RCDs as the switching element. I have a three phase board equipped with a 100mA RCD and a split single phase board with the ring mains on a 30mA RCD. By having emergency stop buttons with two n/o contacts, each with a suitable resistor switching a phase to ground I can introduce a momentary fault current which trips the appropriate RCD but leaves the lighting circuits with power. . Very handy having a button by the exit to ensure everything is off when leaving.
Seems very simple, and I've proved it works, can anyone see any drawbacks to this scheme?
AWEM
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Andrew Mawson explained :

The usual way to achieve this would be using a contactor - a button to energise it, using a pair of n/o contacts to maintain it and a another button to break the circuit releasing the contactor.
The supplies turned of should not include lighting.
Instead of (or to supplement) buttons, you could install a 'washing line' - pull it anywhere along its length and it as the same effect as a stop button.
Whatever method you use, there should be no possibility of anything starting up when the supply is restored.
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Harry (M1BYT) (L)
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Agreed, the act of breaking a circuit to disconnect the power makes it fail-safe, it avoids the possible problems of dirty contacts or damaged cables making a safety circuit fail to operate.
The pull cord idea sounds a bit OTT for a diy workshop though.
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fred
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It happens that fred formulated :

That depends on how big and how many stop buttons you might need to cover it versus one pull cord which can cover an entire bench.
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Harry (M1BYT) (L)
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Supplying the stop buttons from two separate sources may not be safe practice. Also, aren't you disconnecting any dc injection braking?
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writes

An important point if you have your hair wrapped around a pillar drill or lathe. I'd suggest that an emergency stop system is fairly poor without said braking. The 'washing line' idea is a good one but AFAIK normally only employed on larger machines WITH braking where the 'washing line' completely surrounds the workpiece.
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Years ago I used to work on control systems for big production lines and especially power presses. Our mantra was that emergency stop buttons weren't for "mere emergencies", only for real f*ck-ups. In particular, lots of press tooling had a very real risk of several hours downtime and hundreds of pounds damage to tooling if you hit the e-stop and cut the power, wedging the press half-way through a downstroke. Nor was the e-stop necessarily the quickest way to stop a machine. Often there would be a "stop" switch with a controlled braking system that would apply a powerful brake as safely and speedily as possible, causing a stop rapidly and in a safe position. E- stops would frequently work by just dumping the power (and waiting for a spin-down) or by applying full brake instantly, which was sometimes enough to smash a machine to pieces, if used from full speed and at the wrong time. This was highly machine dependent - if you have a few ton of flywheel, nothing is going to stop it immediately and most attempts to do so will generate such a torque that they're likely to break something off its mountings.
So use of the e-stop as a control was seriously discouraged. Unless you knew that it was safe to use it so, there was a risk that using it lazily to shut down at the end of the day would be an expensive mistake, So don't _ever_ assume that it's safe to be so cavalier with an e-stop. That's one of the reasons they might be fitted with key- release buttons - just to discourage this careless use. Sometimes we even provided a stop button of identical function to the e-stop, but trained to operators to treat it in a completely different fashion - next week they might be using a different machine.
The real use of the e-stop was to shut things down _reliably_, not particularly quickly or safely. If the control system had gone crazy, this might be the only way to apply any sort of brake, because the computer-controlled "stop at the top" brake had crashed in an unpredictable and hence uncontrollable manner. This is why an e-stop circuit is designed to be _simple_ and _reliable_ as a priority over its effectiveness and even its safety.
I wouldn't contemplate using RCDs as e-stops. * It's weird. Weird is inherently bad here. * RCDs aren't intended to break high currents repeatedly and they do start going unreliable if used in that way. * There's a risk that resetting an RCD might start a machine, which would be unexpected and thus unsafe.
* I'm assuming that you were at least achieving NVR behaviour by some robust contactor-based circuit. (and if you are, then why not use that overall contactor?)
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If your hair is that long then you shouldn't be working in a workshop, and if it isn't long then in the odd millisecond you have before your head is rearranged you won't be able to reach for the emergency stop anyway.
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Andrew Mawson wrote:

Only drawbacks are - non-compliance. - if RCD fails, your emergnecy stop also fails, which isnt ideal if someone's getting seriously injured.
The first one can be resolved by taking a wire back to neutral before the RCD rather than using earth.
NT
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On Sat, 14 Mar 2009 15:44:21 -0700, meow2222 wrote:

Surely this is *in additon to* the normal NVR switches on each piece of equipment? A sort of 'switch it all off as you leave' button?
But yes, I'd go for a cross-wired contactor instead of using the RCD as the main cutout. That way you have an overall NVR as well, if you have older equipment that doesn't have them. Are they compulsory in industrial settings anyway?
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Slightly OT to the serious nature of this discussion, but did anyone notice the story about the 'Merkins trying some humour with the Russkies and giving them a switch marked 'Reset' in English and Russian - the point of the story was that they used the wrong Russian word.
However what was more amusing to me was that I'd just got a couple of identical 'Emergency Off' switches - same box, same large button - direct from China (via Ebay); "Made in China".
So the 'Merkans gave the Russians a present which was made in China !!!
Rob
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Andrew Mawson wrote:

As others here have said, it used to be the case that you would have emergency stop buttons around the workshop that would cut power to all the machines (but not the lighting) via a contactor.
However, there is recent safety legislation that requires you to consider each machine separately.
I think the general principle is that cutting all power to a machine may not be the best way to deal with an emergency, particularly if the machine is controlled by any kind of electronics. Better to bring the machine to a controlled stop under defined conditions in a defined time with defined decelleration (e.g. if you cut the power then d.c.injection braking fails to operate). Machines which require controlled stop usually have external e-stop interface, which, when activated initiates the machine's emergency shut-down procedure and inhibits restart.
Also, I believe the idea of stopping the entire power to a workshop in the event of an emergency condition is also deprecated, because it can be disorienting in itself to anybody not affected by the notional emergency.
I am not an expert in this field and can't cite the legislation. Your local HSE office will no doubt be able to help. Also a read of PUWER might shed some light.
HTH
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One would hope that this depended on the workshop context. In a school I'd favour it (one teacher, many pupils) although I doubt there's much more hazardous than an Apple Mac left in any "design" playpen these days. In a workshop of peers, it would be a rare accident where cutting power abruptly to many was warranted by the benefits, compared to appropriate controls per-machine.

Years ago I knew these rules - anyone more up to date? I'm sure we'd all appreciate pointers to the relevant regs and good practice.

Read it, couldn't see much of relevance here in it.
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I suspect that your lecky supplier may have T&Cs that disallow this.

RCDs are for safety, not convenience.

DrawbackS? It's a bloody stupid idea.
Fuck me, how much value do you put on real safety? None. Shorting phases to earth? You are a fuckwit of the first order. Well done.
Al.
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current
with
phases to

Well Al your use of language leaves much to be desired - also read the original post - it's switching a 1500 ohm resistor to either earth or neutral before the RCD.
AWEM
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Neither are permitted by the lecky suppliers.
Could you not just actually do a proper job instead of bodging around the problem?
Al.
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Al wrote on 15/03/2009 :

Turning a motor or light on puts a resistor of sorts across the supply, as does pressing the RCD test button - the difference is?

On that we can agree.
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Harry (M1BYT) (L)
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Harry Bloomfield wrote:

The difference is a "resistor of sorts" between live and earth (RCD test button) as opposed to between live and neutral (normal load).
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Dave Osborne wrote:

My bad, that should read 'The difference is a "resistor of sorts" between live and earth (deliberate remote tripping of an RCD)...'
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wrote:

The difference is that both connections to the load pass through the RCD's sensing current transformer (in a direction such that one cancels out the other and the nett current is zero), whilst the test load is only passed once through the sensor and so it represents an unbalanced current.
Current operated RCDs don't detect the "leakage current" per se (that would be difficult from a remote location), instead they detect an imbalance in Live and Neutral currents, then assume that any difference is a leakage to earth.
In fairness to Al, he's also talking against the variant where the test current is fed to earth, rather than this unbalanced current. My own sloppy wording might have sent the thread in that direction.
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