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?
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.
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.
BBC3, ITV2/3/4, channels going to the DOGs
An important point if you have your hair wrapped around a pillar drill or
I'd suggest that an emergency stop system is fairly poor without said
The 'washing line' idea is a good one but AFAIK normally only employed on
larger machines WITH braking where the 'washing line' completely surrounds
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
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
Only drawbacks are
- 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.
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
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
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
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.
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.
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
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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