Workshop isolator / emergency stop

Hi, all.

A friend has a garage into which he has moved some of his workshop equipment. A seperate dedicated ring for the workshop equipment has been installed.

The house CU is located on the garage wall ( at ceiling level ), and a 32A type B MCB has been added to the RCD side of the split-load CU, in addition to the existing socket circuits. This dedicated ring goes in 2.5 T+E down some trunking to a row of sockets along the workbench. So far, so good.

Now, with kids in the house, he wants to be able to lock off the workshop tools when he's not around, just in case. If we could combine this lockable isolator with an emergency stop, so much the better. At the moment, he has to climb up on the workbench to the CU and flip the MCB, which is a hassle.

So I'm at the design stage.

What's the best way to proceed here?

Should I be aiming to isolate both L+N ( requiring 4 poles of switching for both legs of the ring ) or would switching the lives only be sufficient? We're not trying to achieve 'electrical isolation' in the sense of safe electrical working on the circuits; we are trying to achieve 'machine safe' conditions, where the machines can't operate.

A cheap lockable 4-pole 32 A rotary isolator seems the simplest solution.

For the locakble emergency stop mushroom-head button option, we'd need to add a 4-pole contactor and enclosure to the mix, since the contacts on the e-stop button are only rated typically to operate control gear coils, not full load. So this would be a more expensive option.

What experience to others have of such setups? What would you do?

Reply to
Ron Lowe
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switching for

sufficient?

'machine safe'

solution.

coils, not

In my home workshop, the 13amp ring main power circuit is routed through a large two pole conntactor and only becomes a ring after the contactor. The contactor is wired to latch 'on' when a n/o green button is pressed, and drop out when a red n/c button is pressed. The green button and one red is located by the entrance door, and there are several red buttons located next to machines. I also have a single to three phase converter powered off a separate radial circuit, but this runs through another contactor that is pulled in by the ring circuit, so it can only work if the ring is live, and will be isolated if any emergancy stop button is pressed. The red buttons are mushroom headed 'proper' emergancy stop buttons.

AWEM

Reply to
Andrew Mawson

OK, thanks.

Using a contactor does have the advantage of being able to site multiple e-stop mushrooms at individual machines.

If I choose to do what you have done, what size of cable would be appropriate to drop say 2m from the CU to the contactor, if I leave the MCB as 32A and the ring in 2.5 t+e , leaving the whole circuit with a similar capacity? Equiv of existing 2* 2.5 is 5mm^2, so would I use 6 mm^2?

Reply to
Ron Lowe

Maybe I'm still thinking in the industrial mindset, but I've never really liked lockable or _especially_ self-locking e-stops. It's just too many components involved in something that's supposed to be simple, safety-related and _obviously_ switched off.

Isolate with your isolator, e-stop with your e-stops. Don't mix them up.

I don't have overall workshop power like this, but my fixed machines have individual isolators and e-stops (usually with multiple stop buttons) through their motor starters. A couple of them that didn't have starters but did want e-stops, needed contactors adding. Total cost was peanuts, because it's all S/H industrial scrap.

Obviously the isolators are "upstream" of the contactors. NALOPKT (at least of the people who've wired woodworking workshops I've been in)

Reply to
Andy Dingley

Tell "your friend" (you!) to switch off the RCD or lock the garage. If children want to try tools out they will plug them in elsewhere. Stop complicating things.

Reply to
david ryan

Honestly, my friend is not 'me'. I would have no problem saying if it was.

The tools in question are not portable, and can not be plugged in elsewhere...

Reply to
Ron Lowe

Single pole functional switching is hence adequate for this requirement.

I think I would provide a submain from the existing CU to a second CU style enclosure containing a contactor which in turn feeds MCBs for any circuits that you require to be emergency stopped. (the MCB or fuse in the main CU being sized so as protect the submain and provide discrimination with the downstream circuit breakers). The main switch/RCD in the second CU could act as an isolator for all circuits, or you could have an external locking isolator in the submain before the secondary CU.

The contactor could then be configured such that it is energised from a normally open start switch, and latched on by the switched power routed through any number of stop buttons. That way it will also act as a No volt release mechanism on a power failure of trip of the submain protective device.

Reply to
John Rumm

Tagged: Tiscali Idiot

Reply to
Adrian C

Here in Canada we only switch off the live wire (or wires). In the case of UK there would presumably be one live wire at 230 volts from the circuit breaker in the CU, for this particular 'workshop' tool/ machines circuit. Neutral and earth/ground would be wired through. Right? A dead simple idea once seen was to install an unusual socket and plug; i.e. of a type never used in domestic wiring. The live wire from the circuit breaker (input) was run to one pole of the socket. The live wire to the downstream outlets/sockets (output) was wired to the other pole of the socket. A suitable plug (again; not of a type that is normally used or is ever seen in a domestic situation) was wired with its two poles joined together, inside its cover. The plug was just about the right size to fit in someones pocket. The plug was either carried by the owner or hidden somewhere where the children could not reach/find it. The 'special plug' was only plugged into the 'special socket' when the parent was in the workshop. When plug was out circuit was inactive. Not completely familiar with ring mains but presumably the device could be wired at the start of the ring main; i.e. after the input from the circuit breaker and before the 'tail end' of the circuit returns to the source. Just a thought anyway!

Reply to
terry

On Fri, 17 Aug 2007 11:13:54 -0700, terry mused:

One way not to do it I suppose.

And a bad one, try not to think too much in future.

Reply to
Lurch

For functional switching, that is fine. However if you want true isolation then you need to switch the neutral as well. If you think about the implications of have several properties sharing a feed from a substation, then there is the likelihood that the neutral will be pulled a little away from true ground - so in effect it becomes a low voltage but also potentially low impedance live.

Sounds like something that could be done somewhat better with proper isolator switches or contactors.

Does not really work with a ring circuit since both ends are connected to the breaker.

Reply to
John Rumm

Yes a two pole contactor and or two pole switch to break open both the live and the neutral, would perhaps be a 'more correct' UK way to do it'?

This would entail both ends of both live and neutral ring main wiring being wired via the contactor.

Not both ends ends directly to the circuit breaker panel (CU). As made clear in the first post.

But contactors and switches would then have to be 'locked' in the 'off' position in a childproof manner. Maybe a two pole keyed switch with the key in parents pocket? Sounds complicated and expensive. Or a keyed switch in the primary of the contactor!

Regarding the chance of a neutral floating at a voltage slightly above ground. Here, despite, or maybe because of, the preponderance of overhead distribution, there is great emphasis on multi-grounding of all neutrals; both on the primary side and secondary side of all distribution transformers. The multigrounded neutral is tied to everything in sight! Including telephone guy wires, power guy wires, except on very high voltage transmission lines, transformer grounds, telephone cable overhead messenger (supporting) cables, at each residence by at least one ground rod, bonding to water mains etc. etc. Not sure what is the designation of the same method as used in the UK?

In the last 50 years or so have never seen a problem that seems related to having a neutral with an undue potential, except perhaps momentarily when say a 13Kv. primary phase wire either breaks and or touches something during an ice storm and before something trips out. The emphasis of having everything grounded and the neutral and grounds tied together at the entry to each residence seems to have worked well. Not only domestically but also in commercial situations such as telephone exchanges, radio huts etc.

The most common North American system, domestically is three wire 115

- 0 - 115 volts. The middle (neutral and grounded wire) at zero with, as it were, plus and minus 115 volts, on the two legs. And therefore

230 volts between the two legs for heavier appliances, water heaters and electric heating etc.

We do use two pole switching on many 230 volt devices; for example, am about to install a two pole switch on our bench saw which is 230 volts. The neutral will be solid as will the ground/earth to frame of the motor and metal saw frame.

It would be nice when anyone comments that something is incorrect, not possible, or does not meet code requirements within a certain jurisdiction (say the UK) if they could provide an explanation rather than a dismissive remark! One thing have observed, working both in North America and the Middle East, vis a vis say the UK, there is often more than one way of doing something!

Reply to
terry

As I said before, it depends what you are trying to achieve. Breaking just the live is acceptable for turning something on and off (i.e functional switching). For true isolation however then it needs disconnecting or switching via a dual pole switch with a minimum of 3mm contact clearance.

Which with a ring circuit is exactly how you would expect it to be wired. Note however for this sort of application it would be more normal to use a radial circuit anyway.

Rings are very handy for supplying a large number of sockets over a big area, and where the power usage will be spread around and "diverse" (i.e. not continuous big loads in a single place).

For a bunch of machine tools that you want the ability to emergency stop, then a radial fed via a contactor makes more sense.

The emergency stop buttons are available in locking form, such that they need a key to release them. Alternatively isolators are also available in lockable form.

You describe what is closest to "Protective Multiple Earthing (PME)". More detail here:

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with overhead supplies here, then TT earthing is used - i.e. no earth is provided at all by the electricity supplier, and local earth rods and extensive RCD[1] (GFI) protection is utilised to provide adequate protection.

[1]
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In the last 50 years or so have never seen a problem that seems

The danger (and hence the need for all the equipotential bonding you describe) is should the combined neutral and earth conductor get broken in the supply to a property, but not the live, then all the exposed metalwork in the house is liable to drift toward the live voltage.

The only place we typically use double sided supplies like that here is on building sites, via an isolating transformer giving 55-0-55 outputs for 110V tools. These deliberately don't have the central tap earthed.

Many appliance switches here will be single pole, although most sockets are equipped with double pole switches. Power tools like a table saw would typically have a No Volt Release switch (that opens on power failure) - many of these are also double pole.

I am sorry I had not intended my remarks to be dismissive, I was just highlighting that there are well established ways of achieving what the OP wanted to do, without the need of any inventiveness as such. Having emergency stop and isolation facilities is common practice in commercial or public workshops here.

(If you were referring to Lurch, then don't worry, he is like that with everyone - but he grows on you with time!)

With regard to comments being UK specific, then yes it is generally true in this group that it will be assumed unless someone points out that they are discussing different countries etc.

Indeed. There are quite a number of different practices between the way things are done here and elsewhere. (the wiring regulations here are very detailed, and carefully designed - so in many cases there are situations of right and wrong, or at least "best practice" and "not as good").

Reply to
John Rumm

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