Earthing a metal Clad workbench

[Nozzle]

I have a wooden workbench which has a 4 way socket, a junction box, and bunch of flex, a plug and a pillar drill all wired in. Then it gets plugged into a conventional socket which is in my garage and protected in the normal (recently installed) way by an RCD at the consumer unit.

I plan to be screwing a large sheet of steel to the top to make it more robust when welding/grinding at the bench. Clearly this presents a new electrical hazard that wasn't there when it was just plain old wood.

I was going to earth the steel to the junction box by way of green/yellow cable that is sufficiently large, then possibly replace the standard two way socket on the wall in the garage with one with a built in RCD. If nothing else as it'll be easier to test when it's right there.

With recognition that home projects, or indeed any sized project you might think of are a compromise between what is possible, and what is necessary - what advice can fellow users of DIYnot.com offer please?

Nozzle

 

[JohnW2]

I plan to be screwing a large sheet of steel to the top to make it more robust when welding/grinding at the bench. Clearly this presents a new electrical hazard that wasn't there when it was just plain old wood. ... I was going to earth the steel to the junction box by way of green/yellow cable that is sufficiently large ...

I'll be interested to hear what others think about this, since it's very much a 'swings and roundabouts' situation. If something live were to come in contact with the metal sheet, then it would be safer if that sheet were earthed - since the 'fault' should then cause an RCD and/or MCB to operate before anyone had a chance to touch it (which would be dangerous if there was anything earthed around that they could touch at the same time). On the other hand, if you were to come into contact with something live, then having a big earthed sheet of metal sitting there for you to touch simultaneously would increase the risks. To some extent, the decision might be influenced by whether or not there were other earthed things in the vicinity.

... then possibly replace the standard two way socket on the wall in the garage with one with a built in RCD. If nothing else as it'll be easier to test when it's right there.

As often discussed here, unless you just want the reassurance of two RCDs (in case one fails to work when it should), nothing is to be gained by having two RCDs protecting the same circuit - in the event of a fault (and if both RCDs are non-faulty), there is no telling whether one, the other, or both will trip.

Kind Regards, John

 

[EFLImpudence]

Nozzle, Firstly, do you actually mean 'earth'?

I'm not trying to be clever but I am trying to find out if you have already decided that it does not require 'bonding' but it may be advantageous to 'earth' it.
Or are you just using the term as most people do?



It is up to you, really.
As has been said, if you think a live wire may come into contact with the steel then earth it.
If not, then do not.

It may be better to rearrange the wiring (vertical drops) so that contact with the steel is made as unlikely as possible.

 

[JohnW2]

Nozzle, Firstly, do you actually mean 'earth'? ... I'm not trying to be clever but I am trying to find out if you have already decided that it does not require 'bonding' but it may be advantageous to 'earth' it. ... Or are you just using the term as most people do?

Valid questions, but I wouldn't have thought that it was a situation in which many people would contemplate 'bonding' - and, in any event, I suspect that you would argue that this sheet of metal didn't need bonding, because it (probably/presumably) would have only a very high impedance path to earth.

It is up to you, really. ... As has been said, if you think a live wire may come into contact with the steel then earth it. ... If not, then do not.

As I implied, I think the problem here is the need for a 'crystal ball'. It would presumably be impossible for Nozzle to say that he could be certain that a live wire would never come in contact with the steel. He therefore has to try to work out which is more probable - that, or the 'other scenario' in which earthing the steel would create a hazard.

It may be better to rearrange the wiring (vertical drops) so that contact with the steel is made as unlikely as possible.

I would doubt that the (I presume you mean 'fixed'?) wiring is the likely hazard - I would have thought that leads of power tools (and the power tools themselves) represented the main hazard.

Kind Regards, John

 

[JohnW2]

One way of trying to keep electricity away from workbench surfaces is (look in foreground, not the awful mess behind!):

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• JohnW2
• 17 Sep 2011
• 3

Kind Regards, John

 

[EFLImpudence]

I suspect that you would argue that this sheet of metal didn't need bonding,

Oh, I'm not arguing; it definitely doesn't

I would doubt that the (I presume you mean 'fixed'?) wiring is the likely hazard - I would have thought that leads of power tools (and the power tools themselves) represented the main hazard.

Yes I agree but it may be better to raise the sockets or have suspended ones so that the leads are not draped across the steel.



Also, what is the floor made of?

For Nozzle in case this was the reason for the question; there are no regulations stating that the steel should be earthed.
It is up to you to decide the best course of action.


Edit - hadn't seen the picture when I wrote the above.

 

[EFLImpudence]

Not up to Norm's standard.

 

[JohnW2]

Not up to Norm's standard.

I couldn't hope to compete with Norm, not even in terms of tidiness (is his actually a 'working' workshop? ), but I do have a few (well used!) tools crammed in ...

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• JohnW2
• 13 Jul 2014
• 1

Untitled

• JohnW2
• 13 Jul 2014
• 1

Kind Regards, John

 

[EFLImpudence]

is his actually a 'working' workshop?

Oh yes. Have you not seen him working? He's retired now

 

[Nozzle]

Thanks for your input one and all,

To take the question approx in the right order:

I'm ignorant of the difference between earthing and bonding - only to say that I'm approaching this from the point of view that I believe I need "AN earth" else any protection at the consumer unit is useless??

Also, I expect the most likely way said piece of steel might become energised is by damaging a lead to a power tool either directly on the metal itself, or damaging the lead elsewhere then putting the tool down on the metal. I'd like to think that if the steel were connected to earth, and somehow the earth became energised then the existing protection should operate to disconnect the offending supply such that that local is earth is no longer energised. Similar to a radiator, boiler or any other large metallic component distributed across the house.

Also, when I'm working on electronic projects or power supplies that might be a bit of open-heart surgery on some gadget that is live, then if that should somehow contact the steel, the protection should operate. I would take a precaution by laying an insulator first, piece of wood or plastic, else the work bench would just short contacts out and damage the gadget, but still want to be safe with a blown-up project rather than in hospital/6 feet under.

The installed wiring that feeds the bench is vertically down the wall in plastic conduit and finishes higher than the surface of the table, so there's no chance of crimping any cables between bench and wall. That is the installed cable rather than power-tool flex which might.

Nozzle

 

[JohnW2]

is his actually a 'working' workshop?

Oh yes. Have you not seen him working? He's retired now

Yes, I have - but even though he has some very up-market extraction systems, I get suspicious when I cannot see even a single speck of sawdust

The thing that often occurs to me about all the very fancy/nice/ultra-expensive workshop tools he has, is that the setting up of them to perform a little task (which, of course, gets edited out of the broadcasts) must take a lot more time than actually doing the work - I sometimes wonder whether he wouldn't get some things done quicker by using old-fashioned hand tools (which I know he does sometimes!).

Kind Regards, John

 

[JohnW2]

I'm ignorant of the difference between earthing and bonding - only to say that I'm approaching this from the point of view that I believe I need "AN earth" else any protection at the consumer unit is useless??

Don't worry about the semantics. Yes, if you want a device to disconnect the electricity in the event that something live touches the steel (hopefully before you touch it), then it needs to be earthed.

Also, I expect the most likely way said piece of steel might become energised is by damaging a lead to a power tool either directly on the metal itself, or damaging the lead elsewhere then putting the tool down on the metal. I'd like to think that if the steel were connected to earth, and somehow the earth became energised then the existing protection should operate to disconnect the offending supply ...

Yes, that's true, but don't forget that the steel becoming 'live' only presents a hazard if you can simultaneously touch something at earth potential - that might be a wet solid floor, pipes, earthed electrical equipment (including some tools) etc. As I've said, in other situations (if you touch something live yourself), having the earthed steel there would actually increase the chance of you getting a shock (although an RCD should limit its duration to a fairly small number of milliseconds).

Also, when I'm working on electronic projects or power supplies that might be a bit of open-heart surgery on some gadget that is live, then if that should somehow contact the steel, the protection should operate. I would take a precaution by laying an insulator first, piece of wood or plastic, else the work bench would just short contacts out and damage the gadget, but still want to be safe with a blown-up project rather than in hospital/6 feet under.

Personally, I would not work on any powered-up electronic equipment (whether mains-driven or not) on a metal workbench (earthed or not), even if there were a bit of wood or plastic under it.

From what you've said, it sounds as if you are inclined to the 'earthing' (rather than 'not earthing') approach.

Kind Regards, John

 

[Adam_151]

Assuming its not acetylene welding then the problem with an earthed metal bench when welding is that some welders have the work side of the supply tied to mains earth... the rest is that if the workpiece is on the workbench and the work clamp is not attached, you can still weld... you just melt the CPC in the installation and the welder supply cable!

As to electronics repair, not on a metal workbench, I still remember being told about an incident with a faulty dot matrix printer, being disassembled and powered up in pieces to try and find the fault... BANG... its power supply board had been rested upon a metal biscuit tin lid. That saved some time however, the issue of whether the printer should be relocated to the skip had become a very easily decided question

 

[ericmark]

When I had to do a workshop with metal coated tops I used 50mm² cable to bond as anything less can melt with welding current. Much does depend on the welder used in my case Oxford and due to design with one output bonded as if jolted the internal uninsulated bars can bend and cause connection of output and input.

Today oil filled welding sets are rarely seen most modern sets are fully isolated being air cooled but safest method is non conducting surface to work benches.

 

[JohnW2]

When I had to do a workshop with metal coated tops I used 50mm² cable to bond as anything less can melt with welding current.

What did you bond the worktops to? Presumably it would have to be direct to the welder (otherwise some of the current would go through the installation's CPCs) - which may be practical (with 50mm²) for a 'fixed' welder, but presumably not for a DIY 'portable' welder, would it?

Kind Regards, John