Workshop new bench- Electrical

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JIM_TEAL

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Hi everyone,
I have just acquired an all steel bench for my new workshop/garage. Should I bond it to the earth cables in the sockets, similar to the stainless steel sinks in the adjacent utility room.
My limited understanding of mains suggests that without an earth bond any live contact (accidental) would not trip the circuit breakers.
Will it be OK to wire into the earth terminal in the ring main socket at the bench?
Thanks in advance for any advice.
Regards Jim T
 
I would earth the bench to the earth connection in the socket, just in case, but I'm not an electrician.
You could also earth it to a spike in the garden
 
The reason for the bonding on sinks etc is because they're traditionally connected to metal underground water pipes, so already provide a very good earth.

If a short to earth happens elsewhere in the system then until the breaker trips, the building's earth system will be pulled upwards towards 240v (it won't actually reach it, because it's also being pulled down to 0v by the earth spike/supplier's earth/whatever, but it will float somewhere between the two). In the days of fuses, this situation could last for longer than it would now with breakers, certainly long enough to give you a shock if you had one hand on the washing machine (whose case goes up along with all the other earthed metal in the house) and the other on the tap (which stays down at 0v thanks to its copper piping). By bonding to the house earth system any metal that also has its own earth, you ensure that the dangerous potential between the washing machine and the tap doesn't happen - wherever they go relative to the real 0v, they will be the same and you can't get a shock between them. This applies especially to metal underground pipework, but also metal building frames (my garden workshop, for instance) and any other "extraneous conducting parts".

The same issue doesn't exist with a metal workbench, unless the feet are concreted deep into the floor.

I suppose if you regularly work with mains electrics on your bench then there's a risk of a live conductor touching it and you getting a shock off another part of the bench, and earthing it would help with that. Can't do any harm, anyway.

An alternative to wiring directly into the back-box would be to make up a plug with only the earth pin connected, and plug it into a handy socket.

Pete
 
This is not an authoritative reply, I am not fully up to date with latest regs, but I think both the above posts (i.e. the first two replies if more have appeared!) contain dangers so I am diving in.
I hope a registered spark will come along with a definitive answer.

Is there any other earthed metalwork e.g. a water or gas pipe in the room that it would be remotely possible to touch at the same time as the bench?
Are any of the tools you will use at the bench earthed (i.e. do they have 3 core supply cables, as opposed to double insulated with a 2 wire cable)?
If the answer to both is no the bench need not be bonded as I understand it.
However if there is a mains socket within reach of the bench - I'm sure there will be! - it would be wise to bond the bench.
If the answer to either is yes the bench should be bonded to the building's earthing system, as far upstream as possible, definitely before - or at, if no alternative - the first socket on the circuit in the room. It must be a permanent connection not via a removable plug!
Connection to an earth spike realistically won't protect you at all unless there is a RCD or RCBO on the supply. Hopefully there is. Even solid bonding may not protect you without an RCD/RCBO, depends on fuse blowing characteristics.
 
[An alternative to wiring directly into the back-box would be to make up a plug with only the earth pin connected, and plug it into a handy socket.

Pete[/QUOTE]

That suggestion is verging on the criminal! Never, ever do anything like that. Any earth must, repeat must, be permanent.
Yes your metal work bench should be bonded to earth. I won't tell you how to do it as I haven't seen your property or your electrical system. I don't know what age it is or to what edition of the regs your system was installed under.
Most modern systems have all the water pipes, gas incomer and main electrical feed all bonded. Your sinks should be bonded to the water pipes which should be bonded together i.e. Hot and Cold. The earth bonding for these should go back to the main consumer unit earth bar. But, you may have an older system where this is not the case!
The other point is that any electrical work in a kitchen, bathroom, garage or garden etc must now be carried out by a competent person and is notifiable to your local authority under part "P" regulations.
My advice is consult a qualified electrician and get it done properly. At the most your life or that of a family member could depend on it, not to mention the validity of your insurance! I certainly would not rely on what is said on this or any other forum. Sorry if that comment upsets anyone, I know that all advice is given in good faith but we are talking mains electricity and anything said "Blind" could have very dangerous consequences.
 
prv said:
The reason for the bonding on sinks etc is because they're traditionally connected to metal underground water pipes, so already provide a very good earth.

Pete
Click to expand...

Remember Alkathene? Not many underground metal pipes nowadays!

You need to bond the bench to the supply earth. I would then cover it with a sheet of ply or similar. Then touching the bench while holding a live appliance shouldn't kill you.

The ground in our village is so dry that it is impossible to find a good earth. Consequently, all houses have a long bare cable buried in the earth and that is carried back to the pole. Multiple protective earthing or MPE.
 
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Unlikely to be the case, but some early trips need a fairly large current to operate. They would definitely not save a life!
 
Alex_Blackwood said:
That suggestion is verging on the criminal! Never, ever do anything like that. Any earth must, repeat must, be permanent.
Yes your metal work bench should be bonded to earth. I won't tell you how to do it as I haven't seen your property or your electrical system. I don't know what age it is or to what edition of the regs your system was installed under.
Most modern systems have all the water pipes, gas incomer and main electrical feed all bonded. Your sinks should be bonded to the water pipes which should be bonded together i.e. Hot and Cold. The earth bonding for these should go back to the main consumer unit earth bar. But, you may have an older system where this is not the case!
The other point is that any electrical work in a kitchen, bathroom, garage or garden etc must now be carried out by a competent person and is notifiable to your local authority under part "P" regulations.
My advice is consult a qualified electrician and get it done properly. At the most your life or that of a family member could depend on it, not to mention the validity of your insurance! I certainly would not rely on what is said on this or any other forum. Sorry if that comment upsets anyone, I know that all advice is given in good faith but we are talking mains electricity and anything said "Blind" could have very dangerous consequences.
Click to expand...

+1. Wise words Alex.

Strictly speaking, you can do the work yourself and have it inspected by a qualified electrician and you can do that through your local building control department (I have). However, I wouldn't bother again because in practice it's quicker and no more expensive to pay a suitable qualified electrician to do the work.
 
Alex_Blackwood said:
That suggestion is verging on the criminal! Never, ever do anything like that. Any earth must, repeat must, be permanent.
Click to expand...

A proper earth that's actually necessary - yes, of course. But I don't believe that it is necessary to earth a metal workbench in the first place, and it's certainly not necessary to equipotential-bond it in the same way as a sink, which is what the OP was thinking of.

Pete
 
Graham_Wright said:
Remember Alkathene? Not many underground metal pipes nowadays!
Click to expand...

Exactly, so equipotential bonding is often redundant. But it doesn't do any harm, so is often done anyway. I think the 17th Edition regs may actually have removed some of the requirements for it, but the last time I needed to know any of this stuff they weren't yet in force, and I've forgotten most of it anyway.

Note that equipotential bonding (the green and yellow wires and sheet-metal "do not remove" tags you find under your sink) is completely separate (though related) to protective earthing of appliances.

Pete
 
Pete, Agree wth what you say, Just didn't agree with your plug idea. Hope I wasn't too strong!
Personally I would go down the bonding path. There seems to be some argument for and against. I don't have the 17th edition handy so can't be definitive. Like you I have to refresh my memory with a good read when dealing with this sort of thing nowadays! Plus we don't know what his system is it could be protected by nothing more than wire fuses or it could be 17th edition compliant!
Which is why I would reiterate what I said about using a qualified electrician.
What would concen me is a case where the bench had a resistance to earth of such a value that would allow a "shock" current to pass if you provided a parallel path.
I once worked with a very clever guy who used to say, when confronted with a problem, "Start at the Court of enquiry"
 
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Alex_Blackwood said:
Pete, Agree wth what you say, Just didn't agree with your plug idea. Hope I wasn't too strong!
Personally I would go down the bonding path. There seems to be some argument for and against. I don't have the 17th edition handy so can't be definitive. Like you I have to refresh my memory with a good read when dealing with this sort of thing nowadays! Plus we don't know what his system is it could be protected by nothing more than wire fuses or it could be 17th edition compliant!
Which is why I would reiterate what I said about using a qualified electrician.
What would concen me is a case where the bench had a resistance to earth of such a value that would allow a "shock" current to pass if you provided a parallel path.
I once worked with a very clever guy who used to say, when confronted with a problem, "Start at the Court of enquiry"
Click to expand...

As Alex has said, the best way to ensure compliance is to employ a qualified electrician. Nigel 's point about using an insulated bench when doing electrical work is a good one, but the OP has a steel bench.

Imagine the following;
- you have double insulated power tool with a damaged cable (unknown to you).
- your child or grandchild runs in to see what you're doing and holds the steel bench and peers over the edge.
- you turn off the tool and place it on the unearthed steel bench out of reach of the child.
- the live (phase) conductor in the damaged cable touches the bench - child becomes live.

Hopefully you have a RCCD fitted and the child doesn't notice as the power trips out.

But if the bench is earthed the child is safe.

Yes, I did chose the child scenario to be dramatic but it is not far fetched and compliance with the wiring regulations will also protect the OP.
 
Alex_Blackwood said:
The other point is that any electrical work in a kitchen, bathroom, garage or garden etc must now be carried out by a competent person and is notifiable to your local authority under part "P" regulations.
Click to expand...

Does Part P apply in Northern Ireland, from where the OP appears to come? It doesn't in Scotland.

Edit: PS Some work, even in kitchens, bathrooms and gardens (is a garage a "special place"?) doesn't have to be notified.
 
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When you park a large metal object like a car in your garage do you all immediately phone an electrician to earth bond it for you before hoovering it?

There is unlikely to be much harm in it being unbonded.
There could be an issue with a garage as the local earth could be at a different potential to the 'earthed earth' at the house.
If the bench is bonded to the house earth, and the concrete floor gets wet, you might get current flowing.
 
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