Installing plug sockets in steel furniture.

[LParThree]

Hi all,

Figured I'd come straight to the experts with this one because I don't have a great deal of electrical experience.

I'm a designer at a furniture manufacturing company. We manufacture furniture almost entirely out of folded sheet steel profiles.

Currently I am working on designing a bespoke kitchen for a client, this client would like UK mains sockets installed on a couple of her units.

I'd just like to know if there are any regulations or best practice rules when it comes to installing electrical systems on a metal surface.

As far as I understand I'd need to ensure that the steel units are earth bounded to the structure of the building, is there anything else that I need to be aware of?

I want to make it as painless a job as possible for the electrician doing the install. Any advise would be much appreciated. Thanks in advance!

L

 

[Chivers]

All the metal units will indeed require Bonding. Not sure what others thoughts are but personally I in your installation documentation I feel you must insist any customers already have RCD type protection fitted supplying the power to any sockets fixed or fitted to your units.

 

[loz2754]

All the metal units will indeed require Bonding

Why?
The units are not exposed conductive parts nor are they extraneous conductive parts.

 

[Chivers]

Couldn't the metal cabinets become live under a fault condition and if not bonded give some one a Zap? Especially if sockets are installed in them?

 

[MJPD29]

Couldn't the metal cabinets become live under a fault condition and if not bonded give some one a Zap? Especially if sockets are installed in them?

I don’t think they are metal cabinets, from my understanding it’s the metal back boxes he’s talking about.

 

[Chivers]

"I'm a designer at a furniture manufacturing company. We manufacture furniture almost entirely out of folded sheet steel profiles."

 

[MJPD29]

"I'm a designer at a furniture manufacturing company. We manufacture furniture almost entirely out of folded sheet steel profiles."

oops

 

[pc1966]

Will you be supplying the metal furniture item with the sockets already installed and wired, with just a cable or terminal block for the installer to connect to the house's electrical system?

The two issues I can see are:

1. The metal of the structure has to be connected to the CPC (earth wire) of the supply to the sockets. That might just happen due to the mechanics of attaching the sockets via their fixing screws but that is not good enough - you must ensure there are proper electrical connection point(s) to guarantee it. Typically this means a hole with unpainted metal so a screw and terminal will make a good connection there, or a welded stud that (again) has protection to make sure its thread is not covered in any paint. Ideally that would be pre-wired to any junction box along with the cabling and factory tested.
2. There are two choices of wiring for UK 13A sockets, one is the radial where a single cable from the CU (consumer unit = fuse box) feeds all of them, the other is the "ring final circuit" where the cable loops round and eventually goes back to the CU again. To avoid yet another religious war I won't discuss further, but ideally if you have the unit pre-wired it should be suitable for either radial or RFC use, and than means both ends are available at any junction box where house wiring enters the furniture. That way the installer can do either.

 

[loz2754]

The metal of the structure has to be connected to the CPC

Why? The structure is neither an exposed conductive part nor an extraneous conductive part.

 

[James]

exposed conductive part:

Conductive part of equipment which can be touched and which is not normally live, but which can become live under fault conditions.

If it has cables running through it, could it not be considered by the devils advocate to be an exposed conductive part?

i don't know where this furniture could be fitted, but if it were in my house, i think i would prefer it to be bonded in some way.

 

[mainline]

Why?
The units are not exposed conductive parts nor are they extraneous conductive parts.

Would you say the same for a washing machine or a tumble dryer ?

 

[James]

Would you say the same for a washing machine or a tumble dryer ?

My washing machine and tumble drier are bonded to earth via the cpc in the supply cable.
being an appliance they are subject to different standards but the fundamental principals are the same.

if a metal object can become live during a fault, then it should have a return path so the protective device can operate.

I don't understand why you seem to have such an aversion to earthing / bonding / attaching to cpc the body of a metal object that has cables running through it.

 

[DPG]

Big metal box with wires in it and sockets fitted to it? Yes I'd say the metal case should be connected to earth.

 

[pc1966]

Why? The structure is neither an exposed conductive part nor an extraneous conductive part.

As @James has said, it is conductive object with cables running through it. That is only a missing grommet / misplaced screw away from a Line-chassis fault, so like any Class I item I would expect the metalwork to be earthed.

 

[DPG]

My washing machine and tumble drier are bonded to earth via the cpc in the supply cable.
being an appliance they are subject to different standards but the fundamental principals are the same.

if a metal object can become live during a fault, then it should have a return path so the protective device can operate.

I don't understand why you seem to have such an aversion to earthing / bonding / attaching to cpc the body of a metal object that has cables running through it.

I think @mainline is actually agreeing with us isn't he?

 

[James]

yes he is, I was responding to the previous post I think!

 

[loz2754]

I'm not averse to it. I guess I'm playing devil's advocate. An item of furniture is not in itself electrical equipment no matter if it's made of metal. The electrical equipment is the socket, which is to be attached to the furniture. Any cables supplying the socket would be insulated and sheathed. If you fixed a socket to a cable tray, would you then have to earth the cable tray?
Or if a socket was to be affixed to a stainless steel splashback, would you have to earth the splashback? A metal suspended ceiling light fitting is in direct contact with a metal ceiling grid, would you have to earth the grid?

And as an aside, the term "bonded" was used earlier, which is not the correct term for this sort of situation.

Like I said, I'm not averse to the furniture being earthed, but I also think that it's not an automatic assumption that it has to be, based on the definitions found in BS7671. When referring to equipment that could become live under fault conditions, the definition of such equipment does not include furniture. Unless of course the manufacturer of said furniture has designed it in such a way that it is part of the electrical equipment, and explicitly provides an earthing point to facilitate the connection to earth.

 

[loz2754]

Would you say the same for a washing machine or a tumble dryer ?

No I would not, because a washing machine and dryer are items of electrical equipment.

 

[loz2754]

Just a note to add:

If the furniture manufacturer designs electrical items into their products, do they then, in effect, become electrical equipment manufacturers, with all the legal requirements that that would entail, such as declaration of conformity to product standards etc?

 

[mainline]

My washing machine and tumble drier are bonded to earth via the cpc in the supply cable.
being an appliance they are subject to different standards but the fundamental principals are the same.

if a metal object can become live during a fault, then it should have a return path so the protective device can operate.

I don't understand why you seem to have such an aversion to earthing / bonding / attaching to cpc the body of a metal object that has cables running through it.

If you read my post properly you would see that I don’t have an aversion to earthing in fact it’s the opposite