Outside Metal Enclosure Bonding.

[OuterSpaceHopper]

I have an outside metal box housing an ip rated double gang socket, which isn't earth bonded.
Is this a problem?

Whilst I am chasing walls to add network and TV cables, I was going to add a network connection and maybe an alarm cable to this box(with another network connection on the inside), so that any future additions to the back of the house(CCTV or alarm accessories) can be added without drilling any walls.

I noticed that the box isn't earth bonded with anything.
As an 'extraneous metal part', should this box be bonded with 10mm earth cable? What is typical?
There is RCD protection and the house is on a PME supply.

Next year I plan to get a Electrical Periodic Inspection Report done on the house before I decorate, just in case there is anything wrong in that department before I do any serious decorating.
Is this unbonded box likely to be a fail on the report and require remedial action?
I could bond the box(or get an electrician in to bond the box if I am not allowed to do this myself) while the chase is open.

So, are metal enclosures typically be bonded outside? It is safe? Will I get a non compliance if it is not?
Thanks!

 

[John D v2.0]

If it really is an extraneous metal part it is supposed to be bonded. However assuming it doesn't have a low resistance back to earth, it's not. And if it's not an essential part of the socket, and just happens to be there (ie all live parts inside are insulated and sheathed or in an enclosure) then it doesn't need to be earthed as a metal enclosure.
If it's outside, it's not really on the equipotential zone anyway, so if you're pme, connecting it back to the main earning terminal would be counterproductive in many ways. In that situation you should ideally only use class II equipment outside or put an earthing rod in the ground if you're really worried and ensure appropriate RCD protection.

 

[OuterSpaceHopper]

Thanks for your fast reply John, would a steel conduit back to the earthed internal back box count as a low resistance back to earth?

What are my choices if that low resistance to earth makes it count as an extraneous metal part? Change the steel conduit for plastic?
Is plastic conduit allowed for wires going through the cavity wall?

With IP rated socket and class 2 equipment, I don't think I'm worried enough pay for and maintain an earth rod, although I confess, I don't really understand the risk!

Is the concern a lost neutral on the supply? that doesn't seem very likely with underground lines(rather than overhead)?

Should I be concerned?

I guess I am more trying to make sure I can get compliance and pass a PIR without great expense in the future(the decorating is going to cost enough!) i.e. without doing work twice, or paying for the wall to be chased again!

 

[John D v2.0]

No the conduit wouldn't count, the path to earth has to be independent of other things you've connected to it that are already connected to the MET. So your water pipe If metal has it's own connection to earth independent of your met. That means if someone shorts your section of the water pipe to a live wire, you want to be shorting that pipe to all the earthed metal in your house.
You've actually probably got the situation where putting a metal teaspoon on the washing machine casing doesn't mean the teaspoon needs bonding. Assuming the box isn't screwed into a metal post mounted in the ground, or feeding a metal conduit to an outbuilding.
The concern is that the equipotential may be different from local earth. Lost or high resistance neutral would certainly be catastrophic, but it's possible for the earth potential itself to vary, especially near electric train lines etc.

 

[bernardgreen]

Fir best safety, least hazard the box should be kept at the same potential as the ground around it. So earthing to an earth rod is the least hazardous option.

Connecting ( bonding ) it to the Main Earth Teminal ( MET ) would only create a hazard when, as JohnDv2.0 has mentioned, a disconnected Neutral in the local supply network allows or forces the PME "earth" to be raised above true ground potential.

 

[flameport]

I have an outside metal box housing an ip rated double gang socket, which isn't earth bonded.
Is this a problem?

No, and connecting it to the earth for the electrical installation is not required or desirable.
Neither does it need an earth rod, metal conduit or anything else.

It's a metal box on the wall, unrelated to the electrical installation. No different to a metal box to put mail in, a metal house number or a metal ring to attach dog leads to.

 

[JohnW2]

Fir best safety, least hazard the box should be kept at the same potential as the ground around it. So earthing to an earth rod is the least hazardous option.

This metal box is just a metal box, unrelated to the electrical installation, and has no more need to be earthed than does any other metal object in the garden.

Edit: my goodness. I've just noticed flameport's previous message (hadn't noticed it before), of which mine is almost a word-for-word copy. My apologies.

Kind Regards, John

 

[OuterSpaceHopper]

Thanks for all your feedback folks, I'm going to read up on the lost neutral problem to try to understand it.

I was thinking that the earth was there to protect against faults and that a 10mm bonding was required to make sure of a low resistance path to trip the fuse.


I was going to use another metal conduit to pass my network and alarm cable through. The existing spur from the ring main wire looks like it goes through metal conduit with a brass connector to the internal mains back box and external metal box. So I think this does mean that the external box IS currently connected to the house earth via the circuit cpc.

Is this bad?

Whilst I'm removing the box to add the network cable, should I also replace(or get replaced) the metal conduit for the electrical spur with PVC plastic conduit?

Can you even pass a mains spur through the brick cavity in plastic?
I was going to use metal conduit for the network wire to protect it from rodents or falling masonry where it crosses the cavities.
Maybe I should use metal conduit, but with plastic adapters at the end rather than brass?

 

[EFLImpudence]

I was thinking that the earth was there to protect against faults and that a 10mm bonding was required to make sure of a low resistance path to trip the fuse.

It's NOT an earth and that is not the case. Bonding is to equalise potential in the event of a fault.
Your box has no potential on its own so introducing one is introducing a hazard.

I was going to use another metal conduit to pass my network and alarm cable through. The existing spur from the ring main wire looks like it goes through metal conduit with a brass connector to the internal mains back box and external metal box. So I think this does mean that the external box IS currently connected to the house earth via the circuit cpc.

Is this bad?

It would be better if it were not earthed but as(if) it is then it is already earthed.

Whilst I'm removing the box to add the network cable, should I also replace(or get replaced) the metal conduit for the electrical spur with PVC plastic conduit?

It would be better to electrically isolate the box.

Can you even pass a mains spur through the brick cavity in plastic?

Of course.

I was going to use metal conduit for the network wire to protect it from rodents or falling masonry where it crosses the cavities.
Maybe I should use metal conduit, but with plastic adapters at the end rather than brass?

It's up to you.
It would be better.

Why not use metal conduit, not connected to anything, just to cross the cavity with a plastic conduit inside it.

 

[OuterSpaceHopper]

Thanks for the feedback folks, after all your advice, I've used metal conduit with plastic adapters on the end without adding bonding to the box. I think I was over thinking it!
Thanks again.

 

[bernardgreen]

Your box has no potential on its own so introducing one is introducing a hazard.

One could consider bonding it to the Ground to ensure that it is always equipotential to the Ground on which people touching the box will be standing. In normal ( no fault ) conditions the box will not have any potential from inside the building, but in fault conditions it may be exporting a potential from inside that is different and thus hazardous to a person standing on the Ground and touching the box.

 

[OuterSpaceHopper]

Thanks Bernard, I can see this is the belt and braces approach, in your experience, is this custom and practice for this kind of outside installation? If I install a rod, might I also then be saddled with the future costs of maintenance and testing of the earth rod?
I've heard that they are not always reliable through the seasons, and I then might create a future problem to an original problem that didn't need a solution?
I appreciate everybody's input, as it is interesting to understand the issues!

 

[JohnW2]

Thanks Bernard, I can see this is the belt and braces approach ...

You may regard it as 'belt and braces' but that might be a bit misleading - since, as I and others have said, it's actually a 'swings and roundabouts' situation, about which judgements and opinions will vary.

Earthing the box reduces the risk of shock in the situation bernard describes. However, earthing the box also increases the risk of shock if someone (who is reasonably well insulated from the ground - eg wearing rubber-soled shoes or wellies etc.) somehow comes in contact with something 'live' at the same time as touching the box. It therefore comes down to a question of which of the two scenarios one considers more likely - and opinions about that will vary.

Kind Regards, John

 

[EFLImpudence]

That opinions may vary does not alter the correct view.

No, and connecting it to the earth for the electrical installation is not required or desirable.
Neither does it need an earth rod, metal conduit or anything else.

It's a metal box on the wall, unrelated to the electrical installation. No different to a metal box to put mail in, a metal house number or a metal ring to attach dog leads to.

 

[JohnW2]

That opinions may vary does not alter the correct view.

The problem with that is surely that opinions about what is "the correct view" will, themselves, vary?

Kind Regards, John