17th edition earth bonding - plastic and copper pipe

[David Robinson]

Our house is plumbed in a combination of copper and plastic. Mostly plastic, though all visible radiator pipes are copper (or chrome on copper), and all pipe work near the boiler and around the airing cupboard is copper.

There's some equipotential bonding (but no earth) near the boiler, and main bonding to earth under the stairs (which isn't where the gas supply enters the house at all - might have been once upon a time). I've adder proper 10mm2 earth bonding to gas and water where they enter the house, and 16mm earth to the CU.

Given that everything is now run from RCDs, I understand the hard requirement for equipotential bonding in the bathroom has gone from the 17th edition. I've also read on here in the past that with a few metres of plastic pipe feeding a tap on a plastic bath, it's a bit pointless bonding the tap anyway.

I'm wondering about the chunks of copper in the airing cupboard and near the boiler, the copper pipes to the rads (which sometimes have electrical pipes near-ish to them under the floorboards), and the few reasonable lengths of copper feeding one set of bathroom taps. Shall I equipotential-bond any of this? Shall I earth any of this? All of this?

I want to meet the current regs, and I want to be safe in the event of a leak, fault, etc.

Cheers, David.

[dom]

In my situation (plastic water main, mostly plastic plumbing, no gas in my village, everything on RCD/RCBO) there was no requirement for cross-bonding, so only the TN-C-S supply had a 16mm2 main bond.

However I have an oil-fired Rayburn, outside (plastic) oil tank, a fairly-complex wiring centre set-up around a thermal store system, which is all plumbed in copper between the boiler, thermal store, pumps (3 of them) and the valves (2).

I considered it prudent, even though not required, and everything has it's own earth, to have a 10mm2 earth bond where the copper oil pipe emerges form the building and to the rayburn chasis - in that low possibility that something lost an earth at some future point through the multiple connections in the wiring centre.

I'd recommend the same to you. If a mains-powered something could conceivably loose an earth, and is also connected to a network of copper pipe - add in an earth bond. For isolated copper pipe, don't.

[Dave Osborne]

Your oil pipe is required to be main protective bonded.

17th Edition 411.3.1.2

"In each installation main protective bonding conductors complying with Chapter 54 shall connect to the main earthing terminal extraneous-conductive parts including the following:

(i) water installation pipes (ii) Gas installation pipes (iii) Other installation pipework and ducting (iv) Central heating and air conditioning systems (v) Exposed metallic structural parts of the building"

Oil pipes are specifically mentioned in the On-Site Guide, section 4.2, where part (iii), above is paraphrased as:

"(iii) other metal installation pipes (including oil and gas supply pipes) and ducting"

HTH

[John Rumm]

Indeed. The basic question to ask yourself, is could the bit of exposed metalwork introduce a potential (i.e. voltage) into the room - even if that potential is "earth". Obviously copper tails fed by plastic can't. A rising cold main in copper that emerges from the ground however would be well connected to earth, and hence could.

Well since it sounds like you meet the 17th edition requirement of all circuits used in the "special location" are RCD protected with a trip of

30mA or less, then there is no need for bonding anyway. However there is no reason to remove it if its there. There is no need to earth EQ bonding either (although full EQ bonding that includes the CPC of any circuit used in the room will obviously have a connection to the MET via the CPC in the cables)

Prior to the 17th edition, I would say focus on the reason for it rather than the rules, and you will usually get the right answer. When looking at metalwork in a bathroom for example, ask yourself is it connected to earth, or could it become live due to a fault elsewhere in the house? If the answer to either of those is yes, then you would normally bond them locally since otherwise you have no guarantee that under fault conditions there could not exist a large potential difference between bits of touchable metalwork. So metal pipes connected to incoming services, or that snake all over the house, are obvious candidates. Things in isolation like metal baths, or metal pipe stubs connecting rads to plastic plumbing etc can't really be a threat.

[David Robinson]

[snip]

the radiators are fine. I think I'm going to connect an earth to the copper+chrome pipes feeding the towel radiator - "belt and braces" in case its own electrical heater (with its own earth) goes wrong. Other than that, there's nothing touchable that could take its own potential into the en suite, so even under 16th ed I don't think I'd be adding anything else. (did people supplemental bond the earth going to ceiling lights?!)

I'm still left concerned by the lengths of copper heating pipe that go from room to room (including under floors, with electrical cables nearby), but have no electrical connection to earth because there's a run of plastic pipe between them and the main bonding. I've seen instructions to bridge plastic connections to maintain earth continuity, but what I have aren't just small plastic sections. Does it make sense to run a separate earth to these, where exposed?

Cheers, David.

[John Rumm]

The CPC in the supply to the electrical element can legitimately be used as a bonding conductor - so no additional wire is required.

The other thing to keep in mind, is you need two different potentials to get a shock risk. So a towel rail at 240V and everything else just floating is not actually that risky since you will be hard pushed to find an earth path to conduct any current to!

Yup, earth on all circuits entering the room (so typically lights, electric shower, and perhaps a ring circuit if its feeding something like a towel rail via a switched FCU.

In your case with 17th edition style RCD protection, not really.