Electrical bonding again - Guide

[Dave Plowman (News)]

But you claim to be a heating engineer. Surely as such you'd have a grasp of all pertinent regulations?

[John Rumm]

Well that is kind of what I was getting at... it appeared that you were singling out oil systems as needing bonding when the supply pipe comes from underground, while at the same time saying that "A typical gas CH system with all its pipework above ground level and not in a steel-framed building will not usually require bonding" even though a metallic supply pipe from underground is a common arrangement there also.

Part 413-02-02 (ii) will mean that most gas systems are by default bonded if only at the gas pipe, allowing for the fact that most boilers gas inlet will be made with a BSP taper thread connection and lots of gas PTFE, so there is a chance that there will be electrical isolation between gas pipe and boiler chassis.

[Andy Wade]

I was using the oil system as an example rather than singling it out, but the said oil pipe is part of the heating system and if it constitutes an ECP (extraneous-conductive-part) then it needs bonding to satisfy 413-02-02 (iv).

The point I was making here is that if the gas installation pipe is bonded as an ECP under item (ii) then the remainder of the GCH system will not need bonding under item (iv) unless it separately constitutes an ECP, e.g. if there is heating pipework underground or in contact with the ground. Whether or not there is electrical continuity through the gas connection is irrelevant, so far as bonding is concerned. In practice, regardless of main bonding, heating system pipework will usually be _earthed_ via the CPCs of boiler, pump, valves, immersion heater, etc. (and also via any bathroom supplementary bonding which ties heating pipework to other CPCs).

Thinking about this more I guess external overflow, pressure relief or condensate drain pipes from boilers could be ECPs in some cases, in which case separate main bonding near the boiler is required.

[Andy Wade]

Quite, some common sense has to apply in the end. I think it's fair game to secure bonding conductors to their associated pipes near to the point of bonding, but fastening long runs of cable to pipes that just happen to be handy at the time isn't good practice.

[Doctor Drivel]

The subject says a "guide". Is that so difficult? The lecky nerds here should be able to give one. E.G.,

- typical semi or detached house

- gas CH with copper pipe

- hot and cold water in copper pipe

- boiler in kitchen

- elec meter near front door or under the stairs

- iron gas mains pipe

- metal water mains pipe

- normal shower in bathroom

- rad in bathroom

Now what needs to be done in that situation? Where will the earth cables run to and from?

Exceptions:

- electric shower

- plastic mains gas pipe

- plastic CH pipe

- plastic hot and cold water pipe

- plastic water mains pipe

- any others people can think of

[John Rumm]

Especially as some pipes get hot. That can eat into your temperature budget on the cable!

[Doctor Drivel]

Please eff off as you are a worthless troll.

[John Rumm]

Ah, ok I see where you were going.

Yup, that is fair enough...

Overflows and condensate drains are usually run in plastic, and the pressure relief will not usually make contact with ground, so this ought not be a problem in most cases.

[Frank Erskine]

But it could work the other way, if it's a cold water pipe!

:-)

[John Rumm]

I would love to see the discussion for that one! "Yup, I have rated this

2.5mm^2 T&E at 45A continuous, but it is ok because this is based on an average flow rate of 25 litres per min of water not exceeding 25 degrees through the cold pipe that it is bonded to with cable ties and thermopath grease!"

[Ed Sirett]

Nice one! The trouble is that at some point the shower feed must leave the cold pipe and go to the CU. You can't get around it by upping the cable size for the last section because of the reg (473-01-01) that states you have to have protective gear where the CSA changes. 8-(

[Ed Sirett]

I find in practice GCH is never explicitly bonded even when a portion is set in the ground floor screed.

However quite often the GCH is supplementary bonded regardless of the location of the boiler. The gas installation pipework is always in practice an excellent earth, although in principle you could have joints that were not good electrical connections due to Boss/Hawk White or the gas grade PTFE.

In practice gas fitters usually bond all the pipes at the boiler regardless of the necessity.

[John Rumm]

You can do without the protective device at the change if the head end device is sized to protect the smaller of the two cables though (i.e. the water cooled one!). Either that or you need to plumb in the CU as well ;-)

[Andy Wade]

Pipes in a screed should of course be thermally insulated and/or ducted or wrapped etc., and aren't too likely to be in good electrical contact with the surrounding screed. In any case the screed should be fairly dry. The resistance to the general mass of earth is likely to be too high to deliver a dangerous shock current, however high the touch voltage to the local earthing and bonding, so I can't see to much of a case to be made for explicit main bonding in these circumstances.

Yes, it's always puzzled me why they do that. (Unless the boiler's in a bath or shower room, of course, but the wisdom of doing that could open another can of worms...) You often see it in airing cupboards too.

[Andy Wade]

Easily solved: just route the incoming CW main via the CU enclosure! This will be handy for doing the main water bond too - you could solder the main earth bar straight onto the water pipe.

[Doctor Drivel]

They do it because even electricians don't understand bonding, so they do all in sight.

[Ed Sirett]

Try arguing with the CORGI inspector who takes a simplistic view of the regs...