Spellbound by bonding

[Softus]

Earth bonding does my head in. One moment I think I've understood, the next I'm reading some MIs that say I must install supplementary bonding where I'd least expect it.

Please could the people who know comment on the following? Thanks.

The status quo was a 1st floor flat with an immersion heated copper cylinder (Econony 7) with two elements. The mains water entered the flat in a shared service cupboard (ground floor of course), changed to copper, through a brass stop cock, then up to the flat, appeared in the airing cupboard in copper, through another brass stop cock, then converted to plastic pipework and distributed around the flat (except for one 15mm copper feed to an electric shower).

The hot water from the cylinder was in copper, but like the cold it was distributed in plastic pipework.

Thus far, I can't see any external conductive parts entering the bathroom. BTW, there's no sign of any bonding in the airing cupboard or under the bath.

Then I installed a pump in the airing cupboard - single impeller on the hot output from the cylinder. The copper pipe was cut and the the coupling to the pump were all plastic. The electrical supply to the pump is via a new unswitched fused connection unit with a 3 Amp fuse installed - this FCU is a spur off the peak-time IH supply, which itself is fed in 2.5 mm² T&E and protected by a B16 MCB.

The pump MIs say that supplementary bonding must be applied when a copper pipe is breached by the installation, leading me to ask the following questions:

1. Is this correct in this scenario?
2. Is this correct in any scenario?

 

[Spark123]

I suppose it could be getting at bridging a break in the electrical continuity of the pipework as copper pipe can be used as part of the supplementary bonding providing it is electrically sound and continuous.
To be pedantic, supplementary bonding is different to earth bonding. Supplementary bonding is equipotential bonding i.e. its primary purpose is to keep all items in that location at the same potential.

 

[Softus]

Thanks. I take your point about my terminology - I'd prefer to get it right.

This thing of using copper pipe as part of the supplementary bonding - surely that's a can of worms, because it requires plumbers to understand what an electrician might have done with the supp. bonding, so that bonding is reviewed in the light of any pipework modifications.

If nobody encourages that understanding, let alone enforces it, doesn't that make it bad practice to use copper pipes for that purpose?

Anyhow, back to my problem, and a case in point - is there a way that I can tell whether or not the work I've done has interrupted a path of supplementary bonding?

 

[Pensdown]

I can understand your dilemma.

As Spark123 said, “equipotential bonding i.e. its primary purpose is to keep all items in that location at the same potential”. So if you are installing a pump with plastic connectors in the pipework before the bonding then you do not need to cross bond the inlet and outlet. However, if you installing it in a location where the pipework enters the bathroom and after the bonding point then you may need to cross bond the inlet and outlet.

For example, if the equipotential bonding has been carried out in the loft space say in a bungalow and you install the pump in the loft after the bonding point then you would need to cross bond the inlet and outlet or move the existing bonding point to the outlet of the pump.

The copper pipework within buildings is never used as a protective conductor so there is no need to worry about plastic fitting etc anywhere, even within a bathroom as long as any copper parts are bonded together in the normal way where they enter the room.

The same applies to the main water bonding conductor. If you have 1mtr of copper pipe coming into the building and then plastic throughout, as long as the 1mtr section is bonded to the MET there are no further requirement for cross bonding. If you installed a plastic T before the main bonding point then you would need to move the bonding point to before the T, onto the section that enters the building.

 

[Softus]

Thanks Pensdown; that's immensely helpful.

Given that I can't see any bonding anywhere in the flat, I think what I have to do, given that the flat is being let out, is report to the letting agent that I believe the bonding (nice and vague) to be inadequate, and that the routine PIR should (or should have) picked it up.

Thanks again.