bonding/ earthing confusion

[tom.harrigan]

I don't have any metal pipework coming into my house. The water supply is plastic - followed by a short length of copper, then speedfit. All waste pipes are plastic. The only metallic object that could conceivably be in contact with "earth" is my stove (which does my central heating) which is sat on the ground, arguably making the bathroom towel rails extraneous conductive parts.

As far as I can tell, I have nothing to main bond, which means all my main bonding is in place. The electrician disagrees. He thinks that as I have no bonding, it is not in place. He also wants to bond the short length of copper pipe attached to the plastic water supply for some reason.

It seems that whether or not main bonding is regarded as being in place has implications for supplementary bonding. Due to the configuration of the bathrooms, I think the easiest and most sensible way to supplementary bond in my situation is to run a cable from the towel rails to the consumer unit, letting the CPCs do the rest. In fact I don't think even that is necessary (bathrooms are on RCBOs), but don't mind a bit of belt and braces. I don't see the point in doing any more, but the electrician seems to want to put a lot more in!

The electrician says his mate who is going to "Part P" the work for him insists on all the bonding. So who's right - the sparky or me?

[YAPH]

Are you all electric then? No gas or oil?

If your electrician can't certify the work himself then he's not a member of a competent persons scheme which makes one wonder how he operates as a professional electrician, if that's what he calls himself. Sounds as if you need to be talking to the mate of his who's actually going to be certifying the work.

There does seem to be a breed of electricians who bond everything that doesn't move. Attempting to bond what is practically an all-plastics water supply does seem ludicrous, though probably to the letter of the law, and I'd expect a sensible sparks to omit it with perhaps a note on the deviations section of the installation certificate.

As for supplementary bonding that doesn't need to be taken to the main earthing point: it just needs to connect extraneous conductive metalwork in the special location and the cpcs of all electrical services entering the location.

If all your pipework in the special location is plastic then you can make it an earth-free rather than equipotentially-bonded zone: this is regarded by the IET as preferable.

[BruceB]

Probably you are right. The problem with many electricians is they want a proforma way of tackling problems. They do not have the training as engineers to understand what the regulations are trying to achieve and why.

Two points:

- The OSG on page 27 lists *metal central heating and air conditioning systems* as candidates for main protective bonding if they are extraneous. Note they may be extraneous because they are in contact with enough of a damp building they they can introduce a potential. Normally only applies in old houses.

- There is a test he can do to identify whether a part is extraneous and that is to measure its resistance to earth (the MET). My recollection is 23 kOhms. This factsheet from ECA gives a value of 25 k Ohms

ask him to test whether it is extraneous.

Be impressed if you electrician does already know and understand what is in that fact sheet. Many just take the proforma bond everything option.

Regards Bruce

[Tim S]

[e-mail] coughed up some electrons that declared:

Hi

This covers some of the issues:

(admittedly pre-17th Edition - but the spirit is similar - supplimentary bonding was changed a bit in the 17th)

The On-Site Guide,17th says on page 29:

"There is no requirement to main bond an incoming service where the incoming service pipe and the pipework within the installation are both plastic. Where there is a plastic incoming service and a metal installation within the premises, main bonding is recommended unless it has been confirmed that any metal pipework within the building is not introducing an earth potential. ..."

So that is the question that should be asked of the copper pipe.

The usual test for that is:

1) Run a flying wire back to the MET (main earth terminal) and have a Megger or similar tester at the ready; 2) Test on low voltage continuity range between the wire to the MET and the exposed metalwork in question. If the reading is off scale (high ohms), then repeat the test with the 500V insulation test range. 3) If the reading from (2) is >23k Ohms, then the metal is an extraneous conductive part, capable of introducing an earth potential into the room - so bond. could do a rough test with a multimeter, but the final test is best done with a proper test instrument.

Has his "mate" seen the installation? If not, it would be hard for him to have an informed opinion as whether to bond depends on what's there.

Cheers

Tim

[Andy Wade]

No water main bonding is required then - see page 29 of the OSG (red

17th ed. version). Clearly there's no point whatever in bonding the short metal section, provided it is to remain short and electrically floating. Metal gas or oil pipework in the premises might need main bonding, but you haven't mentioned any.

Where is this stove - outside? If it's in contact with the ground (soil or damp concrete, etc.) then any metal pipework from it which enters the house will need main bonding at or near the point(s) of entry.

If none is required then the main bonding requirements are satisfied and bathroom supplementary bonding can be omitted, provided all bathroom circuits are RCD protected.

Well it would do no harm, and covers his back if additional metalwork is teed-in in the future.

No, that would constitute main bonding and should be applied near the point of entry of anything which might import an external earth potential. Any supplementary bonding which may be required is purely local (although it will be connected to the MET via the CPC(s) of any circuit(s) whose exposed-conductive-parts are included in the bonding.

From what you've said, bathroom supplementary bonding would appear to be unnecessary (but see above re possible need for main bonding incoming metal pipes from an external stove). Page 31 in the OSG refers.

How unusual :~)

[ARWadsworth]

Then your water does not need bonding.

A gas or oil supply will need bonding

That will to make money.

If all the electrical circuits that enter the bathroom have 30mA RCD protection then there is no need for any supplementary bonding.

So he is not an electrician then

Adam

[Tim S]

Tim S coughed up some electrons that declared:

^^^ LESS THAN 23k

I read this over once - must refrain from posting before coffee worked...

[tom.harrigan]

Just electricity and wood I'm afraid...

It just so happens that the CU is a convenient place to connect the supplementary bonding to the CPCs. The central heating is in copper and comes up from below. A 4mm wire can easily be run under the floor from the towel rail to the CU. All the electricity enters the bathrooms from above, making bonding less convenient. Is connecting at the CU OK?

The central heating is copper, so the towel rail is probably "extraneous". The trouble is, the electrician is of the opinion that as there is no bonding, supplementary bonding is required even though the bathrooms are on RCBOs (as is everything). I think that all bonding that should be in place is in place (ie none), so supplementary bonding is not required.