cross bonding

[dag1167]

We recently had a survey for the installation of solar power. One of the questions that was asked was 'where are the water pipe work and gas main earthed'? The gas pipe was no problem as it was clear to see. However I could not find an earth connection to the water incomer. Two years ago the house was completly rewired and so I contacted the builder. He stated that as the water main entering the house is plastic then all that is needed is for the water pipes to be 'cross bonded'. My question is where should this be done as I can find no earth connections to any of the pipes throughout the house.
Thanks Dave

 

[ericmark]

With plastic pipes there is no need for an earth. And where most of the pipework is plastic and metal is only used where visible again no need for an earth.

But where pipes or any other metal work connect different rooms then it should be bonded to ensure an electric fault in one room does not make something live in another room. The problem is this cross bonding does not have to be visible. In most cases it is done where the incoming plastic changes to metal. And it connects the electric earth to the rest of the metal work in the house to ensure everything is at the same voltage.

However with the use of RCD's the requirement is not the same. So with all circuits protected by 30ma RCD's maybe you don't need any bonding. In the main rather than wade through regulations and ensuring everything complies it is easier just to cross bond anyway.

My house has a earth wire going to main incomer but I know under floor boards its all plastic pipes so no real need. But saves any argument so earth wire is still there.

 

[dingbat]

One of the questions that was asked was 'where are the water pipe work and gas main earthed'?

They are earthed via their direct contact with the mass of earth. If that is the terminology used, then your surveyor does not understand what he is asking.

He stated that as the water main entering the house is plastic then all that is needed is for the water pipes to be 'cross bonded'.

This meaningless phrase is a favourite of those who do not understand earthing and bonding.

This appears to include ericmark.

...and riveralt.

[Edited to add "... and riveralt"]

 

[westie101]

So with all circuits protected by 30ma RCD's maybe you don't need any bonding.

If it is a PME supply and not cross bonded the DNO has the right and duty not to connect it or if connected and discovered to disconnect it, whatever any theory is, IEE regs are, it is contained within the laws that govern the DNOs

 

[JohnW2]

If it is a PME supply and not cross bonded the DNO has the right and duty not to connect it or if connected and discovered to disconnect it, whatever any theory is, IEE regs are, it is contained within the laws that govern the DNOs

What sort of 'cross bonding' are you referring to? If,as per this thread, the water supply enters the property as plastic, there would not seem to be any BS7671 requirement for main bonding of the incoming supply (no 'extraneous conductice part') and, depending on the situation, quite possibly no BS7671 requirement for any other bonding. Are you saying that there are nevertheless other regulations which always require some sort of bonding with a PME supply (and, if so, bonding of what to what?).

Kind Regards, John.

 

[dag1167]

Let's explain exactly what I have. The water main underground is plastic. This changes to copper just above ground level outside the property. The pipe work then enters the house in copper. ALL pipe work inside is copper. The only earth connections that I can see are from the consumer unit and the gas main to the electrical incomer. There are no other earth connections. There are no connection to the rising water main.
However if I connect a volt meter between a live connection and the copper pipe I get a reading of approx. 237v which does indicate that the copper pipe is at ground potential.

 

[JohnW2]

However if I connect a volt meter between a live connection and the copper pipe I get a reading of approx. 237v which does indicate that the copper pipe is at ground potential.

That's no surprise. If you have a boiler with an electricity supply, an immersion heater or anything else electrical in your plumbing system, your copper pipework will be connected via the CPC (earth wires) of electrical cables to the 'earth' of your house's electrical system. However, if there is a requirement for bonding, that connection via CPCs will probably not be beefy enough to satisfy the requirements.

Kind Regards, John.

 

[bernardgreen]

However if I connect a volt meter between a live connection and the copper pipe I get a reading of approx. 237v which does indicate that the copper pipe is at ground potential.

It only indicates that the pipework is 237 volts different to the live conductor. The pipe work could be 474 volts different from ground. Very unlikely to be that but that is all your test can tell you.

To test what potential there is on pipe work you need to measure between the pipe work and a known good ground such as a ground rod in damp soil.

Also the pipe in your test could be at or close to ground potential but not connected to ground ( or earth ) in a way that would carry fault currents. It could be a very high resistance between pipe and ground. As the meter does not require any significant current to flow when measuring voltage there is no voltage drop. If there was a live to pipe work fault then the fault current flowing through the high resistance ( pipe to earth ) would create a voltage drop between pipe and earth raising the pipe up to a high voltage.

To check pipe work is correctley and safely "earthed" you need to measure the resistance ( impedence ) between pipe and a known good earth. The test needs to be made with a test current of at least 200 mA and not the few micro amps that digital multi-meters test with.

 

[westie101]

What sort of 'cross bonding' are you referring to? If,as per this thread, the water supply enters the property as plastic, there would not seem to be any BS7671 requirement for main bonding of the incoming supply (no 'extraneous conductice part') and, depending on the situation, quite possibly no BS7671 requirement for any other bonding. Are you saying that there are nevertheless other regulations which always require some sort of bonding with a PME supply (and, if so, bonding of what to what?).

The initial post records the water sup[ply as plastic which is accepted. The implications of that post are that the internal pipework is metal.
The requirement to bond (using whatever description you want) to that pipe work is clear.
In electrical terms is is an equipotential bond to comply with the appropriate regulations.

You will see that the initial poster has confirmed that internal pipework is copper!

 

[westie101]

Obviously if all the interior pipework was plastic any bond would be superfluous!

 

[JohnW2]

[The implications of that post are that the internal pipework is metal.
The requirement to bond (using whatever description you want) to that pipe work is clear.
In electrical terms is is an equipotential bond to comply with the appropriate regulations.
You will see that the initial poster has confirmed that internal pipework is copper!

One problem here is that, at least in terms of BS7671, 'whatever description you want' is more than just a question of vocabulary - since, if it were regarded as 'main bonding', it would usually require a much fatter cable than if it were 'supplementary bonding' installed to ensure equipoential conditions within one part (e.g. room) of the building.

If the supply pipe and much of the internal pipework were plastic, but there were some sections of metal pipework in the house, would you expect those bits of metal pipe to have BS7671 'main-bonding'-style bonding (i.e. usually at least 10mm²) back to the MET?

Kind Regards, John.

 

[riveralt]

If the supply pipe and much of the internal pipework were plastic, but there were some sections of metal pipework in the house, would you expect those bits of metal pipe to have BS7671 'main-bonding'-style bonding (i.e. usually at least 10mm²) back to the MET?
Kind Regards, John.

Yes.

 

[JohnW2]

If the supply pipe and much of the internal pipework were plastic, but there were some sections of metal pipework in the house, would you expect those bits of metal pipe to have BS7671 'main-bonding'-style bonding (i.e. usually at least 10mm²) back to the MET?

Yes.

Interesting. If there were multiple (electrically separate) bits of wholly-internal metal pipes in amongst the plastic ones (not unusual when people start gradually replacing copper with plastic), would you main bond each and every one of them?

Kind Regards, John.

 

[westie101]

The point of all this is to take a step back and understand the basic principles of why bonding is required (whatever name you wish to put to it), it's not about blindly following a book but working out the implications of fitting it or not.
With PME, as I keep commenting, it is all about the Faraday Cage.
Understand how that works and you will "get it"
Also understanding the reasons for the choices of conductor size and why they did actually change from 6mm2.

A lot of the parts of the wiring regs are written to allow compliance with other legislation and are drive by external factors (the supply network) rather than what occurs in the house.

 

[riveralt]

Interesting. If there were multiple (electrically separate) bits of wholly-internal metal pipes in amongst the plastic ones (not unusual when people start gradually replacing copper with plastic), would you main bond each and every one of them?
Kind Regards, John.

Yes.
But.
What is the tipping point at which you do not need to bond?

For example I was doing some work in a house recently that had no main protective bonding to either gas or water. No problem bonding the gas but the incoming plastic water supply went to the brass stop cock and then to plastic inside the house. However there was a single piece of 10m of copper pipe that connected in between the plastic incoming pipes and the plastic pipes to the sinks, baths etc.
I bonded the 10m of copper pipe with 10mm earth cable . Right or wrong?