Another couple of main and supplementary bonding questions!

[FX]

The potential the water brings in may be from the pipe work in adjacent houses. Normally this should be ground but if one house has an earth fault such that its pipe work in not at ground potential then other properties could be affected via the water in the network pipes.

That being the case, then ALL taps and pieces of copper pipe (I'm thinking tails to taps and rads) even though they may be supplied by a complete plastic piping system should also be earthed back to the MET. How many people do this?
Also are the main water supply pipes not earthed by the supplier?

 

[Spark123]

They may be earthed if they are burried in the ground, but the intent of protective bonding is to equipotentially bond all exposed and extraneous conductive parts together so a dangerous voltage between them in your equipotential zone cannot exist.
It is debatable wether a short piece of copper needs bonded, if in doubt measure the resistance between it and the MET. If it is above 22k ohms then it doesn't need bonding.

 

[FX]

It would appear that it is down to the designer to determine if it is necessary. Although the OSG (17th) section 4.4 does offer some advice:

"4.4 Main protective bonding of plastic services:
There is no requirement to main bond an incoming service where the incoming service pipe and the pipework within the installation are both of plastic.
Where there is a plastic incoming service and a metal service 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"

The NIC suggest that it could actually be hazardous to do so as it could actually introduce a potential into the building.
A suggestion is to do an IR between copper pipe and an earth point back to MET. If the reading is greater than 0.02Mohms(22000ohms) then the pipe can be considered not to be an extraneous conductive part and does not need bonding.

 

[bernardgreen]

Snipped

"Where there is a plastic incoming service and a metal service 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"

The NIC suggest that it could actually be hazardous to do so as it could actually introduce a potential into the building.

WHAT ?

RANT WARNING

That suggests that introducing a possible earth potential into the building via pipes may be hazarous.

All things that can be touched inside the building should be at earth potential ( or double insulated from both Live and Neutral ) so touching something in the building and at the same time touching the pipe with the earth potential imported from outside the building should not give a shock as both are at earth.

Or does the hazard come from the MET ( safety earth ) not being assured to be earth potential ( bouncing neutral in a PME system will lift the MET away from earth potential ) so introducing a true earth would mean different "earth" potentials inside the building.

OK so far so good,

But what happens if the MET is NOT at earth potential and Mrs Smith while holding the metal electric kettle at MET ( PME neutral ) potential touches say a metal window frame that is at true earth potential. Minor shock maybe but enough for her to drop the kettle of scolding water.

Or a child barefooted on wet grass grabs with wet hands the outside water tap that is bonded to the MET.

 

[NotHimAgain]

This question has been answered many times. Paul Cook addressed it pre 2004 see here:
http://www.theiet.org/publishing/wiring-regulations/mag/pre-2004/
and open the PDF on earthing plastic pipes.
Note the resistance of tap water in a 15 mm pipe. The Electrical Research Association (ERA) carried out extensive testing to produce those figures.

 

[FX]

Bernardgreen...
You'd need to ask the NIC why they consider it would be hazardous.

However, do you ensure that even if the pipe work in a building is all plastic except for the taps that they are all connected to the MET? If not, then you are working against your own logic.

The answer I think lies in the end of the statement ".... unless it has been confirmed that any metal pipework within the building is not introducing an earth potential"[/quote]

 

[RF Lighting]

Do you have a link to where the NICEIC have said it could be hazardous?

 

[FX]

Do you have a link to where the NICEIC have said it could be hazardous?

Not directly, but see here

 

[Spark123]

The question refers to a plastic service with a metal installation, wether or not you'd consider a short piece of copper as in the OP as an extraneous conductive part is another matter.
I have heard of people purposely installing a short piece of copper after the stop tap on an otherwise all plastic installation so they can bond to it!
It is also debatable wether or not supplementary bonding short pieces of copper in a bathroom is making it any safer, which is probably where you have heard about it.

 

[FX]

Quite Spark123,
I'm not just thinking of short pieces of copper pipe supplementary bonded in a bathroom. It could equally apply to a plastic installation with metal taps in a kitchen, utility or cloakroom. None of which are required to be bonded and I suspect most people don't as they are considered isolated. I can see no reason to consider an isolated piece of copper tube as any different.

 

[Spark123]

It depends where the pipework runs which may be out of your control too, if it goes under the floor (in contact with mother earth), plastered into a wall or is in contact with an electrical appliance it may be considered to be an ecp.

 

[RF Lighting]

Do you have a link to where the NICEIC have said it could be hazardous?

Not directly, but see here

Oh right, so this must be another one of those regulations that every one reckons the NICEIC have invented then, but when it comes down to it, it turns out that they infact don't invent their own regulations, as the myths would have you beleive.