Cant find the water earth bonding....but it's there??

[Deleted User]

Ectophile said:

Have you checked what your incoming water pipe is made of?  If it's plastic, then the bonding is pointless anyway.

Not true.
As Risteard pointed out, people are mistaken in calling this earth bonding, because its purpose is not to provide an earth for the installation. The true purpose is evident from its correct name: equipotential bonding, which is there to ensure that exposed metalwork in the house is always at the same potential regardless of whether it is connected to earth via the electrical supply or any other means.

In  PME systems (also known as TN-C-S) commonly used in the UK, the connection to earth for the electrical system is provided at the substation, where the star point of the transformer is connected to an earth electrode. In the domestic installation, the 'earth' (properly the CPC or circuit protective conductor) is then connected to the neutral line of the service cable at the meter cabinet, which then returns it to the substation. The advantages of this are good overall balance of cost and safety, and low earth impedance, but it has a problem, as do all earthing systems.

With PME, if a fault develops in the neutral conductor of the service cable, all the exposed metal casings of the electrical appliances become live, because the casings are connected to neutral via the CPC, and the neutral is connected to live via the appliance circuits. Now if anyone touches both a live appliance and any other object which is earthed, they will get electrocuted, because the RCD won't provide protection, since the earth leak is on the supply side of it. However, you only get electrocuted by touching two objects that are at different potentials, and this is the reason for equipotential bonding: under this fault condition the bonding ensures that all exposed metalwork remains at the same potential.

Two circumstances now arise, one when the pipes are connected to earth, and one when they aren't. In the latter case, such as when the water main is plastic, the bonding won't stop the appliances becoming live, but since all the other metalwork in the house also becomes live, it is difficult to find two objects at different potentials, and hence more difficult to get electrocuted. The other scenario when pipes are earthed ensures that the metalwork in the house all remains at earth potential, but that has scope for another problem: during a neutral fault, any conductor bonding the CPC to earth will carry all the current drawn from the supply, and the installation will carry on working. An unlikely but possible scenario is having a fault which is common to many houses, but only one house has pipes that are earthed: now one bonding wire overheats because it's carrying the supply current for the whole street.

fenwick458

warwicktate said:

Thanks for all your help. 

Next question. We are going to sell the house once the kitchen is sorted. Would it be a good idea to get a new EICR sorted before sale (considering my BIL can get it up to spec very cheaply), or just let the potential buyer sort all that out?

it adds value if the electrical installation is up to date, so yes do that.
don't be surprised if potential buyers instruct "their electrican" to do an EICR before buying though, that happens sometimes too