Part of the BS7671 Electrical Regulations states that an electrical system must be bonded to the non-electrical services within an installation. That is, the water, gas, or oil services or to structural steelwork if present. The main reason for this is so that a potential cannot develop between extraneous metal parts of an installation. For example, if a live conductor was to touch a cold water pipe; Without the bonding of the water system it would be possible to grab a hold of the cold and hot tap at a sink and receive an electric shock.
It is for the above reason that these services should be bonded to earth and not so that a better value of Ze (Earth Loop Impedance of the supply characteristics to an installation) can be obtained. Therefore the actual earth bonds of the services should be installed within 600mm on the property side of the service in question. For example within 600mm of the gas meter (Gas meter being deemed the point where the service enters the property) but on the output side of the gas meter.
How many services needed bonding to earth?
All of them! If a property does not have a gas supply then clearly it does not need bonding. Based on the reasoning above, ALL services need to be bonded to earth where they are metallic. A question that comes up a lot is whether a water service needs to be bonded to earth if it arrives to the property in a plastic pipe. Yes, it still needs bonding at the point where the system is metallic. If the whole system is plastic then there is NO need to bond the system to Earth.
What is the difference between ‘Earthing’ and ‘Bonding’.
Essentially the difference occurs from the type of thing we are trying to bond or earth. The process is the same, however, as we simply connect the item to be bonded or earthed to an earth terminal within the installation – Ordinarily this would be done in either the consumer unit or at the MET (Main Earthing Terminal) which should be located next to the Electric Meter. We ‘Earth’ parts of the electrical system and we ‘Bond’ things which are NOT part of the electrical system. So we ‘Earth’ an appliance using a cpc (Circuit protective conductor) of a cable supplying a socket outlet, and we ‘Bond’ the Gas service of a property using an earthing conductor to a consumer unit or MET.
What size ‘Earthing’ or ‘Bonding’ Conductors should be used
Within a domestic property which is limited to a maximum demand of 100 amps or lower at the suppliers cut out, the following CSA (Cross Sectional Areas) of conductors should be used;
– Main Earthing Conductor – The Main Earth Conductor between the consumer unit and the
Suppliers Cut Out [for TNS and/or TNCS systems] should be 16mm CSA
– Main Bonding Conductor – The Main Bonding Conductor between the consumer unit or
MET and non-electrical services within the installation should be 10mm CSA