Earthing

earthingWhat is Earthing?

In an electrical installation there is (or rather, should be) a Main Earth, Main Equipotential Bonding and Supplementary Equipotential Bonding. The idea is to protect all the pipework and metalwork in the house, so that in the event of a fault occurring, the fuse/MCB will disconnect within the regulated time (usually 0.4 of a second).

Main Earth

This is the earth usually provided by the Regional Electricity Company (REC), but sometimes (especially if the supply is from overhead cables) an earth rod must be driven into the ground and a cable run from there to the consumer unit. The Main Earth will provide earthing to the circuitry fed via the consumer unit. This Main Earth is usually a 16mm green and yellow cable.

Main Equipotential Bonding

This is usually a 10mm green and yellow cable, which runs from the consumer unit to the house side of the main water stopcock and to the house side of the gas meter. These are necessary nowadays as the water and gas service providers run their supplies in plastic pipes and so there is no longer a “Good earth” from their pipes.

Supplementary Equipotential Bonding

Ok, so we’ve bonded the cold water and gas, but what about the hot water and the central heating? These are “Cross bonded” by attaching 4mm green and yellow cables to each of the pipes in the bath/shower room or in the airing cupboard if it adjoins, or is in the bath/shower room. The bath/shower room is the most important place to be bonded, as this is where you could be wet, naked and vulnerable to electrocution. All metalwork in these areas must be bonded. This includes the earth conductors in circuits to an electric shower; shaver point, lighting and heaters/towel rails, which must also be bonded to the pipework.

Note that from July 1st 2008 the 17th edition of BS7671 states that this bonding is not required in bath/shower rooms on new installations as all circuits must be RCD protected.

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Posted on: October 16th, 2013 by Abigail Andrews No Comments

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