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It is our experience that large sums of money are being wasted making un-necessary improvements to earthing and bonding to existing domestic installations. Whilst we would not recommend that a new installation is provided with other than a 16mm main earthing conductor, and 10mm main equipotential bonding conductors, we do not see any reason to improve existing installations that are compliant with BS7671.

Compliance with BS7671 is mandatory if the earthing system is that known as PME. These regulations are covered by an Act of parliament, and require an earthing system compliant with the electricity supply regulations, and any other conditions imposed by the electricity supply company.

Landlords and employers have an obligation to protect tenants and employees under electricity at work legislation, and a landlord or employer will generally be considered to have fully discharged their responsibilities if their wiring systems are maintained so as to comply with BS7671, the wiring regulations.

In the case of overhead supplies, normally using a TT earthing system, the minimum size of the main earthing conductor can be calculated in accordance with regulation 543-01-03. This calculation invariably indicates an existing 6mm cable to be adequate. However, a 16mm cable will normally be required as it is likely to be underground at some point between the earth electrode and the dwelling. The main equipotential bonding conductors to incoming services are required by regulation 547-02-01 to be at least 50% of the size of the main earthing conductor determined by regulation 543-01-03. Existing 6mm main equipotential bonding conductors can usually be left in situ, compliant with BS7671.

In the case of most older installations using cable sheath earthing, known as TNS, existing 6mm main earthing conductors and 6mm main equipotential bonding conductors normally satisfy regulation 543-01-03, and no improvement is necessary.

PME-O'head supplies

A plumbing friend recently nearly died as he loosened a compression joint on a domestic stop cock and pulled the pipe away from the tap.

Three adjoining rural cottages had been refurbished, and the earthing and bonding brought up to date with the present PME regulations. The earthing system had been changed from from TT to PME in the past. The low voltage supply was overhead, and a concentric two core cable was provided by the REC from the pole outside into the property. The neutral connection on the pole had corroded, and completely disconnected.

The occupant of the cottage had no symptons of the fault. The neutral current of the installation flowed from the PME connection through the main earthing conductor to the main earthing terminal, and thence along the main equipotential bonding conductors to the incoming water service. It then flowed through the common water piework to the adjoining property, through their main bonding and earthing cables, and out through their neutral. No sysmptoms of any kind was evident to occupants of any of the cottages.

The pipe disconnected by our plumber friend was carrying the entire installation current. Bearing in mind the grip he would have used to pull the joint apart, and the inevitable quantity of water that ran from the open pipes, he really is aware of how amazingly lucky he was to survive.

The fault was dealt with professionally, and most promptly by the REC, but this type of fault is not notified by RECs to the DTI or HSE. Our core business is inspecting and testing mainly domestic installations, and we regularly find high resistance joints on LV overhead supplies. Please contact us withinformation of supply problems on overhead low voltage distribution in order we can present a case to the IEE and DTI to recommend that PME earthing is no longer used on low voltage overhead supplies.

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