In this article the technical team at Stroma Certification offers expert guidance on exporting a PME supply to another building.
One of the most common queries to the Stroma help desk concerns running a supply to a garden building where the supply to the house has a PME earthing system. There’s no regulation that says you can’t export a PME earth to another building. However, some Distribution Network Operators (DNOs) may forbid their PME earth being exported from the supplied building.
BS7671 forbids the use of PME earthing systems to some types of installations, such as caravans. It doesn’t recommend PME earthing unless special precautions are taken for some installations, such as swimming pools and agricultural and horticultural premises, which would include greenhouses and stables.
What are the risks?
In the event of a failure of the PEN conductor in the supply cable to the premises, both the neutral and earth supplied may go open circuit or high resistance. This fault will cause exposed conductive parts, such as earth terminals, metal conduits and enclosures and Class 1 connected appliances, to become live.
Touch voltages on these exposed parts may rise to 230V or higher. This can cause the lights in the premises to go off and other appliances to stop working, causing people to believe that the power has gone off when the installation is still live. The extraneous conductive parts, such as gas, water and oil pipes, boilers, metal baths and central heating radiators, will also become live. Although these conductive parts are live, the risk of receiving a fatal shock is reduced providing that adequate bonding is in place, the floor coverings provide some level of insulation and the persons and livestock remain inside the building.
In normal, fault-free conditions, especially if the PME supply cable has a long run into the consumer’s premises, there may be a potential difference between the supply earth and the general mass of earth due to the voltage drop on the supply neutral. In this case, persons touching exposed conductive and extraneous parts may experience small shocks and complain of ‘tingles’ and ‘hot when touched’.
Risk assessment
If considering exporting a PME earth, contractors should assess the risks. If the risk of shock is elevated due to the circumstances, or the risks can’t be fully assessed or quantified, the remote building earthing should be converted to a TT system.
If the remote building has a wooden floor and there are no extraneous conductive parts inside the building, and subject to any other risks present, it may be acceptable to use the exported PME earth.
If the remote building has a wooden floor and there are extraneous conductive parts inside the building, and subject to any other risks present, it may be acceptable to use the exported PME earth, providing the extraneous conductive parts are bonded to a Building Earth Marshalling Terminal (BEMT) inside the remote building. In this case, the distribution circuit CPC will also have to act as a bonding conductor and the CSA must comply with Table 54.8 of BS7671 in addition to meeting the CSA requirement for a CPC.
If the remote building has a concrete earth floor, or conductive floor, or the building is of metal construction or metal outside skin, the shock risk is increased. It’s therefore recommended that the remote building has a TT earthing system.
If the remote building is used to house livestock, and bonding of a metal grid in the floor and all exposed metal work isn’t economically or technically viable, the building should have a TT earthing system.
Applying TT earthing to the remote building
If using an SWA cable to the remote building, the steel armour should be connected to the installation earth at the supply end. This is necessary to prevent shock to anyone damaging the cable or piercing the cable with a tool.
At the remote building, the SWA cable should be terminated at the distribution board or consumer unit using a plastic stuffing gland and not the usual metal SWA gland. The SWA armour should be cut back to under the cable sheath inside the plastic gland so it can’t be touched.
The live supply conductors inside the distribution board or consumer unit should be over-sleeved for their entire length, be kept as short as possible and not in contact with the casing or internal parts. The live conductors should be terminated directly into an RCD main switch.
For small installations, a 30mA RCD main switch is acceptable. For larger installations, a 100mA or 300mA S type RCD main switch should be used with 30mA RCDs for additional protection on final circuits. The Main Earthing Terminal in the remote building must then be connected to an earth electrode with an earth resistance complying with Table 41.5 of BS7671.
