Supply Types
For new properties, or new supplies to older properties, TNCS is the most likely type.
TNCS Supply
The Neutral and Earth wires are combined within the supply cable. Typically this will be a concentric cable, with the live as the central core, and a ring of wires around this for the combined neutral and earth.
At the property, the Neutral and Earth are separated, with the earth terminal usually being on the side of the cutout. Inside the cutout, the earth and neutral are linked.
Throughout the supply network, the combined earth/neutral conductor is connected to the ground in multiple places, either buried underground or at the poles for overhead supplies. This multiple earthing is why a TNCS supply is often called PME (Protective Multiple Earthing).
The advantages of this supply type are cost - a 2 core cable is cheaper than a 3 core. As the outer sheath is usually plastic, there are no problems with corrosion.
The main disadvantage is when the combined earth/neutral conductor is broken. This results in a voltage appearing on the exposed metalwork in the customer's property, which can be a shock risk.
This happens as the earth and neutral are connected in the cutout, and there is no direct connection to the ground other than in the supply network.
Another disadvantage is that in the event of a fault, the current flowing in the customer's earthing conductors can be much greater that for a TNS system. It is also possible to get unusual circulating earth currents between properties, particularly where some properties have metal water pipes and others have plastic.
New colours
This diagram shows the same TNCS installation but with the new wiring colours.
It is also possible that the tails between the cutout, meter and consumer unit will have grey outer insulation, with blue or brown inner insulation.
Caution
Just because the earth connects to the side of the cutout, this does NOT automatically mean it is a TNCS supply. Some TNS supplies have a 3 core cable, and use a separate earth terminal on the cutout. Even worse are 'DIY' conversions to TNCS, where someone has used the earth terminal on the cutout without permission.
The terminal might not even be connected inside the cutout!
