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Supply Types

TNS supplies are usually found in older properties, particularly in urban areas.

TNS Supply

The T signifies Terre or Earth, and the N is Neutral. S stands for Separate.

The Neutral and Earth wires are separate throughout the supply cable. The most common arrangement is a two core cable (Live and Neutral) with a lead outer sheath (Earth). The lead is covered with a spiral steel band for added protection.
This has several advantages, the main one being that for underground supplies, the outer metal sheath is in direct contact with the ground for the whole length of the cable. This ensures the earth connection is at the same potential as the real earth.

One disadvantage is that over time, the outer metal sheath corrodes, until it is no longer an effective conductor. When this happens, the only solution is to replace the cable. You will normally then get a TNCS supply, as the lead cable is not used any more.

The Earth connection is normally made to the outer sheath of the supply cable, either by a clamp or a soldered (sweated) joint. If a clamp is fitted, do NOT attempt to adjust or tighten it. For one thing, it is not yours to adjust (it belongs to the DNO), and it is possible to damage the cable, causing a short circuit between the conductors. This is very dangerous, as hundreds of amps will flow, causing overheating, sparks and flame.

Note that there is a variation where the earth wire connects into the side of the cutout, but is NOT connected to the neutral inside. This looks exactly like the TNCS supply, only it is still TNS, as the cable has three separate cores.

New colours

This diagram shows the same TNS installation but with the new wiring colours.

It is also likely that the tails between the cutout, meter and consumer unit will have grey outer insulation, particularly if they have been replaced recently.

Older supplies

As TNS supplies are generally older, the cutout may be cast iron. Generally these should be replaced, as there is the issue of the wires inside contacting the outer metal casing. If this happens, the wires will burn away, and possibly set fire to other items in the vicinity.

The fuse inside may be a ceramic type which could be brittle or already broken.