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Earthing systems: TN-S, TN-C-S, TTTN-S, TN-C-S or TT. How does fault current get home?
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Every building is supplied from a transformer, and every transformer has a star point. That star point is the source of earth for the installation. Fault current has to get back to the star point for the protective devices to work. How it gets back depends on the earthing system.
| System | How the earth gets back to the star point | Max Ze |
|---|---|---|
| TN-S | Neutral and earth are separate all the way. The earth is the metal sheath or armour of the supply cable. Common in older properties. | 0.8 Ω |
| TN-C-S also called PME | Neutral and earth are combined in the underground supply cable, in one conductor called the PEN conductor (protective earth neutral). They're separated at the cut-out. It's the most common system in the UK, very reliable, with very low resistance to earth. | 0.35 Ω |
| TT | No earth from the supplier. The installation has its own earth electrode (rod), and fault current returns through the ground. Used in rural areas. | up to about 200 Ω (your own electrode) |
Our first installations are drawn as TN-S, but it makes no difference to the bonding rules whether it's TN-S or TN-C-S. The difference only matters for open PEN faults (section 8), which can only happen on TN-C-S.
Electrician's Guide to the Building Regs · Supply systems: expected maximum Ze values (TN-C-S 0.35 Ω, TN-S 0.8 Ω) · p.53
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