Earthing systems: TN-C-S, TN-S, TTTN-C-S, TN-S, TT. What do the letters mean?

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Reading the letters: T = Terra (earth) · N = Neutral · C = Combined · S = Separate.

System	How the earth is provided	Expected max Ze	Where you'll find it
TN-C-S also called PME	Neutral and earth are combined in the supply cable (PEN conductor), then separated at the cut-out. PME = protective multiple earthing, with many earth connections along the network. Provided by the DNO.	0.35 Ω	Most new builds and most homes today
TN-S	Earth and neutral are separate all the way. The earth is the metal sheath or armour of the supply cable, clamped and taken to the MET. Provided by the DNO.	0.8 Ω	Older properties
TT	No earth from the DNO. The installation has its own earth electrode (rod or plate) in the ground. The client pays for it, and you install it.	up to 200 Ω	Rural areas, farms, caravan parks and other places too far from a transformer's star point

Simplified. The transformer's star point is on the left of each drawing and the property on the right. TT has a much higher Ze because the fault current has to travel back through the ground between two earth rods.

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 Electrician's Guide to the Building Regs · Figure: cable sheath earth (TN-S), earthing and main bonding · p.55 On-Site Guide · List of earth electrode types (rods, plates, etc.) for TT

The earth fault path

Suppose a line conductor comes loose and touches the metal case of a load. The fault current travels through the case, along the CPC to the MET, down the earthing conductor, back through the supply to the transformer star point, and round to the protective device, which trips. This is automatic disconnection of supply (ADS).

Earth is not a current-carrying conductor under normal conditions. It only carries current when there's an earth fault. For ADS the circuit breaker or fuse must disconnect within the Table 41.1 time (0.4 s on a TN final circuit). Where an RCD gives the protection, a non-delayed RCD must trip within 300 ms at its rated residual current.

Why TN-C-S has a risk
Because the neutral and earth are combined in the supply cable, damage to that PEN conductor (a nick or a cut) means more than losing your earth: all the earthed and bonded metalwork in the installation can rise towards line voltage. On TN-S, a problem with the sheath earth only affects that one property, so it's easier to spot. On your own circuits, you can use the armour of SWA cable as the CPC.

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18th Edition
Building Regs
PAT
Fundamental Inspection & Testing
Initial Verification
Periodic Inspection & Testing

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