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Earthing systems and ZeTN-C-S, TN-S or TT. What Ze should you expect?
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| System | Key point | Expected max Ze |
|---|---|---|
| TN-C-S (PME) | Neutral and earth combined in the supply (the PEN conductor, protective earth neutral), separated at the cut-out | 0.35 Ω |
| TN-S | Earth and neutral separate the whole way, with the earth via the cable sheath | 0.8 Ω |
| TT | Earth electrode, provided and paid for by the client. Ze is high because of ground conditions: a metal rod in the general mass of earth. Above 200 Ω the earth connection becomes unstable. | up to 200 Ω |
Electrician's Guide to the Building Regs · maximum Ze values: 0.35 Ω TN-C-S, 0.8 Ω TN-S · p.53
Ze only measures the external earth path, from the transformer to the incoming MET or consumer unit. Under a fault the circuit breaker should disconnect within the Table 41.1 time (0.4 s on TN, 0.2 s on TT, for final circuits) and a 30 mA RCD trip in 300 ms.
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