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Ze, R1 + R2 and ZsHow do you work out the total earth fault loop?
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- Ze
- External earth fault loop impedance: everything from the transformer, through the supply, up to the intake. Measured live with test equipment.
- R1
- Resistance of the circuit's line conductor.
- R2
- Resistance of the circuit's CPC (circuit protective conductor): the green/yellow cable from the consumer unit to every socket, light, spur and so on. (Rn is the neutral.)
- R1 + R2
- Found by a continuity test (method 1) on each final circuit, done dead.
- Zs
- Total earth fault loop impedance.
The formula
Zs = Ze + (R1 + R2) Example (made-up figures, for practice): TN-C-S supply, measured Ze = 0.30 Ω Ring circuit continuity test, R1 + R2 = 0.52 Ω Zs = 0.30 + 0.52 = 0.82 Ω → compare with the maximum permitted Zs for that protective device
This is covered again in more depth in the 18th Edition and inspection & testing modules. For now, just know what each term means.
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- 18th Edition
- Building Regs
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- Fundamental Inspection & Testing
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