United Kingdom · Wiring Regulations examHow do you read the max Zs values 18th Edition table?
Tables 41.2, 41.3 and 41.4 give the maximum earth fault loop impedance, Zs, that shouldn't be exceeded at a conductor operating temperature of 70 °C. Regulation 411.4.204 says Table 41.5 may be applied in some cases.
- Find the table for your protective device and read the maximum Zs for its type and rating.
- Measure the earth fault loop impedance. The reading is at ambient temperature, so it can't be compared directly with the 70 °C table value.
- Compare your reading with the On-Site Guide's quick-reference values, which are already corrected to 80%.
- If your reading is close to or above that figure, do a detailed calculation with the maximum values and the correction factors in Appendix 3 of BS7671.
- If the maximum you write on the schedule of test results comes from somewhere other than the Chapter 41 tables, such as manufacturer's data, say so on the schedule.
Maximum permitted Zs: an example from the IET
In a May 2025 Wiring Matters article, the IET gave the maximum earth fault loop impedance for an 80 A Type C circuit-breaker to BS EN 60898 as 0.27 Ω. The same article explains that the values in Table 41.5 are derived from a touch voltage of 50 V, not 230 V, and that for TT systems the condition is RA × IΔn ≤ 50 V.
Use your own copy of the current amendment for real work and in the exam.
Why Zs matters: disconnection
Checking Zs confirms that enough fault current will flow to operate the protective device within the maximum disconnection times in Table 41.1 of BS7671. Downstream of an RCD, the IET article says continuity of the protective conductors is enough to confirm automatic disconnection works, since a general non-delay RCD disconnects within 300 ms at its rated residual operating current.
Typical Ze values
Distributors declare typical maximum external earth fault loop impedances, Ze. The IET stresses these are typical values only, and what you measure on site will differ.
| System | Typical maximum Ze |
|---|---|
| TN-S | 0.8 Ω |
| TN-C-S | 0.35 Ω |
| TT | 21 Ω |
Sources: IET · IET Wiring Matters and 2 more. Checked October 2026.
- IET: Inspection and testing FAQs ↗
- IET Wiring Matters: Minimizing unnecessary live testing for initial verification (Michael Peace, May 2025) ↗
- IET: Earthing and bonding FAQs ↗
- IET: Consumer units and protective devices FAQs ↗
Rules and fees can change, so check the official site before your test.
Explore