If you work to BS 7671 (IET Wiring Regulations), you’ll meet maximum earth fault loop impedance Zs on virtually every job. The famous “max Zs table” in the 18th Edition (BS 7671:2018+A2:2022) tells you the highest permissible earth loop impedance that still guarantees automatic disconnection of supply (ADS) within the required time.
In this guide, we’ll demystify what Zs is, where the 18th Edition tables live, how they’re derived (not just looked up), how Cmin and temperature assumptions affect the numbers, what changes with RCDs/RCBOs and MCCBs, and how to use measured vs. tabulated values correctly on inspection and testing.
What Zs Actually Means (and Why It Matters)
Zs is the impedance of the earth fault loop starting and ending at the point of fault supply transformer through line conductor, the fault path, and back via the protective conductor and supply earth. Lower Zs means higher fault current, faster operation of the protective device, and safer disconnection.
BS 7671 defines it in Part 2, and industry primers explain the loop path clearly. In practice, your measured Zs must not exceed the permissible maximum for the protective device and circuit, otherwise disconnection time may be too slow.
Where the “Max Zs Table” Lives in the 18th Edition
The limiting Zs values that most installers look up are set out in Chapter 41 of BS 7671 specifically Tables 41.2, 41.3 and 41.4, and (for special cases) Table 41.6. These tables give max Zs for commonly used protective devices BS EN 60898 MCBs (Types B/C/D), BS 88/BS 1361 fuses, and the overcurrent characteristics of RCBOs on 230 V (Uo) systems, for the standard disconnection times (typically 0.4 s for final circuits in TN/TT systems supplying socket outlets, 5 s for distribution circuits, etc.). The IET’s Wiring Matters reminders are explicit that these tables are aligned to the assumptions elsewhere in the regs (current-carrying capacity tables, voltage drop, temperature).
Key point: The tables are not arbitrary “magic numbers”, they’re derived using the same assumptions used across BS 7671 so that design, verification, and device operation knit together consistently.
The Formula Behind the Table: Zs = (Uo × Cmin) / Ia
Even though most of us look up max Zs, BS 7671’s Appendix 3 gives the underlying equation:
Zs = (Uo × Cmin) / Ia
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Uo is the nominal line-to-earth voltage (230 V in the UK).
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Cmin is a minimum voltage factor that accounts for real-world supply variation, transformer tap changes, etc. In the UK (ESQCR supplies), Cmin = 0.95.
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Ia is the current that ensures instantaneous (or required time) operation of the device. For MCBs to BS EN 60898, Ia is taken from Appendix 3 time/current curves; the tabulated values in 41.3 already embed this.
The presence of Cmin (introduced in late 17th and carried into the 18th) is why older “max Zs crib sheets” don’t match 18th Edition values—multiply 230 V by 0.95 first and you’ll see the newer, slightly lower permissible Zs.
Temperature Assumptions and Why “Designer’s Zs” May Differ
BS 7671’s max Zs tables assume conductor operating temperatures consistent with the current-carrying capacity data, so that your comparisons are apples-to-apples. That’s why the IET points out the values are “useful atthe design stage” under those common conditions.
If your installation conditions differ (ambient, grouping, insulation, conductor temperature), the real Ia for the device and the loop resistance at fault may shift; designers can calculate from first principles using the formula instead of copying a table entry verbatim.
RCDs, RCBOs, MCCBs, and “Devices Not in the Table”
The 18th Edition tables primarily cover fuses and MCBs/RCBOs in common ranges. For MCCBs and some higher-rated fuses, you won’t find a direct row. In those cases, use manufacturer time-current data to establish Ia, then apply Zs = (Uo × Cmin)/Ia. The IET specifically notes that to obtain Ia for devices to BS EN 60898 or BS EN 60947-2, you need the manufacturer’s curve or Appendix 3 graph, which is what Table 41.3 is based on anyway.
For RCD-protected circuits (e.g., an MCB + 30 mA RCD in series), ADS may rely on the RCD. In TT systems particularly, fault protection is often provided by the RCD rather than the overcurrent device, so the Zs limit is driven by the RCD residual operating current (IΔn) and disconnection time requirements rather than the MCB’s instantaneous curve. (You’ll still verify Ze, Zs, and Ra as appropriate.) The tabulated 41.x values are for overcurrent devices under standard assumptions; RCDs follow a complementary route.
Measured vs. Tabulated: The 80% “Pocket Guide” Convention
When you measure Zs on-site, you’re measuring at (usually) ambient conductor temperature, whereas the tables assume conductors at operating temperature. To build in headroom, long-standing guidance (e.g., NICEIC/Electrical Safety First pocket guides) publishes “maximum measured values” equal to 80% (rounded down) of the Chapter 41 table values.
This isn’t a regulation but a pragmatic convention so that a pass at test is likely still safe at operating temperature. Many inspectors therefore compare measured Zs to 80% tables during EICRs and initial verification.
Worked-Through Method (Without Needing to Memorise Every Row)
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Identify the protective device and its standard (e.g., Type B MCB to BS EN 60898, 32 A).
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Look up the max Zs in Table 41.3 (for MCBs/RCBO-characteristics) or 41.2/41.4 (for fuses) for the required disconnection time (0.4 s for final socket circuits in TN/TT; 5 s for distribution, etc.).
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If not tabulated (e.g., MCCB), find Ia from the manufacturer’s time/current curve (or Appendix 3 graphs for 60898/61009 devices), then calculate Zs = (Uo × 0.95) / Ia.
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On test, compare your measured Zs to the 80% “max measured” value (from a pocket guide) to allow for temperature rise; if you prefer to use the full table value, document your assumption and ensure you’ve considered conductor temperature.
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In TT systems or where RCDs provide ADS, check the RCD disconnection time/Trip (IΔn) meets BS 7671, and verify Zs/Ra accordingly—don’t rely solely on the overcurrent device’s table.
Common Pitfalls with Max Zs in the 18th
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Using pre-18th crib sheets that don’t include Cmin (0.95)—you’ll over-estimate permissible Zs. Modern calculators and tables build Cmin in.
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Confusing Ze and Zs: Ze is external loop impedance at the origin; Zs = Ze + (R1+R2) for radial circuits (plus parallel effects). Don’t compare a Ze measurement to a final-circuit max Zs row.
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Forgetting disconnection time context (0.4 s vs 5 s): Table choice and row interpretation depend on the circuit type.
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Assuming RCD presence changes table values: The 41.x tables are for the overcurrent device. If ADS relies on the RCD, validate to RCD criteria instead.
How Updates and Guidance Tie Together
Amendments and IET articles over the 18th Edition era clarified why max Zs values look the way they do, harmonising with temperature assumptions, voltage tolerances, and real device curves. If two Zs tables from different sources don’t match, check: (a) whether Cmin is applied; (b) whether they’re “max measured = 80%” tables; (c) which device standard and disconnection time is assumed. The IET’s 2023–2024 Wiring Matters pieces make this logic explicit and show how to revert to first principles with Ia and Cmin when needed.
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Takeaway: Don’t Just Copy Numbers, Know the Logic
The 18th Edition max Zs table is a brilliant shortcut because it encodes the underlying physics and regulatory assumptions Cmin, device curves, and operating temperatures. Use the tables in Chapter 41 for common devices; fall back to Zs = (Uo × 0.95) / Ia with manufacturer curves when you’re off-piste; and, on testing, remember the 80% “max measured” convention to avoid borderline passes that fail in service. That way, your design, verification, and certificates align with the letter and spirit of BS 7671 and most importantly, faults clear fast enough to keep people safe.
Further reading & quick references (non-exhaustive):
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IET Wiring Matters on max Zs differences and device curves in BS 7671:2018+A2:2022.
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Electrical Safety First/NICEIC pocket guides with 80% “max measured” values.
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Cmin rationale (0.95) under UK ESQCR supplies (Schneider Electric FAQ).
Note: BS 7671 text and the 41.x tables are copyright works always consult the official standard (and your device manufacturer’s data) for design and certification.