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Verify BS 7671 Zs compliance before you test on site. Enter Ze, conductor sizes and circuit length to calculate Zs and check against Table 41.3 limits for any MCB or fuse type.
Typical TN-C-S: 0.15–0.40Ω. Measure at board origin using a loop tester.
Calculated Zs
0.791 Ω
Ze + Ct(R1+R2)
Max permitted Zs
1.440 Ω
BS 7671 Table 41.3
Margin: 0.649 Ω — circuit complies with BS 7671 disconnection time requirements.
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The calculation follows BS 7671 (18th Edition Wiring Regulations) and uses standard copper conductor resistivity at operating temperature. All values are conservative to ensure compliance rather than borderline compliance.
The resistance of each conductor is calculated using R = (ρ × L) / A, where ρ = 0.01724 Ω·mm²/m (copper resistivity at 20°C), L is the one-way circuit length in metres, and A is the conductor cross-sectional area in mm². This gives R1 (phase) and R2 (CPC) at reference temperature.
Standard 70°C PVC-insulated conductors operate at up to 70°C under load. Copper resistance increases with temperature. BS 7671 applies a correction factor Ct = 1.20 to convert from 20°C reference to 70°C operating temperature. This increases R1+R2 by 20% and ensures worst-case Zs is calculated. For 90°C XLPE conductors, the factor would be 1.28.
Zs = Ze + Ct × (R1 + R2). Ze is the measured external impedance; Ct × (R1 + R2) is the temperature-corrected internal circuit impedance. The result is compared against the maximum Zs from BS 7671 Table 41.3 for the selected protective device at 0.4s disconnection time. A 10% margin below the limit is good practice to allow for measurement tolerances and system variations.
BS 7671 Table 41.3 tabulates maximum Zs values for Type B MCBs at 0.4s disconnection. Type C MCBs require twice the fault current, so their maximum Zs is halved. Type D MCBs require four times the fault current, reducing maximum Zs to one quarter. The Table 41.3 values assume 230V nominal voltage. For fuses (BS 88-2, BS 1361), the maximum Zs values are derived from the fuse I²t characteristics at the required disconnection time.
WORKED EXAMPLE
An electrician is wiring a 25m spur from a ring final circuit, using 2.5mm² T&E (1.5mm² CPC). Ze at the board is measured at 0.35Ω. Is the spur compliant for a 32A Type B MCB?
Ze
0.350 Ω
R1 (2.5mm², 25m, 70°C)
0.207 Ω
R2 (1.5mm², 25m, 70°C)
0.345 Ω
Calculated Zs
0.902 Ω
Max Zs (32A Type B)
1.440 Ω
Margin
37%
Result
PASS — 37% margin
Zs 0.902Ω vs max 1.44Ω — well within BS 7671 Table 41.3 limits
Always verify with a calibrated loop impedance tester on site. Calculated values assume consistent conductor temperature and good connections throughout. Refer to BS 7671 18th Edition for compliance certification.
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Common questions about earth fault loop impedance and BS 7671 compliance.
Earth fault loop impedance (Zs) is the total impedance of the earth fault loop — the path that fault current follows from the point of fault, back through the circuit protective conductor (CPC), the main earthing terminal, the supply cable, and back through the source. A low Zs ensures a high enough fault current to operate the protective device within the required disconnection time. Zs is measured in ohms and must be verified at every circuit during an Electrical Installation Condition Report (EICR) or new installation certificate.
Ze is the external earth fault loop impedance — the portion of the loop that exists outside the installation, from the supply transformer earthing point through the incoming supply cable to the installation main earthing terminal. Zs is the total loop impedance, which equals Ze plus R1 (phase conductor resistance) plus R2 (CPC resistance) for the specific circuit. Ze is typically measured at the origin of the installation using a loop impedance tester before any circuit breakers are operated. Typical TN-C-S system values are 0.15–0.40Ω; TN-S systems range from 0.10–0.60Ω.
BS 7671 (18th Edition Wiring Regulations) specifies that final circuits supplying socket-outlets or portable equipment in most locations must disconnect within 0.4 seconds under earth fault conditions to protect against indirect contact. This is because a person can touch a live enclosure and the short disconnection time limits the let-through energy to a level that is unlikely to cause ventricular fibrillation. Distribution circuits and fixed equipment may use the 5-second disconnection time, which allows a higher Zs. The 0.4s limit drives the maximum Zs values in Table 41.3 of BS 7671.
Ze is measured using a calibrated loop impedance tester (such as a Fluke 1664, Metrel MI3102, or Megger MFT) connected at the origin of the installation (main distribution board) with the main switch open. The tester injects a small test current and measures the impedance. You must also check with the supply authority whether the installation is TN-C-S (PME), TN-S, or TT — the expected Ze range differs significantly. TT installations will show a very high Ze and require RCD protection rather than relying on overcurrent device disconnection time.
If Zs exceeds the maximum permitted value for the protective device, the fault current under earth fault conditions may be insufficient to operate the device within the required 0.4s (or 5s) disconnection time. This represents a danger to users. Remedies include: reducing circuit length, increasing phase and/or CPC conductor CSA, using a lower-rated protective device, or fitting an RCD (which provides disconnection independently of the Zs). On an EICR, this would be coded C2 (potentially dangerous) requiring prompt remedial action.
Type B MCBs have the highest permitted Zs (they trip at 3–5× rated current), Type C MCBs need twice the fault current (5–10× rated), so their maximum Zs is halved. Type D MCBs need 10–20× rated current for magnetic operation, so their maximum Zs is one quarter of the Type B value. In practice, Type B MCBs are standard for final circuits; Type C are used for motors and fluorescent lighting; Type D are used for high-inrush loads such as welders and UPS systems. Using Type C or D MCBs requires lower Zs — longer or thinner cables may not comply.
An EICR (Electrical Installation Condition Report) requires loop impedance measurements on a representative sample of circuits (or all circuits) to verify they still comply with BS 7671 requirements. Zs values that were acceptable when the installation was first commissioned can increase over time due to conductor corrosion, poor connections, or changes in the external Ze (e.g. the DNO changing the supply network). High Zs is one of the most common reasons circuits receive a C2 (potentially dangerous) observation on an EICR. This calculator lets you pre-check whether a circuit's calculated Zs is likely to be within limits before conducting the physical test.