Calculate actual earth fault loop impedance from Ze + R1+R2. Verify compliance with BS 7671 maximum Zs values and determine if your protective device will disconnect within the required time.
Calculate actual Zs from Ze + R1+R2 and verify compliance with BS 7671 maximum values
Earth fault loop impedance (Zs) is the total impedance of the fault current path from the point of an earth fault, through the earth fault loop, back to the source of supply. It determines how much fault current will flow during an earth fault and therefore whether the protective device (MCB, fuse, or RCBO) will disconnect the supply quickly enough to prevent electric shock.
The earth fault loop impedance is made up of two components: Ze (the external earth fault loop impedance provided by the supply network) and R1+R2 (the resistance of the circuit's line conductor and circuit protective conductor). The fundamental formula is: Zs = Ze + (R1 + R2).
Zs = Ze + (R1 + R2)
PEFC = U0 / Zs
Where U0 = 230V for single-phase UK supply. The PEFC (Prospective Earth Fault Current) determines if the protective device can disconnect within the required time.
The earth fault loop impedance directly controls the fault current magnitude. A lower Zs means higher fault current, which makes protective devices operate faster. BS 7671 sets maximum Zs values for each type and rating of protective device to ensure disconnection within the required time.
| Circuit Type | Max Disconnection Time | Applies To |
|---|---|---|
| TN Final Circuits | 0.4 seconds | Socket outlets, portable equipment, lighting |
| TN Distribution Circuits | 5 seconds | Sub-main cables, distribution boards |
| TT Final Circuits | 0.2 seconds | All circuits (RCD protection mandatory) |
Measure Ze at the origin and R1+R2 using a low-resistance ohmmeter during dead testing, then add them: Zs = Ze + (R1+R2). This method is preferred by NICEIC as it gives more consistent, repeatable results.
Use a loop impedance tester at the furthest point of the circuit. Faster but can be affected by supply fluctuations and electrical noise. High-current testers give best accuracy but will trip RCDs.
BS 7671 Table 41.3/41.4 values assume conductors at their maximum operating temperature (70°C for PVC). When you test at ambient temperature (~20°C), resistance is approximately 20% lower.
The 80% rule accounts for this: your measured or calculated Zs (at 20°C) should not exceed 80% of the tabulated maximum. This ensures compliance even when cables heat up under load.
Testing Limit = Max Zs (table) x 0.80
Example: 32A Type B MCB, Max Zs = 1.37Ω, Testing limit = 1.37 x 0.80 = 1.10Ω
Given:
Step 1: Calculate R1+R2
Step 2: Calculate Zs
Step 3: Temperature correction
Step 4: Check compliance
Step 5: PEFC
These are the maximum permitted earth fault loop impedance values for Type B MCBs in TN systems with a 0.4 second disconnection time. The actual Zs at the furthest point of the circuit must not exceed these values at conductor operating temperature.
| MCB Rating | Type B (max Zs) | Type C (max Zs) | 80% Test Limit (Type B) |
|---|---|---|---|
| 6A | 7.28Ω | 3.64Ω | 5.82Ω |
| 10A | 4.37Ω | 2.19Ω | 3.50Ω |
| 16A | 2.73Ω | 1.37Ω | 2.19Ω |
| 20A | 2.19Ω | 1.09Ω | 1.75Ω |
| 32A | 1.37Ω | 0.68Ω | 1.10Ω |
| 40A | 1.09Ω | 0.55Ω | 0.87Ω |
| 50A | 0.87Ω | 0.44Ω | 0.70Ω |
Values from BS 7671:2018+A3:2024 Table 41.3. For fuse-protected circuits, refer to Tables 41.2 (BS 88) and 41.4 (BS 3036/BS 1361).
The most common system in UK urban areas. The neutral and earth are combined in the supply cable, then separated at the origin.
The earth return path is via the metal sheath of the supply cable. Common in older properties with lead-sheathed cables.
Earth return via an earth electrode at the property. Common in rural areas with overhead supplies.
A “good” Zs reading depends on the protective device. For a 32A Type B MCB (the most common domestic rating), the maximum Zs is 1.37Ω and the 80% testing limit is 1.10Ω. Ideally, your measured Zs should be well below this limit to provide safety margin. In a TN-C-S system with typical Ze of 0.35Ω, most domestic circuits will have Zs values between 0.5Ω and 1.0Ω.
If Zs exceeds the maximum value for the protective device, the fault current during an earth fault will be insufficient to trip the MCB or fuse within the required disconnection time (0.4s for final circuits, 5s for distribution circuits). This means exposed metalwork could remain live at dangerous voltages. It's classified as a C2 code (Potentially Dangerous) on an EICR. Solutions include using larger cable (lower R1+R2), shorter cable run, or adding RCD protection.
Ze (external earth fault loop impedance) is measured at the origin of the installation with the main earthing conductor disconnected from the MET. This isolates the installation earth from the supply earth so you measure only the supply network impedance. Use a loop impedance tester between line and the supply earth. Warning: disconnecting the main earth removes fault protection from the entire installation — keep testing time to a minimum.
PEFC (Prospective Earth Fault Current) is calculated as U0 / Zs (230V / Zs for single-phase). It tells you the fault current that would flow during an earth fault. The protective device must be capable of breaking this current. For a Zs of 1.0Ω, PEFC = 230A. This value must also not exceed the breaking capacity of the MCB (typically 6kA or 10kA for domestic devices — easily satisfied).
Both methods achieve the same result — accounting for conductor heating under fault conditions. The 80% rule is simpler: multiply the tabulated max Zs by 0.80 and compare with your ambient-temperature reading. The correction factor method is more precise: multiply the measured R1+R2 by 1.20 (PVC at 70°C) then add Ze. Use whichever your certification body recommends — NICEIC and NAPIT both accept the 80% method.
On a TT system, the earth fault return path goes through the earth electrode, which typically has a resistance of 21Ω or more. This makes Zs far too high for MCBs or fuses to provide disconnection within 0.4 seconds. Instead, TT systems rely on 30mA RCDs for earth fault protection. The RCD detects the imbalance between line and neutral and trips at 30mA regardless of Zs. The requirement becomes Ra × IΔn ≤ 50V (e.g., with a 30mA RCD: Ra ≤ 1667Ω).
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