Earth Fault Loop Impedance Guide
Zs & Ze testing, max values, 80% rule, earthing systems, and remedial strategies
BS 7671 Reg 411.3–411.4, Tables 41.2–41.5 & IET Guidance Note 3
What is Earth Fault Loop Impedance?
Earth fault loop impedance (Zs) is the total impedance of the fault current path from the point of a fault, through the circuit protective conductor (CPC), back to the supply transformer. It determines how much fault current flows — and therefore how quickly the protective device disconnects. The lower the Zs, the higher the fault current, and the faster the disconnection.
Zs = Ze + (R1 + R2)
Ze = external loop impedance (DNO supply) • R1 = line conductor resistance • R2 = CPC resistance
Earthing Systems & Loop Paths
Loop path: Line → Fault → CPC → Cable sheath/armour → Transformer neutral
Typical max Ze: ≤ 0.8 Ω
Common in older urban areas. Lead sheath provides reliable low-impedance path.
Loop path: Line → Fault → CPC → PME link → Combined PEN → Transformer neutral
Typical max Ze: ≤ 0.35 Ω
Most common modern UK residential supply. Multiple earth points give very low Ze.
Loop path: Line → Fault → CPC → Earth electrode → Mass of earth → Transformer earth
Typical max Ze: > 20 Ω (can be 100s Ω)
RCDs mandatory — overcurrent devices cannot achieve ADS. Common in rural areas.
Maximum Disconnection Times (Reg 411.3.2)
| System | Circuit Type | Max Time | Regulation |
|---|---|---|---|
| TN | Final circuit ≤ 32 A (with sockets) | 0.4 s | Reg 411.3.2.2 |
| TN | Final circuit ≤ 32 A (fixed equipment) | 0.4 s | Reg 411.3.2.2 |
| TN | Distribution circuit | 5.0 s | Reg 411.3.2.3 |
| TT | Final circuit ≤ 32 A | 0.2 s | Reg 411.3.2.4 |
| TT | Distribution circuit | 1.0 s | Reg 411.3.2.4 |
Maximum Zs Values — MCBs (Tables 41.3)
230 V systems, 0.4 s disconnection time. Values calculated from Zs = (0.95 × U₀) / Ia.
| Rating (In) | Type B (Ω) | Type C (Ω) | Type D (Ω) |
|---|---|---|---|
| 6 A | 7.67 | 3.83 | 1.92 |
| 10 A | 4.60 | 2.30 | 1.15 |
| 16 A | 2.87 | 1.44 | 0.72 |
| 20 A | 2.30 | 1.15 | 0.57 |
| 32 A | 1.44 | 0.72 | 0.36 |
| 40 A | 1.15 | 0.46 | 0.29 |
| 50 A | 0.92 | 0.46 | 0.23 |
| 63 A | 0.73 | 0.36 | 0.18 |
Maximum Zs Values — Fuses (Tables 41.2 & 41.4)
| Rating | BS 88-2 gG (Ω) | BS 1361 (Ω) | BS 3036 (Ω) |
|---|---|---|---|
| 5 A | — | 10.45 | 9.58 |
| 15 A | — | 3.28 | 5.35 |
| 20 A | 1.77 | 1.70 | 3.83 |
| 30/32 A | 1.04 | 1.15 | 2.64 |
| 45 A | 0.60 | 0.60 | — |
The 80% Rule (Temperature Correction)
BS 7671 table values assume conductors at maximum operating temperature (70 °C for PVC). On-site tests are done at ambient (~20 °C), so conductor resistance is lower than it would be under full load.
Copper resistance increases by approximately 20% from 20 °C to 70 °C. To account for this, multiply the table value by 0.8 — if your measured Zs is below this 80% figure, the circuit will remain compliant even at full operating temperature.
Test limit = Table Zs × 0.8
| Rating | Type B (100%) | Type B (80%) | Type C (100%) | Type C (80%) |
|---|---|---|---|---|
| 6 A | 7.67 | 6.13 | 3.83 | 3.07 |
| 10 A | 4.60 | 3.68 | 2.30 | 1.84 |
| 16 A | 2.87 | 2.30 | 1.44 | 1.15 |
| 20 A | 2.30 | 1.84 | 1.15 | 0.92 |
| 32 A | 1.44 | 1.15 | 0.72 | 0.58 |
| 40 A | 1.15 | 0.92 | 0.46 | 0.37 |
Ze Measurement Procedure
Isolate the Installation
Follow safe isolation procedure. Prove dead using a GS38 compliant voltage indicator.
Disconnect Main Earthing Conductor
Remove the main earthing conductor from the MET to prevent parallel paths through gas/water pipes.
Connect Tester
Connect loop impedance tester between incoming line and the disconnected earthing conductor.
Perform High-Current Test
Use a high-current test (typically 20–25 A) for maximum accuracy at the origin.
Compare Against DNO Declared Values
TN-C-S ≤ 0.35 Ω, TN-S ≤ 0.8 Ω. Record the result.
Reconnect Earthing Conductor
Immediately reconnect the main earthing conductor and main bonding conductors.
Zs Testing Methods
Injects ~23 A into the fault loop. Gives the most accurate and repeatable results.
Limitation: Trips any RCD/RCBO on the circuit (exceeds 30 mA threshold). Must bypass or disconnect RCDs first.
Best used at the origin or on non-RCD-protected circuits.
Uses ~15 mA — stays below 30 mA RCD threshold. 3-wire version uses neutral as reference for noise filtering.
Limitation: Less accurate below 1.0 Ω. Can be affected by standing leakage from connected appliances.
Best used on RCD/RCBO-protected circuits where bypassing is impractical.
Calculation Method (Zs = Ze + R1+R2)
Often the preferred method for initial verification. Measure Ze at the origin, then measure R1+R2 at the furthest point of each circuit using a low-resistance ohmmeter during dead testing.
- - More stable and repeatable than live loop testing
- - Preferred by NICEIC — removes “fruit machine” variability of low-current testers
- - Useful for circuits at height (lighting) or where live testing is impractical
- - R1+R2 values measured at ambient — apply 80% rule to the calculated Zs
RCD Protection — Max Zs (Table 41.5)
When a circuit is protected by an RCD, the max Zs depends on the RCD sensitivity, not the overcurrent device.
| RCD Sensitivity (IΔn) | Max Permitted Zs |
|---|---|
| 30 mA | 1,667 Ω |
| 100 mA | 500 Ω |
| 300 mA | 167 Ω |
| 500 mA | 100 Ω |
Diagnosing & Fixing High Zs
| Cause | Symptom | Remedy |
|---|---|---|
| Excessive circuit length | Zs rises with distance from DB | Increase CPC size or add parallel CPC |
| Undersized CPC | High R1+R2 relative to cable length | Upgrade CPC or use larger cable |
| Loose/corroded connections | Erratic readings, higher than expected | Re-terminate, clean, and tighten all joints |
| High Ze from DNO | Ze exceeds declared maximum | Report to DNO; install RCD protection |
| Damaged CPC | Very high or open-circuit reading | Locate and repair/replace damaged section |
EICR Classification for Zs Results
| Condition | Code |
|---|---|
| Measured Zs exceeds 100% table value — ADS cannot be guaranteed | C2 |
| Measured Zs exceeds 80% limit but within 100% table value | C3 |
| Ze exceeds DNO declared maximum | C2 |
| TT earth electrode Ra exceeds 200 Ω (seasonal instability risk) | C3 |
Important Warnings
- - Always reconnect earthing immediately after Ze testing — the installation is unprotected while disconnected
- - High-current loop tests trip RCDs — bypass or disconnect before testing, and reconnect immediately after
- - Never test Zs on a circuit that is dead — loop impedance testers require a live supply
- - TT systems: Earth electrode resistance varies seasonally — test during dry conditions for worst-case values
- - Three-phase: Test each phase individually (L1-E, L2-E, L3-E) and record all values
Key Points
- - Zs = Ze + (R1+R2) — total impedance from fault point back to transformer
- - Compare measured values against 80% of table values (temperature-adjusted limits)
- - TN systems: overcurrent devices (MCBs/fuses) provide ADS within 0.4 s for final circuits
- - TT systems: RCDs are mandatory — overcurrent devices alone cannot achieve ADS
- - The calculation method (Ze + R1+R2) is often more reliable than live loop testing
- - Record all values on the Schedule of Test Results — both Ze and Zs for each circuit