What are typical Ze values for UK installations?

Ze values depend on the system earthing arrangement. For TN-C-S (PME or combined protective and neutral) earthing — the most common domestic arrangement in the UK — typical Ze values are 0.35Ω or less, though the distribution network operator (DNO) may declare a different maximum. For TN-S earthing (separate neutral and earth from the supply), typical values are 0.8Ω or less. For TT earthing systems (where the installation has its own earth electrode), Ze is replaced by the electrode resistance Ra, which may be several ohms.

What are the maximum Zs values for Type B and Type C MCBs?

Maximum Zs limits for MCBs and RCBOs are tabulated in Table 41.3 (Reg 411.4.204). Ia = 5 × In for Type B, 10 × In for Type C, 20 × In for Type D. Type B examples (230 V, Cmin = 0.95): 6 A = 7.28 Ω, 10 A = 4.37 Ω, 16 A = 2.73 Ω, 20 A = 2.19 Ω, 32 A = 1.37 Ω, 40 A = 1.09 Ω, 50 A = 0.87 Ω, 63 A = 0.69 Ω. Type C (10 × In): 6 A = 3.64 Ω, 16 A = 1.37 Ω, 32 A = 0.68 Ω. Type D (20 × In): 6 A = 1.82 Ω, 16 A = 0.68 Ω, 32 A = 0.34 Ω. Tabulated values apply at maximum conductor operating temperature. On site, GN3 recommends: measured Zs ≤ 0.80 × tabulated limit (the GN3 cold-measured site pass criterion). Fuse Zs limits are in Table 41.2 (Reg 411.4.201): BS 88-2 gG 16 A = 2.43 Ω, 32 A = 0.99 Ω; BS 88-3 16 A = 2.30 Ω, 32 A = 0.91 Ω.

How do you calculate prospective fault current?

Prospective fault current (PFC) at the origin of the installation is calculated using PFC = Uo ÷ Ze, where Uo is the nominal voltage to earth (230V for single-phase) and Ze is the measured external loop impedance. For example, if Ze = 0.30Ω, PFC = 230 ÷ 0.30 = 766A. The PFC must not exceed the rated short-circuit capacity (Ics or Icu) of the consumer unit and protective devices. PFC must be measured (not just calculated) and recorded on the schedule of test results.

Why should you not perform live loop impedance tests on RCD-protected circuits?

A loop impedance tester injects a test current of typically 15A to 25A for a brief period to derive the loop impedance. This current flows in the line conductor but not the neutral — creating an imbalance that an RCD will detect as a residual current. A 30mA RCD will trip during the live loop impedance test. Use the calculated method instead: Zs = Ze + (r1 + r2), where r1 and r2 are obtained from the continuity tests. Some modern instruments use a very brief high-current pulse or a special RCD-compatible low-current method — check the instrument manufacturer's guidance.

What is the temperature correction factor for Zs?

The maximum Zs values in Tables 41.2 and 41.3 are given at the maximum permitted conductor operating temperature (typically 70°C for PVC-insulated cables). Tests are carried out at ambient temperature (typically 10°C to 25°C). BS 7671 Appendix 3 (referenced by Reg 411.4.203) provides the method for adjusting measured readings to account for conductor temperature. The GN3-recommended on-site pass criterion is: measured Zs ≤ 0.80 × tabulated limit. This 0.80 site factor provides a margin for conductor temperature rise under load. If the measured Zs does not exceed 0.80 × the Table 41.3 or Table 41.2 limit, the circuit complies. If the measured Zs exceeds 0.80 × the limit, apply the full Appendix 3 temperature adjustment before deciding whether to raise a defect.

Can you use a long-lead method for Ze measurement?

Yes. Where the supply cannot be safely isolated to measure Ze directly, Ze can be measured using the long-lead (or three-wire) method with the supply energised. A long test lead (typically 25m or more) is connected from the line terminal at the origin of the installation to an earth point remote from the installation. This allows the instrument to measure Ze without disconnecting the supply. The supply must be isolated to safely connect the test lead — only a brief disconnection is required. Always follow safe isolation procedures.

TESTING GUIDE

Loop Impedance Testing Guide: Zs & Ze Testing to BS 7671

The complete UK electrician's guide to earth fault loop impedance testing — measuring Ze and Zs, calculating prospective fault current, maximum Zs values for Type B and Type C MCBs, live vs calculated methods, temperature correction, and recording results.

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14 min readUpdated 2026-07-02Andrew Moore, Founder of Elec-Mate

Written and reviewed by Andrew Moore, founder of Elec-Mate, against BS 7671:2018+A4:2026, IET Guidance Note 3 and the IET On-Site Guide.

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What is the difference between Ze and Zs?

Ze is the external earth fault loop impedance — the part of the loop outside the installation (the supply transformer, the supply cable and the earth return). Zs is the total loop impedance at a point in a circuit, where Zs = Ze + (R1 + R2). Zs must not exceed the maximum for the protective device in BS 7671 Tables 41.2 to 41.4 so the circuit disconnects within the required time.

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Key Takeaways

1External loop impedance (Ze) is the impedance of the earth fault current loop outside the installation — from the supply transformer secondary winding, through the line conductor of the supply, back through the earthing system to the point of measurement. Typical values in the UK: TN-C-S (PME) 0.35Ω or less, TN-S 0.8Ω or less.
2Total loop impedance (Zs) is the sum of Ze plus the impedance of the line and CPC conductors within the installation: Zs = Ze + (r1 + r2). The measured or calculated Zs must not exceed the maximum permitted Zs for the protective device on that circuit.
3Prospective fault current (PFC) is the maximum current that would flow under fault conditions, calculated as the supply voltage divided by the total loop impedance: PFC = 230V ÷ Ze. This must not exceed the rated short-circuit capacity of the protective devices.
4Maximum Zs limits for MCBs and RCBOs (Types B, C, D) are in Table 41.3 under Reg 411.4.204; limits for fuses (BS 88-2, BS 88-3, BS 3036, BS 1362) are in Table 41.2 under Reg 411.4.201. The Zs value at the furthest point of every circuit must not exceed the permitted limit for that protective device. On-site, GN3 recommends using a pass criterion of measured Zs ≤ 0.80 × tabulated limit to allow for conductor temperature rise under load.
5Live Zs testing must not be performed on circuits protected by RCDs — the test current trips the RCD. Use the calculated method (Ze + r1 + r2 from continuity tests) for RCD-protected circuits.

01 · Testing Guide

What Is Earth Fault Loop Impedance?

Earth fault loop impedance is the total impedance of the path that fault current would follow in the event of a line-to-earth fault. Understanding this path is fundamental to verifying that protective devices will operate fast enough to prevent electric shock or fire — which is the basis of BS 7671 automatic disconnection of supply (ADS) protection.

When a fault occurs between a line conductor and an exposed-conductive-part, current flows from the supply transformer, along the line conductor of the distribution network, through the fault path within the installation, and back to the transformer via the earthing system. The impedance of this complete loop determines how much fault current flows, which in turn determines how quickly the protective device operates.

The fault loop path: Supply transformer secondary winding → line conductor of the distribution network → line terminal at the installation origin → line conductor of the circuit → fault point → CPC of the circuit → main earthing terminal → earthing conductor → earth electrode or PEN conductor → back to the transformer neutral point. The impedance of this loop must be low enough to allow sufficient fault current to operate the protective device within the required disconnection time.

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02 · Testing Guide

External Loop Impedance (Ze) Testing

Ze is measured at the origin of the installation — typically at the consumer unit or distribution board — before the main switch, using the supply earthing system. It represents the impedance of the fault loop external to the installation.

Method — main switch open: With the main switch open (to disconnect the installation's internal wiring), connect the loop impedance tester between the line and earth terminals at the origin. The test is performed live — the supply to the consumer unit remains energised. The instrument injects a test current and measures Ze directly.
TN-C-S (PME) earthing: Typical Ze ≤ 0.35Ω. The neutral and protective conductors are combined in the distribution network. The supply earth is provided by the PEN (protective earthed neutral) conductor. Most modern domestic properties in the UK have PME earthing.
TN-S earthing: Typical Ze ≤ 0.8Ω. Separate earth conductor in the supply cable sheath. The Ze is higher because the earth path impedance is greater. Found in older urban properties supplied via older distribution cables.
TT earthing: No Ze in the traditional sense — the installation has its own earth electrode with resistance Ra. Ra plus the electrode resistance of the supply transformer gives the total loop impedance. TT systems require RCD protection rather than relying on overcurrent protective devices for ADS.

Record Ze on the schedule of test results and on the face of the consumer unit schedule. Note the earthing arrangement (TN-C-S, TN-S, or TT) alongside the Ze value.

03 · Testing Guide

Total Loop Impedance (Zs) Testing

Zs is the total loop impedance at any point in the installation, including the internal circuit conductors. It is measured (or calculated) at the furthest point of each circuit — this is where the impedance is highest and therefore where the fault current will be lowest and the protective device takes longest to operate.

Live Zs test: With the circuit energised, connect the loop impedance tester between line and earth at the furthest accessible point of the circuit (e.g., the most remote socket outlet). The instrument injects a test current and measures Zs directly. This is the preferred method where practical and where RCDs do not prevent it.
Calculated Zs (dead method): Zs is calculated as Ze plus the r1+r2 value obtained from the continuity tests: Zs = Ze + (r1 + r2). This is used for RCD-protected circuits and where live testing is not practicable. The calculated value must be corrected to account for conductor temperature as described in BS 7671 Appendix 3 (Reg 411.4.203).
Temperature correction: Conductor resistance (and therefore impedance) increases with temperature. Tables 41.2 and 41.3 give Zs limits at the maximum conductor operating temperature. When testing at ambient, GN3 recommends the on-site pass criterion: measured Zs ≤ 0.80 × tabulated limit. Where the measured Zs exceeds 0.80 × the tabulated limit, apply the full Appendix 3 temperature adjustment method (Reg 411.4.203) before deciding compliance.

04 · Testing Guide

Prospective Fault Current (PFC) Calculation

Prospective fault current (PFC) is the maximum current that would flow in the event of a fault — either a line-to-earth fault (PEFC, prospective earth fault current) or a line-to-neutral fault (PSCC, prospective short-circuit current). Both must be determined and recorded, and both must be within the rated short-circuit breaking capacity of the protective devices.

PEFC (prospective earth fault current): PEFC = Uo ÷ Ze, where Uo = 230V and Ze is the measured external loop impedance. If Ze = 0.30Ω, PEFC = 230 ÷ 0.30 = 767A. Consumer unit MCBs and fuses must have a rated short-circuit capacity (Ics) equal to or greater than this value. Most domestic consumer units are rated at 6kA or 10kA breaking capacity, which covers typical UK PME systems.
PSCC (prospective short-circuit current): PSCC = Uo ÷ Zline, where Zline is the impedance of the line-to-neutral loop (line conductor and neutral conductor). Measured by connecting the instrument between line and neutral at the origin with the main switch open. Alternatively calculated from Ze and the line-to- neutral resistance. For a PME system with Ze = 0.30Ω, PSCC will be higher than PEFC because the line-to-neutral path has lower impedance than the line-to-earth path.
Record both values: BS 7671 requires both PEFC and PSCC to be recorded on the schedule of test results. The higher of the two determines the required breaking capacity of the protective devices.

05 · Testing Guide

Maximum Zs Values for Protective Devices

Maximum permitted Zs values for MCBs and RCBOs are in Table 41.3 (Reg 411.4.204); values for fuses are in Table 41.2 (Reg 411.4.201). These tabulated values are at the maximum conductor operating temperature. The Table 41.3 limits apply to both final circuits (0.4 s disconnection, Reg 411.3.2.2) and distribution circuits (5 s disconnection, Reg 411.3.2.3). On site, use the GN3 pass criterion: measured Zs ≤ 0.80 × tabulated limit (the cold-measured site limit).

Type B MCBs (BS EN 60898): Operate at 3–5× rated current (Ia = 5 × In). Maximum Zs at 230V using Reg 411.4.4 formula (Cmin × Uo / Ia, where Cmin = 0.95): 6A = 7.28Ω / 10A = 4.37Ω / 16A = 2.73Ω / 20A = 2.19Ω / 32A = 1.37Ω / 40A = 1.09Ω / 50A = 0.87Ω / 63A = 0.69Ω.
Type C MCBs (BS EN 60898): Operate at 5–10× rated current (Ia = 10 × In). Maximum Zs values are half those of Type B for the same rating: 6A = 3.64Ω / 16A = 1.37Ω / 32A = 0.68Ω. Type C MCBs are common for motor loads and circuits with high inrush currents.
Type D MCBs (BS EN 60898): Magnetic trip at 20 × In (Ia = 20 × In, Reg 411.4.204(c) Table 41.3(c)). Maximum Zs: 6 A = 1.82 Ω / 16 A = 0.68 Ω / 32 A = 0.34 Ω. GN3 site limits (0.80 ×): 6 A = 1.46 Ω / 16 A = 0.54 Ω / 32 A = 0.27 Ω. Very low Zs limits mean Type D devices are unsuitable for long cable runs — typically used for motor or welding loads.
BS 88-2 (gG/gM) and BS 88-3 fuses — Table 41.2 (Reg 411.4.201): BS 88-2 gG maximum Zs at 0.4 s disconnection: 16 A = 2.43 Ω / 32 A = 0.99 Ω / 63 A = 0.44 Ω. BS 88-3 (fuse system C): 16 A = 2.30 Ω / 32 A = 0.91 Ω. Note: Table 41.2 covers 0.4 s disconnection only (Reg 411.3.2.2 final-circuit requirement). Generally higher Zs is permitted than for MCBs of equivalent rating, reflecting the steeper fuse time-current characteristic.

On-site pass criterion (GN3): The measured Zs (at ambient temperature) should not exceed 0.80 × the tabulated Table 41.3 or Table 41.2 limit. This GN3-recommended site factor accounts for conductor temperature rise under normal load. If the measured Zs is between 0.80 × and 1.00 × the tabulated limit, apply the full Appendix 3 temperature adjustment (Reg 411.4.203) before deciding compliance. Where thermosetting insulation is sized per Reg 512.1.5, use 70°C thermoplastic temperatures for Zs assessment (per Note 3 to Reg 411.4.201).

TT systems — Reg 411.5.3 Table 41.5

In TT systems, automatic disconnection is provided by an RCD and Zs compliance is verified using the formula Zs ≤ Uo / (5 × IΔN), where Uo = 50 V (touch voltage limit) and IΔN is the RCD rated residual operating current. Maximum permitted Zs values from Table 41.5 (Reg 411.5.3):

30 mA RCD: Zs ≤ 1667 Ω
100 mA RCD: Zs ≤ 500 Ω
300 mA RCD: Zs ≤ 167 Ω
500 mA RCD: Zs ≤ 100 Ω

For RCDs rated ≤ 100 mA, the earth electrode resistance Ra must not exceed 200 Ω. These Zs limits are far higher than TN system limits — a TT installation is not verified against Table 41.3 MCB Zs limits.

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06 · Testing Guide

Live Zs Testing vs the Calculated (Dead) Method

Both live testing and the calculated method are accepted by BS 7671. Each has advantages and limitations.

Live testing (preferred where RCDs permit): Measures the actual Zs of the complete circuit including all connections and joints. Detects deteriorated connections, corroded terminals, and high-resistance joints that would not be found by the calculated method. More accurate than calculation.
Live testing limitations: Cannot be used on RCD-protected circuits without a special RCD-compatible instrument mode. Requires the supply to be energised and appropriate live working precautions. Test current (typically 15–25A) may be problematic on sensitive circuits.
Calculated method (Zs = Ze + r1 + r2): Safe for use on all circuits. Requires accurate Ze measurement and precise r1+r2 values from continuity tests. Good practice where RCDs prevent live testing. Must be corrected for temperature.
Calculated method limitations: Does not detect high-resistance joints unless r2 is abnormally high. If a connection deteriorates between test day and a future fault, the calculated value may no longer reflect reality. Record on the schedule whether the result is measured or calculated.

07 · Testing Guide

Instrument Settings for Loop Impedance Testing

Loop impedance instruments (and multifunction testers with a loop impedance function) have settings that must be correctly configured before testing.

Standard vs RCD-compatible mode: Select RCD-compatible (low-current or "no-trip") mode when testing circuits with 30mA RCDs. This mode uses a very brief pulse or lower test current that does not trip the RCD. The resolution is lower than the standard high-current mode — note the limitation on the test record.
2-wire vs 4-wire measurement: For Ze measurement at the origin, use the standard 2-wire (L-PE) connection. For accurate Zs measurement at distant points, some instruments support a 3-wire connection using a remote reference lead to compensate for test lead resistance.
Voltage sensing: Confirm the instrument is detecting the correct supply voltage before initiating the test. An instrument set to the wrong voltage range will give an incorrect result. Always check the supply voltage displayed by the instrument before pressing the test button.

08 · Testing Guide

Recording Loop Impedance Results

Loop impedance results are recorded on the Schedule of Test Results (part of the EICR or Electrical Installation Certificate) and on the consumer unit schedule of circuits.

Ze: Record the measured Ze value and the earthing system type (TN-C-S, TN-S, or TT) at the installation origin.
Zs per circuit: Record the measured or calculated Zs value at the furthest point of each circuit. Indicate whether the result is measured (M) or calculated (C). Note if an RCD-compatible instrument mode was used.
PFC: Record the prospective fault current (both PEFC and PSCC) at the origin of the installation.
Instrument details: Record the make, model, serial number, and calibration date of the loop impedance tester on the certificate.

Ze and Zs Explained: Earth Loop Impedance Testing Guide

Ze vs Zs made simple: what each means, how to test earth fault loop impedance, and the maximum Zs values to compare against, to BS 7671.

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09 · Testing Guide

For Electricians: Loop Impedance Testing in Practice

Accurate loop impedance testing underpins the safety of the entire electrical installation. An Zs value above the maximum permitted limit means the protective device will not disconnect fast enough under a fault condition — a potentially lethal situation.

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Frequently Asked Questions About Loop Impedance Testing

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Loop Impedance Testing Guide: Zs & Ze Testing to BS 7671

What is the difference between Ze and Zs?

Key Takeaways

What Is Earth Fault Loop Impedance?

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External Loop Impedance (Ze) Testing

Total Loop Impedance (Zs) Testing

Prospective Fault Current (PFC) Calculation

Maximum Zs Values for Protective Devices

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Live Zs Testing vs the Calculated (Dead) Method

Instrument Settings for Loop Impedance Testing

Recording Loop Impedance Results

Ze and Zs Explained: Earth Loop Impedance Testing Guide

For Electricians: Loop Impedance Testing in Practice

Auto-Check Zs Against Device Limits

Temperature Correction Built In

Frequently Asked Questions About Loop Impedance Testing

What is the difference between Ze and Zs?

What are typical Ze values for UK installations?

What are the maximum Zs values for Type B and Type C MCBs?

How do you calculate prospective fault current?

Why should you not perform live loop impedance tests on RCD-protected circuits?

What is the temperature correction factor for Zs?

Can you use a long-lead method for Ze measurement?

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