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 411.4.202). Ia = 5 × In for Type B, 10 × In for Type C, 20 × In for Type D (0.4 s). 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, 0.4 s): 6 A = 1.82 Ω, 16 A = 0.68 Ω, 32 A = 0.34 Ω. Tabulated values apply at maximum conductor operating temperature. For readings taken at ambient temperature, BS 7671 Appendix 3 gives the acceptance criterion: measured Zs ≤ 0.80 × tabulated limit. Fuse Zs limits for 0.4 s 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. Fuse limits for 5 s are in Table 41.4 (Reg 411.4.203).

How do you calculate prospective fault current?

Prospective earth fault current at the origin of the installation is calculated using PEFC = 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, PEFC = 230 ÷ 0.30 = 767A. The PFC must not exceed the rated short-circuit capacity (Ics or Icu) of the consumer unit and protective devices. Reg 643.7.3.201 requires the prospective short-circuit current and prospective earth fault current to be measured, calculated or determined by another method at the origin and other relevant points. The model forms carry a single "Prospective fault current, Ipf" box under supply characteristics — Appendix 14 states that in a single-phase system the prospective fault current is the greater of the line-to-neutral and line-to-earth values.

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 apply when the line conductors are at their maximum permitted operating temperature (Table 52.2 — typically 70°C for thermoplastic-insulated cables) and the CPCs are at the assumed initial temperature of Tables 54.2 to 54.5. Tests are carried out at ambient temperature. Note 2 to those tables says that if the conductors are at a different temperature when tested the reading should be adjusted accordingly, and refers to Appendix 3. Appendix 3 states that Reg 411.4.4 is considered to be met when the measured Zs ≤ 0.8 × (Uo × Cmin ÷ Ia) — in other words, measured Zs ≤ 0.80 × the tabulated limit, the 0.8 factor allowing for the rise in conductor resistance with load current. If the measured Zs exceeds 0.80 × the limit, work the figures through properly against the actual conductor temperature 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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The short answer

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

01External 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.
02Total 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.
03Prospective fault current (PFC) is the maximum current that would flow under fault conditions. At the origin the prospective earth fault current is PEFC = Uo ÷ Ze (230V ÷ Ze); the prospective short-circuit current (PSCC) is taken across line and neutral. Reg 434.1 allows PFC to be determined by calculation, measurement or enquiry, and the greater of the two values must not exceed the rated short-circuit capacity of the protective devices.
04Maximum Zs limits for MCBs and RCBOs (Types B, C, D) are in Table 41.3 under Reg 411.4.202; 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. BS 7671 Appendix 3 gives the on-site acceptance criterion for readings taken at ambient temperature: measured Zs ≤ 0.80 × tabulated limit, the 0.8 factor allowing for conductor temperature rise under load.
05Live Zs testing on an RCD-protected circuit will trip the RCD unless the instrument's no-trip (RCD-compatible) mode is used — the test current flows in the line conductor but not the neutral. Otherwise use the calculated method (Ze + R1 + R2 from continuity tests).

01 · Testing Guide

What Is Earth Fault Loop Impedance?

Jump to the Zs calculator — put in your Ze and R1+R2 and it works out Zs, the maximum permitted for your protective device and the fault current. Free, no sign-up.

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 off, means of earthing disconnected: GN3 2.29 gives the sequence — turn the main switch off and secure it with a safety locking device, then disconnect the means of earthing from the main earthing terminal and from extraneous-conductive-parts so that no parallel paths remain. Connect the loop impedance tester between the incoming line conductor and the isolated means of earthing. The supply side remains energised; the instrument injects a test current and measures Ze directly. If parallel paths through bonded metalwork are left in place the reading will be artificially low and is not a true Ze.
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. Reg 411.5.2 lists both an RCD and an overcurrent protective device as permitted devices for a TT system, an RCD being preferred — an overcurrent device may only be relied on where a suitably low Zs is permanently and reliably assured.

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 value must be adjusted for conductor temperature — Note 2 to Tables 41.2 and 41.3 refers you to BS 7671 Appendix 3 for the method. Our earth fault loop impedance calculation guide works through the full method with worked examples.
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. For readings taken at ambient temperature, BS 7671 Appendix 3 gives the acceptance criterion: measured Zs ≤ 0.8 × (Uo × Cmin ÷ Ia), i.e. measured Zs ≤ 0.80 × the tabulated limit. Where the measured Zs exceeds 0.80 × the tabulated limit, work the Appendix 3 figures through against the actual conductor temperature before deciding compliance.

04 · Testing Guide

Work Out Zs and Check It Against the Device Limit

Free to use, no sign-up. Enter the Ze you measured at the origin and the R1+R2 from your continuity test — or switch to measured mode and enter the Zs straight off the instrument. It returns Zs = Ze + (R1 + R2), the maximum permitted Zs for the protective device you select, the Appendix 3 site limit (0.80 × tabulated) and the resulting fault current. TT circuits are handled on the R_A × IΔn ≤ 50 V basis of Reg 411.5.3.

Earth Fault Loop Impedance Calculator

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Earthing System

Measurement Method

Ze (External Impedance)

Measured at origin

R1+R2 (Circuit Resistance)

To furthest point

Required Disconnection Time

0.4 s applies to a TN final circuit at 230 V rated up to 63 A with one or more socket-outlets, and up to 32 A supplying only fixed connected current-using equipment (Reg 411.3.2.2, Table 41.1). 5 s is permitted for a TN distribution circuit and any circuit not covered by 411.3.2.2 (Reg 411.3.2.3). In a TT system the corresponding allowance is 1 s (Reg 411.3.2.4).

Protection Device (for compliance check)

Device Type

Curve Type

Empty: Ze (External Impedance), R1+R2 (Circuit Resistance), Curve Type.

Earth Fault Loop Formulas

Zs = Ze + (R1 + R2)

Zs= Earth fault loop impedance (Ω)

Ze= External earth fault loop impedance (Ω)

R1+R2= Line + CPC resistance to furthest point (Ω)

05 · 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.
Determine both values: Reg 643.7.3.201 requires both the prospective short-circuit current and the prospective earth fault current to be measured, calculated or determined by another method. Appendix 14 confirms that in a single-phase system the prospective fault current is the greater of the two, and it is that value which is entered in the single "Prospective fault current, Ipf" box on the certificate and which determines the required breaking capacity of the protective devices.

06 · Testing Guide

Maximum Zs Values for Protective Devices

Maximum permitted Zs values for MCBs and RCBOs are in Table 41.3 (Reg 411.4.202); values for fuses are in Table 41.2 for 0.4 s (Reg 411.4.201) and Table 41.4 for 5 s (Reg 411.4.203). These tabulated values are at the maximum conductor operating temperature. Table 41.3 covers both final circuits (0.4 s disconnection, Reg 411.3.2.2) and distribution circuits (5 s disconnection, Reg 411.3.2.3): Types B and C print a single row valid for both times, while Type D — Table 41.3(c) — prints separate 0.4 s and 5 s rows. For readings taken at ambient temperature, use the BS 7671 Appendix 3 criterion: measured Zs ≤ 0.80 × tabulated 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 band 10–20 × In; Table 41.3(c) (Reg 411.4.202) uses Ia = 20 × In for the 0.4 s row and 10 × In for the 5 s row. Maximum Zs for 0.4 s: 6 A = 1.82 Ω / 16 A = 0.68 Ω / 32 A = 0.34 Ω (the 5 s figures are double these). Appendix 3 site limits (0.80 ×) for 0.4 s: 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) — the 5 s fuse values are in Table 41.4. Fuse limits sit between the MCB curves rather than always above them: at 16 A, BS 88-2 gG allows 2.43 Ω against 2.73 Ω for a Type B and 1.37 Ω for a Type C.

On-site pass criterion (BS 7671 Appendix 3): The measured Zs (at ambient temperature) should not exceed 0.80 × the tabulated Table 41.3 or Table 41.2 limit. The 0.8 factor accounts for the increase in conductor resistance with temperature under load current, and Appendix 3 states that Reg 411.4.4 is considered met when this is satisfied. If the measured Zs is between 0.80 × and 1.00 × the tabulated limit, work the Appendix 3 figures through against the actual conductor temperature before deciding compliance. Where thermosetting insulation is sized per Reg 512.1.5, use 70°C thermoplastic temperatures for Zs assessment (Note 3 to Tables 41.2 and 41.3).

TT systems — Reg 411.5.3 Table 41.5

Where an RCD provides fault protection in a TT system, Reg 411.5.3 requires R_A × IΔn ≤ 50 V, where R_A is the sum of the resistances of the earth electrode and the protective conductor connecting it to the exposed-conductive-parts. The requirement is met if the loop impedance meets Table 41.5, i.e. Zs ≤ 50 V ÷ IΔn. 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

Note 2 to Table 41.5 (against the 30 mA and 100 mA rows) says the resistance of the installation earth electrode should be as low as practicable, and that a value exceeding 200 Ω may not be stable — see Reg 542.2.4. These Zs limits are far higher than TN system limits, so a TT circuit relying on an RCD for fault protection is not verified against the Table 41.3 MCB Zs limits.

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07 · 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.

08 · 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.

09 · 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: Determine both the prospective earth fault current and the prospective short-circuit current at the origin, and enter the greater of the two in the "Prospective fault current, Ipf" box under supply characteristics.
Instrument details: Record the make, model, serial number, and calibration date of the loop impedance tester on the certificate.

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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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10 · 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.

Auto-Check Zs Against Device Limits

The Elec-Mate testing app automatically compares your recorded Zs against the maximum permitted value for the protective device type and rating on each circuit. Red-flags non-compliant circuits before you leave site.

Temperature Correction Built In

The app applies the correct temperature correction factor based on the cable insulation type selected for each circuit, so you can check compliance at operating temperature without manual calculation.

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

Work Out Zs and Check It Against the Device Limit

Earth Fault Loop Impedance Calculator

Earthing System

Measurement Method

Required Disconnection Time

Protection Device (for compliance check)

Prospective Fault Current (PFC) Calculation

Maximum Zs Values for Protective Devices

Record test results hands-free on site

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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Record Zs Results and Auto-Check Against Device Limits