What does Zs mean in electrical testing?

Zs stands for earth fault loop impedance, measured in ohms. It is the total impedance of the complete fault loop path that current would follow during an earth fault. The loop goes from the point of fault along the circuit protective conductor (CPC) to the main earthing terminal, through the earthing conductor to the means of earthing (earth electrode or supply company earth), through the earth return path back to the transformer star point, through the transformer winding, along the phase conductor of the supply cable and installation to the point of fault. Every component in this loop adds impedance. Zs is critical because it determines how much fault current flows during an earth fault — and therefore how quickly the protective device operates to disconnect the supply. A lower Zs means a higher fault current and faster disconnection. A higher Zs means a lower fault current and slower disconnection. BS 7671 sets maximum Zs values for each type and rating of protective device to ensure the disconnection time is fast enough to prevent electric shock.

How is Zs measured on site?

Zs is measured using a multifunction tester or a dedicated loop impedance meter. The instrument is connected between the phase and earth terminals at the furthest point of the circuit being tested (for example, at the last socket on a ring or radial circuit). The instrument injects a brief test current, measures the voltage drop, and calculates the impedance using Ohm's law. The result is displayed in ohms. Zs must be measured at every circuit during initial verification and at representative points during periodic inspection. The test should be performed with the circuit energised and all connections made. A no-trip loop impedance test mode is available on most modern instruments — this is essential for circuits protected by RCDs, because a standard loop test may trip the RCD. The measured Zs must be compared with the maximum Zs value from BS 7671 Chapter 41 — Table 41.2 for fuse-protected circuits (Reg 411.4.201) or Table 41.3(a)/(b)/(c) for Type B/C/D MCB-protected circuits (Reg 411.4.204).

What is Ze and how is it different from Zs?

Ze is the external earth fault loop impedance — the impedance of the supply-side portion of the fault loop, measured at the origin of the installation with the main earthing conductor disconnected from the MET (main earthing terminal). Ze includes the impedance of the supply transformer, the supply phase conductor, and the supply earth return path. It does not include any impedance from the installation itself. Zs is the total earth fault loop impedance at any point in the installation, which includes Ze plus the impedance of the installation cables: Zs = Ze + (R1+R2), where R1 is the resistance of the phase conductor and R2 is the resistance of the protective conductor from the origin to the point of measurement. Ze is typically 0.2 to 0.8 ohms for a TN-S supply and 0.35 to 0.8 ohms for a TN-C-S (PME) supply. The DNO provides a maximum Ze value for each supply.

What is R1+R2 and how do I measure it?

R1+R2 is the combined resistance of the phase conductor (R1) and the circuit protective conductor (R2) from the origin of the installation (the distribution board) to the furthest point of the circuit. R1+R2 is measured during the dead tests as part of the continuity of protective conductors test (BS 7671 Regulation 643.2). The standard method uses a low-resistance ohmmeter: the phase and CPC are linked together at the distribution board, and the resistance is measured at the furthest point of the circuit. The reading gives R1+R2 directly. For ring final circuits, the R1+R2 value at each socket should be approximately one-quarter of the total end-to-end resistance (for a properly wired ring). R1+R2 depends on the cable length and the conductor cross-sectional area — longer circuits and thinner conductors give higher R1+R2 values.

Why do I need to apply temperature correction to Zs?

The resistance of copper and aluminium conductors increases as temperature rises. When you measure R1+R2 during dead testing, the cable is cold (approximately 10 to 20 degrees Celsius ambient). When you measure Zs during live testing, the cable may be slightly warm from the test current but is still not at its normal operating temperature. However, when a real fault occurs, the cable will be at its normal operating temperature (which could be 70 degrees Celsius for PVC insulated cables). At this higher temperature, the resistance — and therefore Zs — is higher than the cold measured value. BS 7671 Appendix 3 provides a correction factor of 1.2 for PVC cables (to convert from 10 degrees to 70 degrees). When comparing a calculated or measured Zs with the maximum permitted value, you must apply this correction to ensure the Zs will still be within limits at the worst-case operating temperature. The rule is: Zs (corrected) = Zs (measured) x 1.2 for thermoplastic cables or x 1.28 for thermosetting cables.

Where do I find the maximum Zs values in BS 7671?

The maximum Zs values are listed in BS 7671 Chapter 41. Under BS 7671:2018+A4:2026: Table 41.2 (Reg 411.4.201) covers all fuse types — BS 88-2 gG/gM, BS 88-3, BS 3036, and BS 1362 fuses all appear in this single table. Table 41.3(a) (Reg 411.4.204) covers Type B MCBs and RCBOs; Table 41.3(b) covers Type C; Table 41.3(c) covers Type D. Each sub-table lists the maximum Zs for each protective device rating (6 A, 10 A, 16 A, 20 A, 32 A, etc.). Importantly, MCB table values in Table 41.3 apply to both 0.4 s (final circuit) and 5 s (distribution circuit) disconnection — the instantaneous magnetic trip governs MCB operation, so a single Zs limit covers both circuit types. GN3 (Guidance Note 3: Inspection and Testing) provides 80% of maximum Zs as the cold-measured site limit — for example, 1.37 Ω maximum becomes 1.10 Ω site limit for a 32 A Type B MCB. The IET On-Site Guide also reproduces these values. When using the tables, remember the tabulated maximum Zs values assume conductors at their maximum operating temperature — so you must apply temperature correction when comparing values measured at ambient temperature.

What happens if Zs is too high?

If the measured Zs at any point on a circuit exceeds the maximum Zs value from the BS 7671 tables for that protective device, the protective device may not operate quickly enough during an earth fault. This means the fault could persist long enough to cause electric shock (the whole purpose of the disconnection time limit is to prevent lethal electric shock). On an EICR, this would be classified as a C2 (Potentially Dangerous) observation — or C1 (Danger Present) if the risk is immediate. The remedial action depends on the cause: if Zs is high because R1+R2 is high (long cable run, thin CPC), the solution may be to use a cable with a larger CPC, reduce the circuit length, or change the protective device to one with a higher maximum Zs (for example, fitting an RCD or RCBO, which has a much higher maximum Zs of approximately 1667 ohms at 30 mA). If Zs is high because Ze is high, the issue is with the supply and should be reported to the DNO.

TECHNICAL GUIDE

Earth Fault Loop Impedance: The Zs Calculation Explained

Zs determines whether the protective device will disconnect fast enough to prevent electric shock. If it is too high, the circuit is unsafe. This guide walks through the formula, Ze, R1+R2, temperature correction, the maximum Zs tables from BS 7671, and worked examples for real circuits.

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15 min readUpdated 2026-06-10Andrew 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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How do you calculate earth fault loop impedance (Zs)?

Zs = Ze + (R1 + R2): the external loop impedance Ze (measured at the origin, or the DNO declared maximum — 0.35 Ω for TN-C-S/PME, 0.8 Ω for TN-S) plus the resistance of the circuit line and protective conductors (R1 + R2). The measured Zs must not exceed the maximum value for the protective device in BS 7671 Tables 41.2–41.6, and good practice applies the 80% rule to allow for conductors heating during a fault. If Zs is too high the device will not disconnect within the required 0.4 s (final circuits up to 63 A) or 5 s (distribution circuits) per Reg 411.3.2.2 and 411.3.2.3.

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Key Takeaways

1Earth fault loop impedance (Zs) is the total impedance of the fault loop path from the point of fault, through the circuit protective conductor (CPC), back to the transformer, and via the phase conductor to the point of fault: Zs = Ze + (R1+R2).
2Zs must be low enough to ensure the protective device (MCB, RCBO, or fuse) operates within the required disconnection time — 0.4 seconds for final circuits and 5 seconds for distribution circuits under BS 7671.
3The measured Zs at the furthest point of each circuit must not exceed the maximum Zs value listed in BS 7671 Chapter 41 — Table 41.2 for fuses (BS 88, BS 3036, BS 1362) and Table 41.3(a)/(b)/(c) for Type B, C, and D MCBs respectively.
4Temperature correction must be applied when comparing designed (calculated) Zs values with the maximum permitted values, because conductor resistance increases as temperature rises during normal operation.
5Elec-Mate includes a Zs calculator that checks measured values against the BS 7671 maximum Zs tables, plus 50+ other calculators including cable sizing, voltage drop, PFC, max demand, and adiabatic equation.
6MCB Zs limits (BS 7671:2018+A4:2026 Table 41.3) cover both final circuits (0.4 s) and distribution circuits (5 s) — a single table entry applies because the instantaneous magnetic trip governs, not the thermal element. For installations with BS 88 or BS 3036 fuses, use Table 41.2 (Reg 411.4.201): the 32 A BS 88-2 gG limit is 0.99 Ω, compared with 1.37 Ω for a 32 A Type B MCB — a lower limit that catches circuits which would otherwise pass.

01 · Technical Guide

What Is Earth Fault Loop Impedance?

Earth fault loop impedance (Zs) is the total impedance of the path that fault current follows during an earth fault. It is measured in ohms and is one of the most important values in electrical installation design and testing.

When a fault occurs between a live conductor and an exposed-conductive-part (such as a metal appliance enclosure or a conduit), the fault current flows in a complete loop: from the point of fault, along the circuit protective conductor (CPC) back to the distribution board, through the main earthing conductor to the means of earthing, through the earth return path back to the supply transformer, through the transformer winding, and along the phase conductor of the supply and installation cables back to the point of fault.

The total impedance of this loop determines the magnitude of the fault current — and therefore how quickly the protective device (MCB, RCBO, or fuse) operates to disconnect the supply. A lower Zs means higher fault current and faster disconnection. BS 7671 sets maximum Zs values for each protective device type and rating to ensure the disconnection time is fast enough to prevent electric shock (0.4 seconds for final circuits and 5 seconds for distribution circuits).

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02 · Technical Guide

The Formula: Zs = Ze + (R1+R2)

The earth fault loop impedance at any point in the installation is calculated using:

Zs = Ze + (R1+R2)

Zs = total earth fault loop impedance at the point of measurement (ohms)
Ze = external earth fault loop impedance — the supply-side portion (ohms)
R1 = resistance of the phase conductor from the origin to the point (ohms)
R2 = resistance of the protective conductor from the origin to the point (ohms)

This formula shows that Zs increases as you move further from the origin of the installation. At the origin, R1+R2 is zero (or negligible), so Zs approximately equals Ze. At the furthest point of a long circuit, R1+R2 can be significant, making Zs much higher than Ze. This is why the Zs measurement is taken at the furthest point of the circuit — it is the worst case.

The formula can be used in two ways: during design (calculating Zs from known cable data and Ze to verify that the maximum Zs will not be exceeded) and during testing (measuring Ze and R1+R2 separately, then adding them to predict Zs, or measuring Zs directly with a loop impedance tester to verify the result).

03 · Technical Guide

Ze Explained: The External Loop Impedance

Ze is the external earth fault loop impedance — the portion of the fault loop that is outside the installation, belonging to the supply company. It includes the impedance of the supply transformer winding, the phase conductor of the supply cable from the transformer to the property, and the earth return path (which varies depending on the earthing arrangement).

TN-S (separate earth): Ze is typically 0.2 to 0.8 ohms. The earth return path is via the metallic cable sheath or a separate earth conductor in the supply cable. This provides a reliable, low-impedance earth return.
TN-C-S (PME): Ze is typically 0.35 to 0.8 ohms. The earth return path uses the combined neutral/earth conductor (PEN) of the supply cable. This is the most common supply arrangement in the UK for newer installations.
TT (earth electrode): Ze can be very high — typically 20 ohms or more — because the earth return path is through the general mass of earth via an earth rod. TT systems almost always require RCD protection because the high Ze means Zs will exceed the maximum values for MCBs and fuses alone.

Ze is measured at the origin of the installation with the main earthing conductor disconnected from the MET (main earthing terminal), so that only the external portion of the loop is measured. This value is recorded on the EICR and EIC. The DNO also declares a maximum Ze value for each supply — the measured value should not exceed this.

04 · Technical Guide

R1+R2 Explained: The Installation Loop Impedance

R1+R2 is the combined resistance of the phase conductor (R1) and the circuit protective conductor (R2) from the origin of the installation to the furthest point of the circuit. It represents the installation's contribution to the total earth fault loop impedance.

How R1+R2 Is Measured

During dead testing, the phase and CPC are linked together at the distribution board (using a long lead or a wander lead method). A low-resistance ohmmeter is then used to measure the resistance at the furthest point of the circuit. This gives R1+R2 directly. For ring final circuits, the figure-of-eight test method measures R1+R2 at each socket to verify the ring is continuous and correctly wired.

What Affects R1+R2

R1+R2 depends on the cable length, the conductor cross-sectional area, and the conductor material. Longer circuits have higher R1+R2. Thinner conductors (1.0 mm² CPC in a 2.5 mm² T+E cable) have higher R2 than thicker ones. The R1+R2 value per metre for common cables is published in the IET On-Site Guide (Table I3) — for example, 2.5/1.5 mm² T+E cable has an R1+R2/m of 19.51 milliohms per metre at 20°C.

A high R1+R2 value means a high Zs — which may cause the circuit to exceed the maximum Zs for the protective device. This is particularly common on long lighting circuits with 1.0 mm² conductors or long radial circuits to remote sockets. The solution is to use a cable with a larger CPC, reduce the circuit length, or use a protective device with a higher maximum Zs (such as an RCD or RCBO).

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05 · Technical Guide

Temperature Correction: Why It Matters

Conductor resistance increases with temperature. Copper has a positive temperature coefficient — at 70°C (the maximum operating temperature for PVC insulated cables), the resistance is approximately 20% higher than at 10°C (a typical ambient temperature when cold measurements are taken).

Temperature Correction Factors (BS 7671 Appendix 3)

PVC (thermoplastic) cables: Multiply measured R1+R2 by 1.20 to correct from ambient to 70°C operating temperature.
XLPE (thermosetting) cables: Multiply measured R1+R2 by 1.28 to correct from ambient to 90°C operating temperature.

When you calculate Zs at the design stage, you must use the corrected R1+R2 value:

Zs (design) = Ze + (R1+R2) x 1.20

For PVC cables. Use 1.28 for XLPE cables.

When comparing a measured Zs (taken during live testing at ambient temperature) against the BS 7671 maximum values, you should check that the measured value does not exceed 80% of the tabulated maximum. GN3 (Guidance Note 3: Inspection and Testing) formalises this as a 0.80 site factor — the cold-measured limit is 80% of the BS 7671 maximum Zs. For example, the 32 A Type B MCB maximum of 1.37 Ω becomes a GN3 site limit of 1.10 Ω. The IET On-Site Guide also reproduces these adjusted values. This accounts for the fact that measured Zs will increase when the cable reaches operating temperature under normal load.

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06 · Technical Guide

Maximum Zs Values from BS 7671

BS 7671 Chapter 41 lists the maximum earth fault loop impedance (Zs) for each type and rating of protective device. Under BS 7671:2018+A4:2026, Table 41.2 (Reg 411.4.201) covers fuses (BS 88-2, BS 88-3, BS 3036, and BS 1362), and Table 41.3(a)/(b)/(c) (Reg 411.4.204) covers Type B, C, and D MCBs respectively. The MCB table values apply to both 0.4 s (final circuit) and 5 s (distribution circuit) disconnection, because the instantaneous magnetic trip governs — a single Zs column covers both circuit types.

Maximum Zs for MCBs — BS 7671:2018+A4:2026 Table 41.3 (0.4 s and 5 s, 230 V)

Rating (A)	Type B (ohms)	Type C (ohms)	Type D (ohms)
6	7.28	3.64	1.82
10	4.37	2.19	1.09
16	2.73	1.37	0.68
20	2.19	1.09	0.55
32	1.37	0.68	0.34
40	1.09	0.55	0.27
50	0.87	0.44	0.22

For circuits protected by a 30 mA RCD (in addition to the overcurrent device), the maximum Zs is approximately 1667 ohms (calculated from 50V / 0.03A). This much higher limit means that circuits with high Zs values — such as long runs on TT systems — can still achieve the required disconnection time when RCD protection is provided.

GN3 (Guidance Note 3: Inspection and Testing) provides cold-measured site limits at 80% of the BS 7671 maximum Zs values — for example, 1.37 Ω becomes 1.10 Ω for a 32 A Type B MCB. These GN3 0.80-factor limits are the values to compare your measured Zs against on site. The IET On-Site Guide also reproduces these adjusted limits.

BS 88 and BS 3036 fuses: lower Zs limits (Table 41.2)

Older installations protected by BS 88-2 gG fuses have stricter maximum Zs limits than MCBs of the same rating (Reg 411.4.201, Table 41.2). At 32 A, a BS 88-2 gG fuse has a maximum Zs of 0.99 Ω, compared with 1.37 Ω for a 32 A Type B MCB. On EICRs, circuits that pass the MCB limit may fail if the protective device is a fuse — always identify the device type before selecting the table.

07 · Technical Guide

Worked Examples

Example 1: Domestic Ring Final Circuit

Circuit: 32 A ring final circuit, Type B MCB
Cable: 2.5/1.5 mm² T+E, total ring length 50 m
Ze (measured): 0.35 ohms (TN-C-S supply)
R1+R2 (measured at furthest socket): 0.56 ohms
Zs (calculated): 0.35 + 0.56 = 0.91 ohms
Maximum Zs (32 A Type B, 0.4s): 1.37 ohms
80% of maximum: 1.10 ohms
Result: 0.91 ohms is less than 1.10 ohms — PASS

Example 2: Lighting Circuit (Long Run)

Circuit: 6 A Type B MCB, lighting radial
Cable: 1.5/1.0 mm² T+E, 35 m run
Ze (measured): 0.45 ohms
R1+R2/m (from On-Site Guide): 30.20 milliohms/m
R1+R2 (calculated at 20°C): 0.03020 x 35 = 1.057 ohms
R1+R2 (corrected to 70°C): 1.057 x 1.20 = 1.268 ohms
Zs (design): 0.45 + 1.268 = 1.718 ohms
Maximum Zs (6 A Type B, 0.4s): 7.28 ohms
Result: 1.718 ohms is well within 7.28 ohms — PASS

Example 3: Shower Circuit (Close to Limit)

Circuit: 40 A Type B MCB, shower radial
Cable: 10/4 mm² T+E, 18 m run
Ze (measured): 0.50 ohms
R1+R2/m: 6.44 milliohms/m (10/4 mm² from tables)
R1+R2 (calculated): 0.00644 x 18 = 0.116 ohms
R1+R2 (corrected): 0.116 x 1.20 = 0.139 ohms
Zs (design): 0.50 + 0.139 = 0.639 ohms
Maximum Zs (40 A Type B, 0.4s): 1.09 ohms
80% of maximum: 0.87 ohms
Result: 0.639 ohms is within 0.87 ohms — PASS

Note: If Ze were higher (for example, 0.80 ohms on a property far from the transformer), Zs would be 0.939 ohms — still within the maximum but above the 80% rule of thumb. This shows why Ze values matter for high-current circuits.

08 · Technical Guide

Common Mistakes in Zs Calculations

Forgetting temperature correction. Comparing a cold-measured Zs directly with the BS 7671 maximum values without applying the 1.20 correction factor can give a false pass. The cable resistance at operating temperature is higher — if the measured value is close to the limit, it may exceed it when hot.
Using the wrong table. Using the maximum Zs value for a Type B MCB when the circuit is actually protected by a Type C MCB (or vice versa) gives the wrong limit. Type C MCBs require a higher fault current to trip, so their maximum Zs values are lower (more restrictive).
Not testing at the furthest point. Zs must be measured at the furthest point of the circuit — not at the first socket or a convenient mid-point. The furthest point has the highest R1+R2 and therefore the highest Zs.
Ignoring the CPC size. In a 2.5/1.5 mm² T+E cable, R2 (the CPC resistance) is higher than R1 (the phase resistance) because the CPC is 1.5 mm² while the phase is 2.5 mm². The CPC size has a significant impact on R1+R2 and therefore Zs.
Not considering the PFC relationship. Zs and PFC are inversely related. A high Zs means a low earth fault current. While this may still be within limits for disconnection time (especially with RCD protection), it should be considered alongside the adiabatic equation check.

09 · Technical Guide

Using the Elec-Mate Zs Calculator

Elec-Mate includes a dedicated Zs calculator as part of its suite of 50+ electrical calculators for UK electricians:

Automatic Zs Verification

Enter Ze, R1+R2, the protective device type (B, C, or D), and the device rating. The calculator applies temperature correction, computes Zs, and checks it against the BS 7671 maximum values — giving a clear pass or fail result.

Built-In BS 7671 Tables

All the maximum Zs values from BS 7671:2018+A4:2026 are built into the calculator — Table 41.2 for fuses (BS 88, BS 3036, BS 1362) and Table 41.3(a)/(b)/(c) for Type B, C, and D MCBs. Select the device type and rating and the correct maximum Zs is applied automatically, including the GN3 0.80 cold-measured site limit.

50+ Calculators in One App

Zs is one of over 70+ calculators on Elec-Mate. Others include cable sizing, voltage drop, PFC, max demand, adiabatic equation, conduit fill, trunking fill, power factor, diversity factor, and three-phase power.

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Frequently Asked Questions About Earth Fault Loop Impedance

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Earth Fault Loop Impedance: The Zs Calculation Explained

How do you calculate earth fault loop impedance (Zs)?

Key Takeaways

What Is Earth Fault Loop Impedance?

Get the BS 7671 A4:2026 Cheat Sheet — free

The Formula: Zs = Ze + (R1+R2)

Ze Explained: The External Loop Impedance

R1+R2 Explained: The Installation Loop Impedance

How R1+R2 Is Measured

What Affects R1+R2

Check Zs against BS 7671 tables instantly

Temperature Correction: Why It Matters

Temperature Correction Factors (BS 7671 Appendix 3)

Try Elec-Mate free for 7 days

Maximum Zs Values from BS 7671

Maximum Zs for MCBs — BS 7671:2018+A4:2026 Table 41.3 (0.4 s and 5 s, 230 V)

BS 88 and BS 3036 fuses: lower Zs limits (Table 41.2)

Worked Examples

Example 1: Domestic Ring Final Circuit

Example 2: Lighting Circuit (Long Run)

Example 3: Shower Circuit (Close to Limit)

Common Mistakes in Zs Calculations

Using the Elec-Mate Zs Calculator

Automatic Zs Verification

Built-In BS 7671 Tables

50+ Calculators in One App

All BS 7671 tables in your pocket

Frequently Asked Questions About Earth Fault Loop Impedance

What does Zs mean in electrical testing?

How is Zs measured on site?

What is Ze and how is it different from Zs?

What is R1+R2 and how do I measure it?

Why do I need to apply temperature correction to Zs?

Where do I find the maximum Zs values in BS 7671?

What happens if Zs is too high?

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