Calculate earth fault loop impedance (Zs) using Ze + R1+R2. Instantly check compliance with BS 7671 maximum Zs values for MCBs, RCBOs, and fuses with automatic temperature correction.
What is Zs? Zs is the total earth fault loop impedance — the complete impedance of the fault current path from the point of fault, through the circuit protective conductor (CPC), back to the supply transformer and return via the line conductor. It is calculated using the formula Zs = Ze + (R1 + R2), where Ze is the external impedance supplied by the network and R1 + R2 is the combined resistance of the line conductor and CPC within the installation. To verify BS 7671 compliance, the corrected Zs (adjusted to conductor operating temperature) must not exceed the maximum values in BS 7671:2018+A4:2026 Tables 41.2–41.4 for the protective device fitted — confirming that automatic disconnection of supply will operate within the required disconnection time (GN3 9th Ed, Chapter 8).
Verify TN/TT system compliance with BS 7671
Measured at origin
To furthest point
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)Earth fault loop impedance, commonly written as Zs, is the total impedance of the path that fault current takes when a line conductor comes into contact with an earthed part of an electrical installation. It is one of the most critical measurements in electrical testing because it determines whether the protective device — the MCB, RCBO, or fuse — can disconnect the supply quickly enough to prevent electric shock in the event of an earth fault.
The concept is rooted in a fundamental principle of electrical safety: automatic disconnection of supply. BS 7671 Regulation 411.3.2 requires that, in the event of a fault between a line conductor and an exposed-conductive-part connected to the protective earthing, the protective device must disconnect the faulty circuit within a specified time. Under Reg 411.3.1.2, the 0.4 second disconnection time applies to two categories of final circuit: (i) final circuits rated up to 32 A that supply only fixed connected current-using equipment, and (ii) final circuits rated up to 63 A that include one or more socket-outlets. For distribution circuits and other final circuits not falling within those two categories, the maximum disconnection time is 5 seconds. The lower the earth fault loop impedance, the higher the fault current, and the faster the protective device will operate.
If the earth fault loop impedance is too high, the fault current will be insufficient to trip the protective device within the required time. This leaves the faulty circuit energised, creating a serious risk of electric shock and fire. This is why verifying Zs is a mandatory part of initial verification and periodic inspection under BS 7671 and the IET Guidance Note 3.
In practical terms, earth fault loop impedance testing is performed on every circuit during initial verification (when a new installation is being commissioned) and during periodic inspection and testing (EICR). The results are recorded on Schedule of Test Results forms and compared against the maximum values published in BS 7671 Tables 41.2, 41.3, and 41.4. Use the disconnection time calculator to verify 0.4s and 5s compliance, and the EICR certificate tool to record your test results digitally.
To understand earth fault loop impedance, you need to understand the path that fault current takes. When a line conductor touches an earthed enclosure (an earth fault), current flows in a complete loop — the earth fault loop. This loop consists of several elements, each contributing impedance:
The total impedance of this complete loop is Zs. It is split into two components for practical measurement purposes: Ze (the external part, from the transformer through the supply cables and back) and R1+R2 (the internal part, the line conductor and CPC within the installation).
The fundamental formula for earth fault loop impedance is straightforward:
Zs = Ze + (R1 + R2)
Zs = total earth fault loop impedance in ohms
Ze = external earth fault loop impedance in ohms
R1 = resistance of the line conductor from origin to fault point
R2 = resistance of the CPC from origin to fault point
A domestic ring final circuit is protected by a 32 A Type B MCB. The supply is TN-C-S with a measured Ze of 0.25 ohms. The R1+R2 of the ring at the furthest point is 0.72 ohms, measured at an ambient temperature of 20 degrees C.
Zs at ambient = 0.25 + 0.72 = 0.97 ohms
Zs corrected = 0.97 x 1.20 = 1.16 ohms
From BS 7671 Table 41.2, the maximum Zs for a 32 A Type B MCB (0.4 s disconnection) is 1.37 ohms.
Result: PASS — 1.16 ohms is within the 1.37 ohms limit.
A lighting circuit is protected by a 6 A Type B MCB. The supply is TN-S with a measured Ze of 0.62 ohms. The R1+R2 measured at the furthest luminaire is 1.85 ohms at ambient temperature.
Zs at ambient = 0.62 + 1.85 = 2.47 ohms
Zs corrected = 2.47 x 1.20 = 2.96 ohms
From BS 7671 Table 41.2, the maximum Zs for a 6 A Type B MCB (0.4 s disconnection) is 7.28 ohms.
Result: PASS — 2.96 ohms is well within the 7.28 ohms limit.
A 20 A radial circuit supplies socket outlets in a workshop. It is protected by a 20 A Type B MCB on a TN-S supply with Ze = 0.72 ohms. The circuit is wired in 2.5 mm² twin and earth cable (1.0 mm² CPC) running 38 metres. The measured R1+R2 at the furthest socket is 1.22 ohms.
Zs at ambient = 0.72 + 1.22 = 1.94 ohms
Zs corrected = 1.94 x 1.20 = 2.33 ohms
From BS 7671 Table 41.2, the maximum Zs for a 20 A Type B MCB (0.4 s disconnection) is 2.19 ohms.
Result: FAIL — 2.33 ohms exceeds the 2.19 ohms limit. Solutions: increase cable size to 4 mm² (which reduces R1+R2), shorten the cable run, or add RCD protection (which gives a maximum Zs of 1667 ohms for a 30 mA RCD, easily achieved).
When you measure earth fault loop impedance on site, the cable conductors are at ambient temperature — perhaps 15 to 25 degrees Celsius on a typical UK day. However, the maximum Zs values given in BS 7671 Tables 41.2, 41.3, and 41.4 are calculated at the maximum conductor operating temperature: 70 degrees C for thermoplastic (PVC) insulated cables and 90 degrees C for thermosetting (XLPE) insulated cables.
Conductor resistance increases with temperature. Copper has a positive temperature coefficient of resistance — approximately 0.4% per degree Celsius. This means that a cable at 70 degrees C has significantly higher resistance than the same cable at 20 degrees C. As a result, the Zs of a circuit under full load (when cables are hot) will be higher than the Zs you measure on a cold, unloaded circuit.
There are two accepted methods for accounting for this:
Compare your measured Zs against 80% of the tabulated maximum Zs. If your measured value (at ambient) is no more than 80% of the BS 7671 maximum, the circuit complies. This is the quick method used by most electricians on site. For example, if the maximum Zs for a 32 A Type B MCB is 1.37 ohms, then 80% is 1.10 ohms — your measured Zs must be 1.10 ohms or less.
Multiply your measured Zs by a correction factor to obtain the estimated Zs at operating temperature. For 70 degrees C thermoplastic cables, the factor commonly applied on site is approximately 1.20; for 90 degrees C thermosetting cables, approximately 1.28. The corrected value is then compared directly against the BS 7671 maximum. GN3 9th Ed Tables B1–B6 tabulate maximum Zs values at a reference temperature of 10 °C; consult GN3 9th Ed Table B2 for the precise correction factors applicable to your measurement conditions and cable type.
The Elec-Mate calculator applies the correction factor automatically. Enter your measured values, select the cable insulation type, and the calculator shows both the ambient and corrected Zs alongside the BS 7671 maximum for your protective device.
BS 7671 provides maximum earth fault loop impedance values in Tables 41.2, 41.3, and 41.4 for circuit breakers to BS EN 60898 and RCBOs to BS EN 61009. These values ensure the protective device will disconnect the circuit within 0.4 seconds (for final circuits up to 32 A) or 5 seconds (for distribution circuits). Below are the commonly referenced values for each MCB type.
These values are from BS 7671:2018+A4:2026 Tables 41.2–41.4 and represent the maximum Zs at conductor operating temperature. Cross-referenced against GN3 9th Ed (2022, incorporating A4) Tables B1–B6, which present maximum Zs values at a reference temperature of 10 °C and provide the correction factors required for site measurements taken at ambient temperature. When comparing against site measurements, use the 80% rule or apply the appropriate correction factor from GN3 9th Ed Table B2 as described above.
Residual current devices (RCDs) operate on a fundamentally different principle from overcurrent devices. An RCD detects an imbalance between the current flowing in the line conductor and the current returning through the neutral. When some current leaks to earth through a fault (or through a person), the RCD senses the difference and disconnects the supply — typically within 30 to 40 milliseconds for a 30 mA device.
Because the RCD operates on differential current rather than magnitude of fault current, it does not require a high fault current to trip. The maximum earth fault loop impedance for an RCD-protected circuit is calculated from the formula:
Zs = 50 / Idelta n
50 = maximum touch voltage in volts (from BS 7671 Regulation 411.3.2.1)
Idelta n = rated residual operating current of the RCD in amperes
For a 30 mA (0.03 A) RCD: Zs = 50 / 0.03 = 1667 ohms. For a 100 mA (0.10 A) RCD: Zs = 50 / 0.10 = 500 ohms. For a 300 mA (0.30 A) RCD: Zs = 50 / 0.30 = 167 ohms.
The 1667 ohm limit for 30 mA RCDs is so generous that virtually any circuit within a building will meet it. This is why RCD protection is so valuable on TT installations (where Ze can be very high due to the earth electrode resistance) and as a solution for circuits where the Zs is too high for the OCPD alone to disconnect within time.
However, it is important to note that even on RCD-protected circuits, you should still verify that the overcurrent protective device can disconnect a line-to-earth fault. If the RCD fails (sticks), the OCPD is the backup. BS 7671 recommends (but does not require for all cases) that the Zs still permits the OCPD to operate within 5 seconds as a secondary measure. The Elec-Mate calculator checks both the RCD limit and the OCPD limit, flagging any discrepancies.
On TT installations, the return path from the installation earth back to the supply transformer is through the general mass of earth rather than a metallic conductor. The resistance of this path — the earth electrode resistance, Ra — can be 20 ohms or more, making Zs far too high for any overcurrent protective device to clear a fault within the required disconnection time. This is why BS 7671 Regulation 411.5 requires TT installations to use RCD protection: Regulation 411.6.5(b) states that the condition Ra × I ≤ 50 V must be satisfied (where Ra is the sum of the resistances of the earth electrode and the protective conductor, and I is the current causing automatic disconnection). Because RCDs operate on differential current rather than fault-current magnitude, even a very high Ra will not prevent disconnection — the 1667 ohm limit for a 30 mA RCD easily encompasses any practical TT Ze value.
Ra is measured on site using a proprietary earth electrode resistance tester (or the 3-terminal fall-of-potential method). BS 7671 Regulation 643.7.3 requires that where the earthing system incorporates an earth electrode, the electrode resistance to earth (Ra) shall be measured and recorded. Typical TT Ze values obtained from the distributor or measured on site should be used for design verification; on-site measurement is required if the distributed value is unavailable or suspect (OSG Reg 1.3).
Alongside Zs, BS 7671 Regulation 643.7.3.201 requires that the prospective short-circuit current and prospective earth fault current shall be measured, calculated, or determined at the origin of the installation and at other relevant points. This is a mandatory requirement for initial verification — not an optional check. The prospective fault current (commonly written as Ipf or PSCC at the origin) must be recorded on the Schedule of Test Results and on the Electrical Installation Certificate (EIC).
Ipf is related to Zs but is a separate quantity. While Zs tells you whether a fault will be cleared quickly enough, Ipf tells you how severe the fault current could be at a given point. All protective devices (MCBs, fuses, RCBOs) have a rated short-circuit capacity (Ics or Icn) — the maximum fault current they can safely interrupt. If Ipf exceeds the device's rated breaking capacity, the device may fail catastrophically during a fault. BS 7671 Appendix 14 provides further guidance on determination of prospective fault current.
Ipf (prospective earth fault current) = Uo ÷ Zs, where Uo is the nominal line-to-earth voltage (230 V). Prospective short-circuit current (line-to-line) uses the line-to-line voltage (400 V) and the relevant loop impedance. For typical TN-C-S supplies with Ze of 0.35 ohms, Ipf at the origin is approximately 230 ÷ 0.35 ≈ 657 A — well within the 6 kA breaking capacity of standard domestic MCBs. On installations with very low Ze (close to a substation), Ipf can be significantly higher.
The OSG (On-Site Guide, Reg 1.2.7) confirms that installers shall document verification of prospective fault current on installation records, certificates, and schedules of tests to demonstrate compliance. When completing an EICR, the maximum Ipf recorded at the time of original installation should be checked against device ratings; a code C2 or C3 may be appropriate if devices with insufficient breaking capacity are found.
Amendment 4 adds Regulation 411.3.4, which requires that AC final circuits supplying luminaires within domestic (household) premises shall be provided with additional protection by an RCD with a rated residual operating current not exceeding 30 mA. This directly affects Zs planning on lighting circuits (such as Example 2 above): the circuit must now have a 30 mA RCD regardless of whether the Zs easily meets the OCPD limit. The 1667 ohm RCD Zs limit will always be satisfied, but the presence of the mandatory RCD must be reflected on the Schedule of Test Results and the EICR.
Regulation 421.1.7 recommends the installation of arc fault detection devices (AFDDs) in AC final circuits of a fixed installation to mitigate the risk of fire due to arc fault currents. The wording is recommendatory (not mandatory with 'shall'), but specifiers and assessors should consider AFDDs — particularly on socket-outlet circuits — when planning protection at design stage. AFDDs combine OCPD and RCD functions with arc detection; their Zs requirements follow the integrated OCPD type fitted.
Measuring Ze: The external earth fault loop impedance is measured at the origin of the installation. Disconnect the main earthing conductor from the earthing terminal (this isolates the installation's earth from the supply earth). Connect your loop impedance tester between the incoming line terminal and the disconnected earthing conductor. The reading is Ze. On a TN-C-S (PME) supply, you should expect a Ze of 0.35 ohms or less. On a TN-S supply with a cable sheath earth, typical values are 0.80 ohms or less. On a TT system, Ze depends on the resistance of the earth electrode and can be 20 ohms or more.
Measuring R1+R2: The resistance of the line conductor and CPC combined is measured using the long lead method (also called the wandering lead method). At the distribution board, temporarily link the line and CPC of the circuit together. Then, using a low-resistance ohmmeter, measure the resistance from the line terminal at the distribution board to the line and CPC linked together at each point on the circuit. The reading at the furthest point is the R1+R2 value.
The long lead method effectively measures the resistance of a conductor loop consisting of the line conductor going out to the point and the CPC coming back. This is why the value is called R1+R2 — it is the sum of the line conductor resistance (R1) and the CPC resistance (R2) over the length of the circuit.
Common R1+R2 values for domestic circuits depend on the cable size and length. Always obtain the precise mΩ/m figures from GN3 Table B1 (copper conductors at 20 °C) — the full table must be used because fragmentary extracts can be misleading. Note that 25.51 mΩ/m refers to aluminium 2.5 mm² conductors, not copper twin and earth; verify the correct copper 2.5 mm²/1.0 mm² CPC value from Table B1 before use in calculations.
Determine the external earth fault loop impedance (Ze) at the origin of the installation. This can be measured with a loop impedance tester at the incoming supply with the main earthing conductor disconnected, or obtained from the electricity distributor. Typical values are 0.35 ohms for TN-C-S (PME) supplies and 0.80 ohms for TN-S supplies.
Perform a continuity test on the circuit using the long lead method. Connect a long test lead between the line and CPC at the distribution board, then measure the resistance at each point on the circuit. The reading at the furthest point gives the R1+R2 value in ohms at ambient temperature.
Add the Ze value to the R1+R2 value to obtain the total earth fault loop impedance (Zs) at ambient temperature. For example, if Ze = 0.35 ohms and R1+R2 = 0.86 ohms, then Zs = 0.35 + 0.86 = 1.21 ohms at ambient.
Multiply the calculated Zs by the appropriate correction factor to account for conductor resistance at operating temperature. GN3 (Chapter 5) notes that where reduced csa protective conductors are used, maximum EFLIs may need further reduction; the general temperature correction requirement stems from the fact that the tabulated Zs values in BS 7671 Tables 41.2–41.4 (and GN3 Tables B1–B6) are based on conductors at their maximum normal operating temperature, not at the ambient temperature at which site measurements are taken. The correction factors commonly used on site are 1.20 for thermoplastic (PVC) insulated cables and 1.28 for thermosetting (XLPE) cables — verify the precise values for your measurement conditions against GN3 9th Ed Table B2. Using the previous example with 1.20: 1.21 x 1.20 = 1.45 ohms at operating temperature.
Look up the maximum permitted Zs from BS 7671 Tables 41.2 (Type B MCBs), 41.3 (Type C MCBs), or 41.4 (Type D MCBs) for the protective device rating and disconnection time. If your corrected Zs is less than or equal to the tabulated maximum, the circuit complies. If it exceeds the maximum, you must reduce the impedance by increasing the cable size, reducing the cable length, or adding RCD protection.
Purpose-built for UK electricians working to BS 7671. Faster and more reliable than flipping through tables on site.
Enter Ze and R1+R2 values and get the total earth fault loop impedance instantly. Automatic pass/fail check against BS 7671 maximum Zs tables.
Automatically applies the correct temperature correction factor for thermoplastic and thermosetting cables.
Maximum Zs lookup for Type B, Type C, and Type D MCBs from BS 7671 Tables 41.2, 41.3, and 41.4. Also covers RCBOs and fuse types to BS 88 and BS 3036.
Clear colour-coded result showing whether your circuit meets the BS 7671 maximum Zs requirement. Green for pass, red for fail, with the margin shown.
All maximum Zs values verified against the current 18th Edition wiring regulations including Amendment 4. Updated tables for all protective device types.
All calculation logic and BS 7671 tables run locally on your device. Calculate Zs on site with no internet connection — results sync when you reconnect.
Free Zs calculator: Zs = Ze + (R1+R2). Check earth fault loop impedance against the maximum for your device and disconnection time, to BS 7671.
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