Earth Fault Loop Impedance Explained: Ze and Zs Guide

Earth fault loop impedance is the total impedance of the complete path that fault current takes when a line conductor comes into contact with earth — either directly or through a metallic enclosure. This path forms a loop: from the supply transformer, through the line conductor to the point of fault, through the fault itself, through the circuit protective conductor (CPC) back to the main earthing terminal, through the external earth return path back to the supply transformer, and through the transformer winding back to the line conductor.

The total impedance of this loop determines how much fault current flows when an earth fault occurs. By Ohm's law, fault current equals supply voltage divided by loop impedance: If = Uo / Zs. The higher the loop impedance, the lower the fault current. The lower the fault current, the longer the protective device takes to operate — and if the impedance is too high, the device may not operate at all within the required time, leaving a dangerous situation where metalwork is energised at a voltage that could cause lethal electric shock.

This is why earth fault loop impedance testing is one of the most critical measurements in electrical testing. It is test number five in the GN3 testing sequence — the first live test, carried out after all dead tests have confirmed that the wiring is intact and safe to energise.

Ze represents the portion of the earth fault loop that is external to the installation — the part that you, as the electrician, cannot change. It includes the impedance of the supply transformer winding, the supply line conductor from the transformer to the origin of the installation, and the external earth return path from the installation back to the transformer.

The external earth return path differs depending on the earthing system:

Ze is measured at the origin of the installation with the main earthing conductor temporarily disconnected from the main earthing terminal. This isolates the installation earth from the supply earth so you measure only the external loop. The value is recorded on the EICR or EIC.

Zs is the total earth fault loop impedance measured at a specific point in the installation. It includes everything in Ze, plus the impedance of the installation wiring from the distribution board to the point of measurement. The relationship is:

R1 and R2 are the resistances of the line conductor and CPC respectively, measured from the distribution board to the point of test. These are the same R1 and R2 values obtained during continuity testing (test 1 in the GN3 sequence). By adding the measured R1+R2 to the measured Ze, you can calculate the expected Zs and compare it against the measured Zs as a verification check.

Zs is measured at the furthest point of each circuit because this gives the highest (worst-case) value — the point where the conductor lengths are longest and therefore the impedances are highest. If Zs passes at the furthest point, it will pass at every other point on the circuit.

Zs lookup by protective device — instant maximum values

Select the protective device type (B, C, D MCB, BS 3036 fuse, BS 88 fuse) and rating…

Try it free for 7 days

The connection between loop impedance and disconnection time is fundamental to electrical safety. BS 7671 requires that in the event of an earth fault, the protective device must disconnect the supply within a maximum time to prevent electric shock. The required disconnection times are:

The protective device has a time-current characteristic: it operates faster when more current flows through it. An MCB rated at 32 A might take 5 seconds to trip at 100 A but only 0.01 seconds at 500 A. The maximum Zs values in BS 7671 are calculated so that the resulting fault current (Uo/Zs) is sufficient to make the device operate within the required disconnection time.

For example, a Type B 32 A MCB trips magnetically at 5 times its rated current (5 x 32 = 160 A). BS 7671 applies a voltage factor Cmin = 0.95 to account for supply voltage tolerance, giving 0.95 x 230 = 218.5 V. The maximum loop impedance is therefore 218.5/160 = 1.37 ohms. This is exactly the value you find in BS 7671 Table 41.3 for a B32 MCB.

The maximum permitted Zs values are found in BS 7671 Tables 41.2 (BS 3036 fuses), 41.3 (Type B MCBs), and 41.4 (Type C and Type D MCBs). These tables give the values at the maximum conductor operating temperature. For a complete reference, see our dedicated maximum Zs values guide.

The regulatory framework behind Zs verification is set out in Reg 643.7.3.1, which requires that where protective measures depend on a knowledge of earth fault loop impedance, the relevant impedances shall be measured or determined by an alternative method, and the measured values shall comply with Chapter 41. Reg 411.4.202 sets out how compliance is demonstrated for circuit-breakers: the maximum Zs shall be determined by the formula in Reg 411.4.4 (Zs = Uo × Cmin / Ia), or — for a nominal voltage of 230 V — the values in Table 41.3 may be used directly as an alternative to calculation. Using Table 41.3 directly is the standard on-site approach; the formula route is used where a device is not listed in the table or where a non-standard voltage applies.

The "corrected" values in brackets are the tabulated maximum multiplied by 0.8 — the values your measured Zs should not exceed when testing at ambient temperature.

Earth fault loop impedance measurements are live tests — they require the circuit to be energised at mains voltage. They can only be performed safely after all dead tests (continuity, insulation resistance, polarity) have been completed satisfactorily.

Voice to test results — speak Zs values on site

With your MFT in one hand, speak: 'Ring 1, Zs 0.89 ohms.' Elec-Mate fills in the schedule of test results and validates the value against the maximum Zs…

Try it free for 7 days

The maximum Zs values in BS 7671 tables are given at the maximum conductor operating temperature — typically 70 degrees Celsius for PVC-insulated cables. However, when you measure Zs on site, the conductors are usually at ambient temperature (approximately 10 to 25 degrees Celsius). As the installation operates and cables carry their design current, the conductor temperature rises and their resistance increases.

Copper conductor resistance increases by approximately 0.4% per degree Celsius. Between 20 degrees Celsius (ambient) and 70 degrees Celsius (maximum operating temperature for PVC), this is a 20% increase. The 0.8 factor compensates for this: if your measured Zs at ambient temperature is no more than 80% of the tabulated maximum, the actual Zs under load (when conductors are hot) should still be within the tabulated maximum.

The 0.8 correction factor appears as the cold-measured site limit within GN3 and is embedded alongside every entry in Table 41.3 of BS 7671 (Reg 411.4.204). For example, the B20 MCB entry states a tabulated maximum of 2.19 ohms and a cold-measured site limit of 1.75 ohms. This makes the 0.8 factor part of the standard's normative guidance package, not merely an optional IET practice — it should be treated as the working limit for on-site testing. Elec-Mate applies the 0.8 correction automatically when validating Zs measurements.

The earthing system of the supply significantly affects the Ze value and therefore the achievable Zs. Understanding the typical values for each system helps you anticipate whether circuits are likely to pass or fail before you start testing.

If the measured Zs exceeds the maximum permitted value (after applying the 0.8 correction factor), you must take action. The circuit does not comply with BS 7671 and the protective device will not disconnect within the required time in the event of an earth fault.

On an EICR, a Zs value exceeding the maximum permitted value is recorded as an observation. The classification code depends on the severity — C2 (potentially dangerous) or C3 (improvement recommended) depending on whether RCD protection is present and functioning.

Auto-validated Zs in the schedule of tests

Elec-Mate validates every measured Zs against BS 7671 maximum permitted values for the specific protective device on each circuit.

Try it free for 7 days