Zs (earth fault loop impedance) is the total impedance, in ohms, of the path a fault current takes from the point of a line-to-earth fault, back through the circuit protective conductor, the installation earthing and the supply transformer, and it determines whether the protective device will disconnect fast enough under BS 7671 Chapter 41. It is measured at the furthest point of every circuit and recorded on the schedule of test results, then compared against the maximum values in BS 7671 Tables 41.2 to 41.4 for the device protecting that circuit.
- Zs = Ze + (R1 + R2): supply impedance plus the circuit's line and protective conductor resistances.
- The value must not exceed the maximum in BS 7671 Chapter 41 for the protective device.
- Compare measured readings to 80% of the tabulated maximum to allow for conductor temperature.
- A high Zs on an EICR is normally a C2, because disconnection times cannot be guaranteed.
What is Zs in practice?
When a live conductor touches earthed metalwork, current flows around the earth fault loop. The lower the impedance of that loop, the higher the fault current and the faster the fuse or circuit-breaker operates. BS 7671 Regulation 411.3.2 requires disconnection within 0.4 s for final circuits up to 63 A on TN systems and 5 s for distribution circuits, and Zs tells you whether the device will achieve it.
The loop has three parts: Ze, the external part back through the distributor's network to the transformer; R1, the line conductor of the circuit; and R2, its circuit protective conductor. R1 + R2 is measured dead during continuity testing, and Zs is the sum, which is why a long lighting circuit in 1.0 mm² cable has a much higher Zs than a short cooker circuit in 6 mm².
When is Zs measured?
Zs is recorded for every circuit on an EIC, a Minor Works Certificate and an EICR. For new work it confirms the design achieves disconnection; on periodic inspection it reveals deterioration such as a corroded earth connection or a loose terminal.
| Situation | What to measure | Where it goes |
|---|---|---|
| New circuit (EIC) | Zs at the furthest point | Schedule of test results |
| Addition to a circuit (Minor Works) | Zs at the new or altered point | Minor Works Certificate |
| Periodic inspection (EICR) | Zs at the furthest accessible point of each circuit | Schedule of test results |
How is Zs measured and what are the limits?
Zs is measured live with the loop function of a multifunction tester, line to earth, at the furthest point of the circuit. On circuits without an RCD the high-current test gives the most accurate reading; on RCD-protected circuits the no-trip test avoids operating the device but is less precise, so where the reading is close to the limit, calculate Zs from Ze and R1 + R2 as a cross-check.
The measured value is compared against BS 7671 Chapter 41: Table 41.3 for circuit-breakers to BS EN 60898 and RCBOs to BS EN 61009, Table 41.2 for fuses, Table 41.4 for 5 s disconnection. For Type B devices at 0.4 s:
| Type B MCB rating | Maximum Zs (BS 7671 Table 41.3) | 80% test limit |
|---|---|---|
| 6 A | 7.28 Ω | 5.82 Ω |
| 16 A | 2.73 Ω | 2.18 Ω |
| 32 A | 1.37 Ω | 1.10 Ω |
The 80% column is the rule of thumb from IET Guidance Note 3 and the On-Site Guide. Tabulated values assume conductors at operating temperature; a cold circuit reads lower, so the limit is reduced to compensate. The alternative is the temperature correction method in BS 7671 Appendix 14.
On TT systems, where Ze is typically tens of ohms, an RCD provides fault protection instead and the requirement becomes Zs × IΔn ≤ 50 V, so a 30 mA RCD needs a Zs of no more than 1667 Ω.
Common mistakes with Zs
- Not testing at the furthest point. A reading at the first socket tells you nothing about the end of the circuit.
- Comparing the measurement to the full tabulated value without the 80% adjustment or a temperature correction.
- Calculating Ze plus R1 + R2 without ever measuring live. Calculation is a cross-check, not a substitute.
- Recording Zs from an old certificate. Loop impedance changes as connections age.
- Confusing Zs and Ze on the form. Ze goes on the supply characteristics page; Zs against each circuit.
Related terms
- External earth fault loop impedance (Ze)
- Prospective fault current (PFC)
- Multifunction tester (MFT)
- RCD
- Schedule of test results
- BS 7671
How Certio helps
Certio records this on the BS 7671 model forms without the typing: photograph the board to draft the circuit schedule, point the camera at a Megger MFT-X1 to capture readings, or dictate them. Starter is free for 7 days.