Earth fault loop impedance: Ze and Zs
The protective device only disconnects fast enough if the fault current is big enough — and that depends on the impedance of the earth fault loop.
Ze — at the origin
Ze is the part of the loop outside the installation. Measure it at the origin between line and the main earthing conductor, disconnected from the MET so parallel paths through bonding don’t lower the reading — and reconnect it straight after. Typical maximum values declared by distribution network operators:
| Earthing arrangement | Typical maximum Ze |
|---|---|
| TN-C-S (PME) | 0.35 Ω |
| TN-S | 0.8 Ω |
| TT | 21 Ω (supply side only — the installation earth electrode is extra) |
Zs — at the furthest point
Zs is the whole loop. Measure it live at the furthest point of each circuit, line to earth, or calculate it as Zs = Ze + (R1+R2). On RCD-protected circuits use the tester’s no-trip (low-current) loop setting.
Always cross-check a measured Zs against Ze + (R1+R2). A measured value much higher than the calculated one points to a loose or high-resistance connection.
Comparing with the maximum
BS 7671 Table 41.3 gives maximum Zs for each device. Those values are at conductor operating temperature, so when you test at room temperature compare your reading with roughly 80% of the tabulated value (the rule of thumb used in IET Guidance Note 3). See the max Zs tables.
Prospective fault current (Ipf)
At the origin, also measure PSCC (line–neutral) and PEFC (line–earth). Record the higher as Ipf and check it doesn’t exceed the breaking capacity of the protective devices. For a three-phase supply, a common approximation is twice the line–neutral PSCC.
iZapp’s 3D Testing Bench lets you do this test on a virtual board with real-looking kit: place the probes, read the tester and record the result on a practice schedule. Some attempts have a fault planted for you to find.
Frequently asked questions
What is the difference between Ze and Zs?
Ze is the external earth fault loop impedance at the origin of the installation. Zs is the total loop impedance at a point in a circuit — Ze plus the circuit’s R1+R2.
Why disconnect the earthing conductor to measure Ze?
To remove parallel earth paths through the bonding (gas, water pipes), which would give a falsely low reading. Reconnect it immediately afterwards.
What is the 80% rule?
Tabulated max Zs values assume hot conductors. Testing at room temperature, compare your measured Zs with about 80% of the table value to allow for the rise in resistance.
Last updated 2026-10-14. Training summary of BS 7671 and IET Guidance Note 3 practice, written by NAPIT-registered electricians. Always check the current edition and amendment of BS 7671 and your scheme’s requirements.