Earth fault loop impedance testing measures, with the supply live, the impedance of the path a fault current would take from a line conductor to earth and back to the transformer. Two figures are recorded: Ze, the external loop impedance measured at the origin with the installation's earthing conductor disconnected, and Zs, the total loop impedance at the furthest point of each circuit. Zs must not exceed the maximum in Tables 41.2 to 41.4 of BS 7671:2018+A4:2026 for the protective device, compared at 80 per cent for a cold measurement (1.10 Ω for a 32 A Type B MCB). The same instrument reports prospective fault current, Ipf, which must be within the breaking capacity of every device on the board.
- Zs = Ze + (R1+R2). Ze is measured once at the origin with the earthing conductor disconnected; Zs is measured at the far end of every circuit.
- Isolate and lock off before disconnecting the earthing conductor, because the installation has no earth while it is off.
- Use a no-trip loop test on RCD-protected circuits and the standard high-current test everywhere else.
- Record Ipf as the higher of the line-neutral and line-earth prospective fault currents and check it against the kA marking on every device.
- Compare measured Zs with 0.8 × the tabulated maximum; if it fails, check Ze + (R1+R2) before condemning the circuit.
What is the difference between Ze and Zs?
The earth fault loop on a TN system runs from the point of fault along the cpc to the main earthing terminal, through the earthing conductor to the supply earth (the cable sheath on TN-S, the combined neutral on TN-C-S), back through the transformer winding and out along the line conductor to the fault. On a TT system the return path is through the earth electrode and the general mass of earth.
- Ze (external earth fault loop impedance) is the part of the loop that belongs to the supply: transformer winding, service cable and DNO earth. It is measured at the origin with the installation's earthing conductor disconnected so that nothing inside the installation contributes.
- Zs (earth fault loop impedance) is the whole loop as seen from a point on a circuit: Ze plus the line conductor resistance R1 plus the cpc resistance R2 up to that point.
Ze is recorded once on the front of the certificate with the earthing arrangement and Ipf. Zs is recorded per circuit on the schedule of test results; the tables and the 80 per cent rule are covered in Maximum Zs Values BS 7671: Tables, 80% Rule and Ze.
Typical Ze figures: up to 0.8 Ω on TN-S and 0.35 Ω on TN-C-S (both commonly quoted from Energy Networks Association guidance on what a DNO will declare), and on TT whatever the electrode gives, often tens of ohms. A Ze far above the expected figure for the supply type is a matter for the DNO.
How do I measure Ze at the origin safely?
The test measures the supply's earth path alone, so the parallel paths inside the installation (main bonding to gas, water and structural steel) must be removed from the circuit. That means disconnecting the earthing conductor from the main earthing terminal, and while it is off the installation has no earth at all.
- Isolate the installation at the main switch, lock off and prove dead. Nothing downstream should be live while the earth is disconnected.
- Disconnect the earthing conductor from the main earthing terminal (or disconnect the main bonding conductors at the MET instead; either way the parallel paths are removed).
- Connect the loop tester between the incoming line at the main switch supply terminals and the disconnected earthing conductor. On a three-phase supply test each line to earth and record the highest.
- Read Ze and the prospective earth fault current shown with it, using the high-current test.
- Reconnect the earthing conductor and bonding, tighten and check, then remove the lock-off and re-energise.
The reconnection step is the one that gets forgotten when the phone rings; if a later loop test at a socket reports no earth, check the MET before anything else.
On an EICR where the customer will not accept isolation, measuring with the earthing conductor connected is a limitation and must be stated. The figure obtained will be lower than true Ze because of the parallel paths and should not be presented as Ze.
How do I measure Zs on each circuit?
Zs is measured live at the point electrically furthest from the board: the last socket on a radial, the furthest socket on a ring, the last luminaire on a lighting circuit, or the outlet of fixed equipment. Probe line, neutral and earth, run the loop test and record the ohms.
Compare against 80 per cent of the tabulated maximum for the device. If a circuit fails, do not condemn it immediately: add Ze to the R1+R2 measured during the dead tests. If the calculation passes and the measurement fails, the discrepancy is either no-trip instrument accuracy, a loose connection that passed at the low current of the continuity test, or a fault introduced since. If both fail, the circuit is too long for the device, has a high-resistance joint or sits on a high Ze, and needs an observation. A Zs below Ze is not a bonus: it means a parallel earth path, usually bonded pipework or SWA armour, that was absent during the Ze measurement.
Where a loop test is not practicable, Zs may be calculated from Ze + (R1+R2) and recorded as calculated, with a remark; see Schedule of Test Results: How to Fill In Every Column.
What is no-trip loop testing and when should I use it?
A conventional loop tester measures Zs by drawing a substantial current (typically 20 to 25 A) between line and earth for a few milliseconds and calculating the impedance from the voltage drop. That current flows to earth, so on a circuit protected by a 30 mA RCD it trips the device. No-trip modes get round this by using a low test current below the RCD's threshold, or by shaping the test current so the RCD does not register it as a residual fault.
The trade-off is accuracy. Low-current tests are more sensitive to supply noise and to the instrument's resolution at low impedances, so a no-trip reading of 0.30 Ω might really be 0.25 or 0.35 Ω, and manufacturers quote wider tolerances for these modes. Practical rules:
- Use no-trip mode only on RCD-protected circuits. Elsewhere, use the standard high-current test.
- Where a no-trip reading is close to the limit, cross-check with Ze + (R1+R2).
- Some older or electronic RCDs still trip in no-trip mode. If so, calculate Zs and note it rather than repeatedly tripping the device.
- Never bypass or link out an RCD to get a high-current reading on a live installation.
Should I use a 2-lead or 3-lead loop test?
Most multifunction testers offer both. A 3-lead test connects to line, neutral and earth; the neutral connection lets the tester check polarity and gives the no-trip algorithm a reference so the earth measurement is more accurate. A 2-lead test connects to line and earth only.
| 3-lead | 2-lead | |
|---|---|---|
| Connections | L, N, E | L, E |
| Where it suits | Socket-outlets, boards, anywhere a neutral is accessible | Lighting points, switch drops, Ze at the origin, ceiling roses without a neutral at the switch |
| No-trip accuracy | Better on most instruments | Reduced on most instruments |
| Polarity check | Included | Not possible from the test alone |
Use 3-lead wherever the wiring lets you, particularly for no-trip tests. Use 2-lead where only line and earth are available, and expect a slightly less certain reading.
How do I check prospective fault current against breaking capacity?
Prospective fault current, Ipf, is the current that would flow if a fault of negligible impedance occurred at the point of measurement. Two values are relevant:
- PSCC (prospective short-circuit current): line to neutral, or line to line on three-phase. Measured with the tester's L-N or PSC function.
- PEFC (prospective earth fault current): line to earth, equal to U0 ÷ Ze at the origin. The loop tester displays it with the Ze measurement.
Ipf is the higher of the two; on TN-C-S they are usually very close because neutral and earth share a conductor upstream of the cut-out. Measure at the origin and at each distribution board. On three-phase, the line-to-line figure can be roughly double the line-to-earth figure; measure rather than estimate.
Every overcurrent protective device on a board must have a rated short-circuit breaking capacity, marked in kA on the device (Icn for BS EN 60898 MCBs, usually 6 kA on consumer units and 10 kA on commercial boards; BS 88 fuses typically 80 kA), not less than the Ipf at that board. Chapter 43 permits a lower-rated device where an upstream device, such as the DNO's 100 A BS 88-3 cut-out fuse, provides back-up protection confirmed by the manufacturer; most consumer unit manufacturers publish such a declaration, which is why a measured Ipf of 6.5 kA in a flat near a substation is usually acceptable rather than a fail. Record the breaking capacity of each device in the OCPD columns so the comparison is visible.
Typical domestic figures are well below 6 kA. On a TT supply the PEFC can be a few amps and the PSCC is the figure that matters.
Where does loop testing fit in the test sequence?
Loop impedance and prospective fault current are the first live tests, after all dead tests (continuity, ring test, insulation resistance, dead polarity) are complete and live polarity has been confirmed at the origin. RCD testing follows, because you want to know Zs is acceptable before deliberately tripping the device, and functional testing comes last. The dead tests are covered in Insulation Resistance Testing Guide: Voltages, Minimums, Method and Ring Final Circuit Testing: r1, rn, r2 and R1+R2 Explained, and the RCD tests in RCD Testing Explained: Types, Test Currents and Trip Times.
How Certio helps
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