Earth fault loop impedance and prospective fault current are the two live tests most likely to come apart under assessment conditions — not because the meter is difficult, but because candidates cannot say what the reading actually represents or where in the installation it was taken. Everything below is one live test sequence, and it comes at the end.
First: these are live tests, and they come last
Regulation 643.1 sets the order, and it is not negotiable. Dead tests before live tests: continuity of protective conductors, continuity of ring final circuit conductors, insulation resistance, polarity by continuity — all of these happen with the installation isolated and proved dead. Only when the dead tests have passed do you energise and carry out earth fault loop impedance, prospective fault current and RCD testing.
The reason is straightforward: insulation resistance testing on a circuit with a line-earth fault would be dangerous to energise. If insulation resistance is below the 1 MΩ minimum of Table 64.1, you do not proceed to the live tests — you find the fault first.
The Ze test itself needs isolation before it can be done live. To remove parallel paths you must disconnect the main earthing conductor, and that means:
- Isolate at the main switch and lock off.
- Prove the voltage indicator on a known source, test the conductors dead, prove the indicator again — the GS38 prove, test, prove sequence.
- Disconnect the main earthing conductor from the main earthing terminal.
- Connect the loop tester between the incoming line side of the main switch, the neutral and the disconnected earthing conductor.
- Energise for the test, then de-energise and reconnect the earthing conductor before anything else.
The installation is unearthed while the main earthing conductor is off. Nobody works on it and nothing else is energised in that window. The full test sequence and the reasoning behind it is covered in the BS 7671 test sequence post.
What Ze actually measures
Ze is the earth fault loop impedance external to the installation. It is the path from the main earthing terminal, out of the building, through the earth return, back through the supply transformer winding, and along the line conductor to the incoming side of the main switch.
The question it answers is worth stating in the assessor’s words:
Is there a continuous path from the earth in the consumer unit all the way to the supply transformer and back to the main switch, and what is that impedance measurement?
The “back to the main switch” half is the part candidates drop. A fault current that leaves the installation but does not return through the protective device is a fault current that never operates it. Automatic disconnection under Regulation 411.3.2 depends on the whole loop, not on the earth connection alone.
It is called impedance rather than resistance because the test is performed on an energised AC supply, so reactance is part of the measurement.
The three earthing arrangements, and what to expect
| System | Earth path | Typical Ze at origin | Typical PFC | Typical PSCC |
|---|---|---|---|---|
| TN-C-S (PME) — combined then separated at the intake | Copper PEN conductor back to the transformer | ≤ 0.35 Ω | ~1.33 kA (1330 A) | Several hundred A to > 1 kA |
| TN-S — earth and neutral separate throughout | Separate copper/armour earth back to the transformer | ≤ 0.8 Ω | ~0.96 kA (960 A) | Several hundred A to > 1 kA |
| TT — electrode at the installation, electrode at the transformer, soil between | Through the ground | Tens to hundreds of Ω | ~0.003 kA (3 A) | ~1.19 kA (1190 A) |
The TN-S and TN-C-S figures are the maximum values quoted in the DNO’s declaration and reproduced in the On-Site Guide; they are what you compare a measurement against, not what a measurement must equal. A TN-S reading of 0.18 Ω is perfectly fine — the limit is a ceiling.
TT has no fixed maximum in BS 7671. Regulation 411.5.3 sets the real requirement: RA × IΔn must not exceed 50 V, where RA is the resistance of the electrode and protective conductor. With a 30 mA RCD that arithmetic permits up to 1667 Ω, but treat that as a mathematical limit rather than an acceptance criterion. The IET recommends 200 Ω as the point beyond which electrode resistance is considered unstable, and instability is the operative word — a TT electrode that measures well in a wet Shropshire winter can measure very differently after a long dry summer.
Ze and PFC are the same measurement, twice
Prospective fault current and earth fault loop impedance are two corners of the same Ohm’s law triangle. Given U₀ and Ze, the meter calculates PFC; given PFC and U₀, it can work back to Ze.
If Ze increases, PFC decreases. If Ze decreases, PFC increases. Ze must change before PFC changes.
That last clause is the one worth holding onto. Impedance is the cause; fault current is the effect. You cannot change the loop impedance by changing the fault current — the fault current is what the impedance and the voltage produce between them.
The arithmetic goes both ways, and assessors ask for it in both directions. A TT installation reading 0.003 kA is 3 A. At 230 V nominal that implies roughly 77 Ω of loop impedance — a plausible figure for a soil-return path. Get comfortable converting between kA and A, because most instruments display in kA and a misplaced decimal point turns a healthy reading into a nonsense one.
PSCC and why we measure it at the origin
Prospective short-circuit current is the current that would flow in a short circuit between line and neutral. It is measured at the origin because that is the worst case: every point further into the installation adds conductor length, adds impedance and reduces the fault current. If the origin is within the device’s capability, everywhere downstream is too.
Regulation 643.7.3.201 allows the prospective fault current to be taken as the greater of the line-neutral and line-earth values measured at the origin, and that is the figure recorded on the certificate.
What the figure is for is the point assessors most often press on. Look at the front or side of any circuit breaker or RCBO and you will find a number in a rectangular box — 6000 in most domestic devices, 10000 in commercial and industrial work, and occasionally domestically where the substation is at the end of the garden. That is the rated short-circuit capacity in amperes under BS EN 60898, and Regulation 434.5.1 requires it to be not less than the prospective fault current at the point of installation. A 6 kA device on a supply with a 7 kA PSCC is not a paperwork problem. It is a device that may not clear a fault it is asked to clear.
Note the TT row in the table above: a TT installation with a PFC of 3 A still has a PSCC around 1190 A. The line-neutral loop is copper all the way back to the transformer and knows nothing about the earthing arrangement. Anyone who expects a low PSCC on a TT system has confused the two paths.
Zs at the points of use
Zs is the earth fault loop impedance for the whole loop measured at a point of use — Ze plus the circuit’s own (R1 + R2). It is measured at the furthest point of each circuit, because that is where the impedance is highest and the disconnection time longest.
Every circuit gets its own Zs, and each is compared against the maximum for its protective device and disconnection time — Tables 41.2, 41.3 and 41.4 of BS 7671, reproduced in the On-Site Guide. Under an open-book exam, knowing which table to turn to for a Type B 32 A MCB on a 0.4 s final circuit is worth more than memorising the value itself.
Two practical points:
- Socket and lighting test adaptors let you take Zs without dismantling the accessory. They are the sensible approach for live testing — fewer exposed conductors, less time with a lid off.
- The tabulated maxima are for conductors at their operating temperature. Where you have measured on a cold installation, apply the 0.8 rule-of-thumb correction (or the On-Site Guide’s temperature-corrected tables) before deciding a value passes.
A measured Zs above the tabulated maximum means automatic disconnection is not achieved in the required time. On an EICR that is a C2 — potentially dangerous, not a C3, because the protective measure is not working.
Where each value goes on the paperwork
- Ze and PFC/PSCC at the origin — the supply characteristics section of the EIC or EICR. One Ze per installation.
- Zs — the Schedule of Test Results, circuit by circuit.
- If you have used the DNO’s declared maximum figure instead of measuring, say so on the certificate. That is permitted, but it must be declared as enquiry rather than presented as a measurement.
Blank cells read as “not tested”. If a value could not be obtained, record why.
How 2391 Prep Fits Into This
The 2391-52 is 60 questions in 120 minutes at a 75% pass mark, and it is open book — the questions that bite are the ones where you know the principle but lose two minutes hunting for Table 41.3.
2391 Inspection & Testing Pro has 556 practice questions across 7 subject areas, with the Test Procedures and Initial Verification areas covering Ze, Zs, PFC and PSCC directly — including the kA-to-A conversions and the “which value do I compare against” questions that assessors favour.
The 4 working calculators include Zs from Ze + (R1 + R2) and the temperature correction, so you can check your arithmetic against the tabulated maxima rather than trusting a mental sum. The 15 quick reference cards put the maximum Zs tables and the earthing-system limits where you can find them in seconds, and the 6 field tools cover the on-site side of the same work.
If you are building a revision plan around this, how to revise for the 2391-52 without wasting weeks sets out the order to tackle the material in.
