Loop impedance is the one calculation on the 2391-52 that everything else leans on. Get the path itself clear in your head and the numbers stop being arbitrary — you know what each term is, where it comes from, and why a high reading means what it means.
Walk the path, conductor by conductor
The source video traces the fault path on a TN-S installation step by step, and it is worth following exactly that order, because the path is what the formula describes.
A line-to-earth fault occurs — line conductor to an exposed-conductive-part. Then:
- The exposed-conductive-part becomes live.
- It is connected to the circuit protective conductor (CPC), which becomes live.
- The CPC runs back to the main earthing terminal in the consumer unit.
- That terminal is connected to the earthing conductor, which arrives with the meter tails.
- On TN-S, the earthing conductor is connected to the lead or steel sheath of the distributor’s cable.
- The sheath returns to the star point of the supply transformer, which is earthed at source.
- The line conductor from the transformer back out to the fault completes the circuit.
That is the complete loop — out through the line conductor, back through the protective conductor and the distributor’s earth return. Every metre of it has impedance, and that impedance is the only thing limiting the fault current.
“This is the full earth fault loop path and needs to be a very low impedance in order to generate a very high fault current to cause rapid operation of the overcurrent protection device.”
That sentence is the whole subject in one line. Low Zs → high fault current → fast disconnection.
Why the cutout fuse does not blow
The video makes a point that catches people out in written answers: the fault current passes through the DNO’s cutout fuse, but that fuse does not operate. The device that operates is the one closest to the fault — the protective device in the consumer unit.
The cutout is rated at 60, 80 or 100 A depending on the supply. A 32 A MCB or a 32 A BS 88-3 fuse reaches its disconnection threshold at a small fraction of the current the cutout needs. This is discrimination (co-ordination) working correctly, and it is also why you must never treat the cutout as backup protection for a final circuit.
The formula, and what each term is
| Term | What it is | Where you get it |
|---|---|---|
| Ze | External earth fault loop impedance — everything upstream of the origin | Measured at the origin with the main earthing conductor disconnected, or taken from the distributor’s declared maximum and declared as such |
| R1 | Resistance of the circuit’s line conductor | Measured during continuity testing, dead |
| R2 | Resistance of the circuit’s circuit protective conductor | Measured during continuity testing, dead |
| Zs | Total earth fault loop impedance at the furthest point of the circuit | Measured live with a loop tester, or calculated as Ze + (R1 + R2) |
Zs = Ze + (R1 + R2)
The brackets matter conceptually even though they change nothing arithmetically: R1 + R2 is the circuit’s own contribution, measured as a single value on your continuity test, and Ze is everything the distributor owns.
Two points the video is precise about and candidates often are not:
- Zs is the value at the furthest point of the circuit. That is where the loop is longest and the impedance highest, which is the worst case for disconnection time. A reading taken at the board proves nothing about the far end.
- The example is a radial circuit. On a ring final circuit, the two legs in parallel mean the highest R1+R2 sits roughly at the mid-point of the ring, not at the last socket — one of the reasons the ring continuity test in Regulation 643.2.1 has its own step-by-step method.
TN-S and TN-C-S: the figures that change
The path differs only in the earth return, and that changes Ze:
| Earthing arrangement | Earth return path | Maximum declared Ze |
|---|---|---|
| TN-S | Lead or steel sheath / armour of the distributor’s cable, separate from the neutral | 0.8 Ω |
| TN-C-S (PME) | Combined neutral and earth (PEN) conductor back to the transformer star point | 0.35 Ω |
The formula does not change. Only the first term does:
“This drawing shows a TN-C-S earthing arrangement and the supply authority’s cable now has a maximum external impedance Ze of 0.35 of an ohm. Total circuit impedance Zs is still Zs = Ze + R1 + R2.”
TN-C-S is lower because a PEN conductor is a full-sized conductor sharing the neutral, not a sheath. Note that both are maximum declared values from the distributor. They are figures to design against when you have nothing better; they are not what you should expect to read. A TN-C-S supply frequently measures 0.1 Ω or less. Two related points our post on Ze, PEFC and PSCC covers in more detail: a TT system has no metallic return at all, so Ze there is dominated by the electrode and the mass of earth and is typically tens or hundreds of ohms; and where you use the declared figure rather than your own measurement, say so on the certificate.
Disconnection times are the point of the whole exercise
A low Zs is not a goal in itself. It exists to satisfy the disconnection times in Regulation 411.3.2, which the video refers to as “either a maximum time of five seconds or a maximum time of nought point four”:
- 0.4 s — Regulation 411.3.2.2, for final circuits up to 63 A with one or more socket-outlets, and final circuits up to 32 A supplying only fixed connected current-using equipment, on a TN system
- 5 s — Regulation 411.3.2.3, for distribution circuits and circuits not covered above, on a TN system
Those times are what the maximum Zs tables in Chapter 41 are derived from. Table 41.2 gives maximum Zs for fuses and Table 41.3 for circuit-breakers at 0.4 s; Table 41.4 covers the 5 s cases. Every value in them is just the loop impedance at which the device’s characteristic curve delivers the required time.
Two things to have straight before you sit the exam:
- The tabulated maximums assume a conductor temperature. If you compare a measured Zs against a tabulated value, apply the 0.8 rule-of-thumb correction (or the rule-of-thumb factors in the On-Site Guide) — measured cold, compared against a table derived at operating temperature.
- An RCD does not fix a high Zs for overcurrent purposes. Regulation 411.4.204 sets its own loop impedance requirement where a 30 mA RCD provides fault protection on a TN system, and that is a separate check.
Where this sits in the test sequence
R1 and R2 come from a dead test. Zs measured directly comes from a live test. They are at opposite ends of the sequence for a reason, and the exam expects you to know the order:
- Continuity of protective conductors, including R1 + R2 — dead
- Continuity of ring final circuit conductors — dead
- Insulation resistance — dead
- Polarity — dead
- Then, and only then: polarity confirmed live, Ze, PFC, Zs and RCD operation — live
The order is not administrative. Insulation resistance after continuity means you have already proved the CPC is connected before you apply 500 V. Polarity dead before any live test means you are not about to energise a circuit with line and neutral reversed. Ze measured at the origin before circuit Zs means you know what the baseline is.
And nothing in that live column happens until safe isolation has been proved and reversed deliberately. You isolate, lock off, prove the voltage indicator on a known source, test, prove again — GS38-compliant probes throughout — and you only re-energise when you intend to and the installation is safe to energise. There is no version of this where you take a shortcut to get a loop reading faster. Our posts on the BS 7671 test sequence and safe isolation and GS38 go through both in full.
Measure or calculate?
Regulation 643.7.3 allows Zs to be determined by measurement or by calculation. Both are valid; the certificate needs to say which you did.
- Measured with a loop tester at the point of use: quick, real, includes everything — but needs the circuit live, and a no-trip/low-current test on an RCD-protected circuit reads with less resolution.
- Calculated as Ze + (R1 + R2): takes no live testing beyond the Ze measurement, and is the approach where a loop reading is impractical — but the R1+R2 must be corrected to operating temperature, or you will under-report the real Zs.
On the Schedule of Test Results, a Zs cell is read as a measured value unless you note otherwise. If it was calculated, say so.
How 2391 Prep Fits Into This
The loop impedance path is the sort of topic where the 2391-52 rewards knowing exactly where a figure lives. The exam is 60 questions in 120 minutes with a 75% pass mark, and it is open book — which means the marks go to whoever can find Table 41.3 or Regulation 411.3.2.2 in seconds, not to whoever memorised a number and hopes it is the current one.
2391 Inspection & Testing Pro is built for exactly that. It has 556 practice questions across 7 subject areas, including the test procedures and earthing arrangement questions this topic sits in, so you practise the Zs = Ze + (R1 + R2) reasoning under the same time pressure you will face. The 4 working calculators cover the loop impedance arithmetic — Ze plus a corrected R1+R2, and the comparison against maximum Zs — so you check your method rather than just your answer. The 15 quick reference cards put the maximum Zs figures and the disconnection times where you can reach them in one tap, and the 6 field tools are for the van, not the exam hall, when you are on site with a reading you need to sanity-check.
Learn the path first. The formula is only shorthand for it.
