Free notes
Ze and prospective fault current on three phaseWhy double the short circuit reading?
Free from the notes: Sparky’s video on it, the section word for word as members see it, and a PDF to keep.
The notes
Exactly as they are on the site.
Same tests, once per line. Earthing conductor off the MET, earth lead on it, then:
| Test | L1 | L2 | L3 |
|---|---|---|---|
| Ze (line to earth) | 0.80 Ω | 0.78 Ω | 0.75 Ω |
| PEFC (line to earth) | 0.288 kA | 0.295 kA | 0.307 kA |
| PSCC (line to neutral) | 0.297 kA | 0.287 kA | 0.300 kA |
| PSCC × 2 | 0.594 kA | 0.574 kA | 0.600 kA |
- Record the highest Ze — 0.80 Ω in the example.
- The worst short circuit on three phase is a fault across all three lines — and many testers can’t be connected across 400 V. So measure line to neutral and double it. Guidance Note 3 says that, as a rule of thumb, the three-phase short-circuit level is about twice the single-phase value, and that this errs on the side of safety.
- Record the highest of the six (after doubling) as Ipf — 0.600 kA here.
Then reconnect the earthing conductor. At each sub-board, repeat it all as Zdb, doubling the PSCCs again.
Guidance Note 3 · 2.6.16 · prospective fault current · three phase, about twice the single-phase value · p.88
Free section from Sparky Study Notes. The full Level 3 notes, 1,463 practice questions and the video lessons: sparkystudynotes.co.uk
Test yourself on it
The tests are in the members’ area. Get one wrong and it shows you why, and where to revise.
Get new notes and exam tips from Sparky
About one email a fortnight. Unsubscribe any time. Privacy noticeThat was one section. There are 583.
Sparky Study Notes is the whole Level 3 syllabus written up like this: the explanation, the worked example, and where it is in your book. With a video lesson for every part and a practice paper for every exam.
- 583sections of written notes
- 35video lessons
- 1,463practice questions
- 146diagrams, redrawn
- 18th Edition
- Building Regs
- PAT
- Fundamental Inspection & Testing
- Initial Verification
- Periodic Inspection & Testing