SPARKY STUDY NOTES sparkystudynotes.co.uk/learn/checking-zs-for-a-fuse
Free notes
Checking Zs for a fuseA 16 amp fuse reads 3.25 ohms. Pass?
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.
Fuses have their own table, and a different disconnection time.
A 16 A BS 88-2 (gG) fuse on a distribution circuit
Disconnection time = 5 seconds (distribution circuit)
BS 7671 · Table 41.4 · p.76
Tabulated Zs = 4.0 Ω
80% figure = 4 × 0.8 = 3.2 Ω
Reading taken = 3.25 Ω
✋That one fails. A 3.25 Ω reading sits just above the 3.2 Ω figure the 80% rule gives, so it is not acceptable — however close it is. Find the tabulated value in your own book, apply the 80%, and compare. Over is over.
Different device, different table. Table 41.3 is circuit breakers and RCBOs. Table 41.4 is fuses with a 5 second disconnection time. Read the question for which device you have.
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