Free notes
Circuit breaker types and fusing factorWhat current trips a B6 in 0.4 seconds?
Free from the notes: Sparky’s video on it, the section word for word as members see it, and a PDF to keep.
The question at the end: What current trips a B6 in 0.4 seconds? The answer is in the notes below.
The notes
Exactly as they are on the site.
| Type | Trips instantly at (within 0.4 s) | Used for |
|---|---|---|
| B | up to 5 × In | Domestic and small commercial |
| C | up to 10 × In | Commercial and light industrial (motors, fluorescent lighting) |
| D | up to 20 × In | High inrush loads: X-ray machines, welders, transformers |
On-Site Guide · Table 7.2.7(ii): circuit-breaker types, tripping currents and applications · p.102 Worth tabbing: you use it a lot
In is the device's rated current, e.g. 6 A or 32 A.
Worked examples
B6 lighting circuit (domestic lighting): current to trip within 0.4 s = 5 × In = 5 × 6 = 30 A B32 ring final circuit (domestic sockets): current to trip within 0.4 s = 5 × 32 = 160 A A non-delayed RCD must trip within 300 ms (0.3 s) at its rated residual current
A CB has two ways of tripping:
- A magnetic coil handles big fault currents fast.
- A bimetallic strip heats up and bends under overload, so it trips more slowly.
Fusing factor
Circuit breakers have a fusing factor of 1.45 → I2, the current that ensures effective operation of the device on overload, is 1.45 × In B6: 6 × 1.45 = 8.7 A So between 8.7 A and 30 A the B6 trips on a time curve; at 30 A or more (5 × In) it trips within 0.4 s
For lighting, work out the circuit current from the lamps' power, e.g. 10 × 20 W = 200 W, then I = P ÷ V.
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