SPARKY STUDY NOTES sparkystudynotes.co.uk/learn/three-phase-supplies-and-boards
Free notes
Three-phase supplies and boardsStar or delta, and how does an RCD's test button work?
Free from the notes: the section word for word as members see it, and a PDF to keep.
The notes
Exactly as they are on the site.
- A generator has 3 windings, making 3 sine waves 120° apart. Line to neutral is 230 V on each phase. Line to line is 400 V, not 460 V, because the phases never peak together.
- Star (the transformer secondary) has a star point, so it gives a neutral, which carries any out-of-balance current when each phase feeds different loads. Delta (usually the primary) has no star point, so there is no neutral.
- Three-phase is mainly commercial and industrial (big machinery, plant, welders). Very large sites may take an 11 kV supply. One transformer can feed a street, a village or a single factory. A house normally takes one phase, and you usually won't know which.
- Triple-pole (TP) main switch: switches off all three phases at once, so you can't turn off just one. A TP&N (4-pole) switch also breaks the neutral.
- You can mix three-phase and single-phase circuits on one board. Example: 4 three-phase breakers fill 12 ways, or 2 three-phase plus 6 single-phase uses the same space for 8 circuits.
- For a whole supply there are 3 DNO fuses (all the same rating: 63, 80 or 100 A), a three-phase meter with L1, L2, L3 and N in and out, and an isolator. The earth doesn't go through the meter.
- Isolation: for a consumer unit change with no isolator, the DNO must pull the fuse. For work on one circuit, lock off that breaker or the main switch. Safe isolation is covered in detail later.
- Mixed colours: when joining old and new three-phase cables, e.g. red→brown, yellow→black, blue→grey, black→blue, make sure every conductor is correctly identified. A caution label is no longer required by BS 7671 (Reg 514.14 was deleted in A2:2022), though you can still fit one.
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