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Two-wire or three-wire: the loop impedance testTwo wire or three wire?
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Back over the earth fault loop impedance test, and the decision that drives it:
| Test | When | Current |
|---|---|---|
| Two-wire high current | Always — unless there is an RCD in circuit | Enough to need care (see below) |
| Three-wire non-trip | Where an RCD or RCBO is present | Under 15 mA, so the RCD does not trip |
There is also a two-wire non-trip test on some instruments — no neutral lead, measuring across line and earth of the supply only. The advantage is that you can take a Zs at a lighting point without reaching in among exposed conductors. The trade-off is that it is less accurate and more prone to error, and it is not on every tester.
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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.
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- 18th Edition
- Building Regs
- PAT
- Fundamental Inspection & Testing
- Initial Verification
- Periodic Inspection & Testing