Free notes
R1 + R2 as a designer: Table B3Why multiply by Table B3?
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.
What it's for: the designer's next question is "will ADS happen on my circuit?" Automatic disconnection of supply can't happen without a path to earth from the exposed-conductive-parts back to the MET. So the designer needs the circuit's Zs, and for that they need R1 + R2.
Design, install, verify
This one overlaps with testing. The designer works out R1 + R2 on paper. The installer builds it. Months later the inspector measures R1 + R2 with a low-resistance ohmmeter (test method 1) and checks it against the design. Design → install → verify → back to the design. That's why several books print the same table.
Where to find the resistance per metre
Table B1 gives the resistance of line + CPC per metre (mΩ/m) at 20 °C, for each combination of sizes. The same table appears in three books, with identical values. For 2.5 mm² line with 1.5 mm² CPC (standard 2.5 mm² twin and earth), all three give 19.51 mΩ/m.
Guidance Note 3 · Appendix B · Table B1 R1 + R2 per metre (mΩ/m) · p.174 On-Site Guide · Appendix I · Table I1 (the same table) · p.226 Electrical Installation Design Guide (IET, Level 4 book) · Appendix F · Table F1 (the same table)
You have GN3 in your inspection and testing exams, so use Table B1. The On-Site Guide isn't used in most exams. Strictly, GN3 is an inspection and testing book, not a design book. But it's the one everyone knows, and the values are the same.
Designer's formula
B1 in mΩ/m · B3 = multiplier · L = length in m · ÷ 1000 turns milliohms into ohms
Table B3 is a short table of just six multipliers. It corrects the resistance to the conductors' operating temperature. Earth faults only happen while a circuit is on (a live part touching an earthed part). A working circuit's conductors are warmer, and warmer copper has more resistance. B3 lets the designer copy those operating conditions on paper.
| Table B3 multipliers used here | Value |
|---|---|
| 70 °C thermoplastic, CPC incorporated in a cable (e.g. twin and earth) or bunched | 1.20 |
| Conductors run on their own, separated (e.g. singles spaced out on cable tray) | 1.04 |
| 90 °C thermosetting, incorporated in a cable | 1.28 |
Guidance Note 3 · Table B3 operating-temperature multipliers · p.176 Six multipliers in total. Check the rest in your copy
Worked example: radial, 2.5/1.5 mm² twin and earth, 17 m
Table B1 (2.5 mm² line + 1.5 mm² CPC) = 19.51 mΩ/m
Table B3 (70 °C thermoplastic, in a cable) = 1.2
L = 17 m (the designer knows it from the drawings)
R1 + R2 = (19.51 × 1.2 × 17) ÷ 1000
= 398.0 ÷ 1000
= 0.40 ΩA common slip is to give the answer as 0.33 Ω. That's 19.51 × 17 ÷ 1000, which leaves out the 1.2 from Table B3 (even though it's written in the working). With the multiplier included it's 19.51 × 1.2 × 17 ÷ 1000 = 0.398 ≈ 0.40 Ω. The method is right. Only the final number slips.
The designer is saying: "If you follow my design, when the inspector comes to verify it, they should measure this value or something close to it."
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