Expected Readings: R1+R2, Bonding and Zs
Why This Matters
- You measure R1+R2 on every circuit as a dead test.
- Check each reading against the expected value for the cable size and length. It proves the cable is what you think it is and there is no fault. If it is wrong, find out why before you carry on.
- Later you measure Ze with the supply on.
- Zs is normally calculated, not measured:
Zs = Ze + (R1 + R2)
A wrong R1+R2 gives a wrong Zs, so the R1+R2 reading has to be right.
- Compare Zs with the maximum for the protective device in BS 7671.
How to Use the Table
- R1+R2 is the resistance of the line conductor plus the cpc, from the board to the furthest point.
- Longer cable gives a higher reading. Bigger cable gives a lower reading.
- The value for 1 metre is the value for each metre. Multiply it by the length. For 2.5/1.5 mm² twin and earth, about 0.02 Ω for each metre, so 18 m is about 0.36 Ω.
Twin and Earth, R1+R2
| Cable (line / cpc) | Each metre | At 10 m |
|---|---|---|
| 1.0 / 1.0 mm² | 0.036 Ω | 0.36 Ω |
| 2.5 / 1.5 mm² | 0.02 Ω | 0.2 Ω |
| 4 / 1.5 mm² | 0.017 Ω | 0.17 Ω |
| 6 / 2.5 mm² | 0.01 Ω | 0.1 Ω |
| 10 / 4 mm² | 0.006 Ω | 0.06 Ω |
The full table, for more lengths, is in the IET On-Site Guide, Table 10.1.
Main Bonding Conductors
| Conductor | At 10 m |
|---|---|
| 6 mm² | 0.03 Ω |
| 10 mm² | 0.02 Ω |
Worked Example
A circuit wired in 25 m of 2.5/1.5 mm² twin and earth.
- The table gives about 0.02 Ω each metre, so about 0.5 Ω at the furthest point (the published table gives 0.49 Ω).
- If Ze is 0.35 Ω (an example only), Zs = 0.35 + 0.49 = 0.84 Ω.
- Compare 0.84 Ω with the maximum Zs for the device protecting the circuit.
If the Reading Is Not What You Expected
- Much higher: a loose connection, a break, or the run is longer than you thought. Check connections and the drawing.
- Open circuit: no link in the board, a switch is off, or a break. Operate the switches, then check the link.
- Much lower: the run is shorter than you thought, or there is another path. Check the earthing conductor and bonding are disconnected.
Source
Values are taken from a published reproduction of Table 10.1 of the IET On-Site Guide. Always check them against your own copy of the current On-Site Guide.