The numbers the 2391-52 expects you to apply correctly: insulation resistance, earth fault loop impedance, disconnection times and RCD operating times.
SELV & PELV — 250 V DC test, minimum 0.5 MΩ. Up to and including 500 V (e.g. most LV circuits) — 500 V DC test, minimum 1.0 MΩ. Above 500 V — 1000 V DC test, minimum 1.0 MΩ.
| Circuit nominal voltage | Test voltage (DC) | Minimum insulation resistance |
|---|---|---|
| SELV and PELV | 250 V | 0.5 MΩ |
| Up to and including 500 V | 500 V | 1.0 MΩ |
| Above 500 V | 1000 V | 1.0 MΩ |
BS 7671 Table 64 / Reg 643.3.2
Maximum Zs depends on the protective device's type, rating and disconnection time — see the Max Zs calculator for the full Table 41.3 figures (0.4 s, 230 V) rather than memorising a single number here.
BS 7671 Table 41.3
TN system — 0.4 s for final circuits up to 63 A, 5 s for distribution circuits and final circuits over 63 A. TT system — 0.2 s for final circuits up to 63 A, 1 s for distribution circuits and final circuits over 63 A.
| System | Final circuits up to 63 A | Distribution circuits and final circuits over 63 A |
|---|---|---|
| TN | 0.4 s | 5 s |
| TT | 0.2 s | 1 s |
BS 7671 Reg 411.3.2.2/.3, Table 41.1
30 mA RCDs are used for additional protection. A general (non-delay) type must operate within 300 ms at its rated residual operating current IΔn; an S-type (time-delayed) device operates between 130 ms and 500 ms at IΔn.
| Device type | Operating time at IΔn |
|---|---|
| General (non-delay) | Within 300 ms |
| S-type (time-delayed) | Between 130 ms and 500 ms |
BS 7671 Reg 415.1.1 / 643.8
GN3's recommended test set applies the residual current twice. At ½ × IΔn the RCD should not trip. At 1 × IΔn the maximum expected tripping time is 300 ms for a non-delay RCD to a harmonized standard, 500 ms for an S-delay type, and 200 ms for a non-delay RCD to BS 4293 or BS 7288:1990. BS 7671 does not require testing of RCDs at 5 × IΔn — the 40 ms figure at 5 × IΔn appears only among GN3's optional fault-finding tests, which are product-standard performance requirements rather than BS 7671 requirements.
| Applied residual current | Harmonized, non-delay | Harmonized, S-delay | BS 4293 / BS 7288:1990, non-delay |
|---|---|---|---|
| ½ × IΔn | Should not trip | Should not trip | Should not trip |
| 1 × IΔn | 300 ms | 500 ms | 200 ms |
IET GN3 10th Ed Table 2.18 (p.96) / §2.6.18 (p.97)
