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RCD testing practical: which circuits need 30 mAWhy is it safe to switch on now?
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RCDs are everywhere now: every socket-outlet circuit used by ordinary people needs 30 mA protection under Reg 411.3.3, and domestic lighting under 411.3.4. This test proves each RCD disconnects fast enough.
It’s a live test with the installation energised — safe to do now, because every test before it has proved the installation is sound. Earth fault loop impedance is tested before the RCD tests.
| Test | On a 30 mA RCD | Expect |
|---|---|---|
| ½ × IΔn | 15 mA | Should not trip — it isn’t over-sensitive · recommended |
| 1 × IΔn | 30 mA | Must trip, within 300 ms (500 ms for an S-type, time-delayed) · recommended, and the one BS 7671 requires |
| 5 × IΔn | 150 mA | Within 40 ms (150 ms S-type) · optional, for fault-finding only |
BS 7671 only requires the 1× test, with the instrument on Type AC, and the result recorded on the schedule of test results. Guidance Note 3 recommends ½× and 1× (Table 2.18(a)); 5× and the Type A, F and B tests are its optional tests (Table 2.18(b)) — for fault-finding, not BS 7671 requirements, and they shorten the RCD’s life.
Guidance Note 3 · 2.6.18 · p.95 Guidance Note 3 · Table 2.18(a) recommended · Table 2.18(b) optional · p.96
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