RCD testing
An RCD only gives extra protection against electric shock if it trips quickly. You test it with an RCD tester at its rated residual current and time how fast it goes.
Method
- Disconnect loadsUnplug equipment on the circuit so leakage doesn’t affect the result.
- Set the testerSelect the RCD’s rated residual current (IΔn), e.g. 30 mA, and its type.
- Test at 1× IΔn, 0°Connect at a socket or point on the load side and test. Record the trip time.
- Test at 1× IΔn, 180°Reset and repeat on the other half-cycle. Record the longer time.
- Test buttonOperate the integral test button to prove the mechanism works.
Maximum trip times
| RCD | At 1× IΔn |
|---|---|
| General (non-delay) RCD / RCBO | 300 ms maximum |
| Type S (time-delayed) | 130 ms to 500 ms |
Most 30 mA RCDs in good condition trip in well under 40 ms. A device that passes but is much slower than it used to be is worth noting.
Recording
On the schedule of test results, record the trip time at 1× IΔn and tick that the test button operated. If the RCD doesn’t trip on its test button, it has failed — record it and recommend replacement.
iZapp’s 3D Testing Bench lets you do this test on a virtual board with real-looking kit: place the probes, read the tester and record the result on a practice schedule. Some attempts have a fault planted for you to find.
Frequently asked questions
What is the maximum trip time for a 30 mA RCD?
300 ms at 1× IΔn for a general (non-delay) RCD or RCBO.
Why test at 0° and 180°?
An RCD can trip faster on one half of the a.c. cycle than the other, so you test both and record the longer time.
Is pressing the test button enough?
No. The test button proves the mechanism works, but only an RCD tester shows whether it trips fast enough.
Last updated 2026-10-14. Training summary of BS 7671 and IET Guidance Note 3 practice, written by NAPIT-registered electricians. Always check the current edition and amendment of BS 7671 and your scheme’s requirements.