Q & A of the Day – Are these RCD rated test values OK?

Question: I've recently changed a consumer unit at a domestic dwelling, supplied from a TT System. The new board is made up with a 100mA time-delayed RCD as the main disconnector, plus a further two 30mA RCDs for the MCB-protected final circuits.

I have two questions:

1) Can I record the max permitted Zs for each circuit as 1667 ohms 50/30mA on the test sheet, as the max values for MCBs in the Regulations is for TN Systems, not TT?

2) When testing the 30mA RCDs, the rated test value is 32ms and the five times test value is 9ms. Seeing that the max disconnection time is 20ms for a TT system, are the values OK, even though only the five times test meets the requirement?

Answer: For a new installation carried out to BS 7671:2008, in a domestic or similar premises with a TT earthing system, you would normally provide a dual board with two 30mA RCDs or RCBOs for each of the circuits.

The recorded Zs values would be based on the protective device in place, i.e. the MCB.

Appendix 3 of BS 7671:2008 gives the requirements for testing RCDs.

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