Eddie Platt asks this question, which concerns odd R1 + R2 readings obtained from testing a lighting circuit. It has been answered by SELECT (Theme – Fire, Safety & Security):
Question: I've recently conducted an R1 + R2 test on a lighting circuit through a dimmer switch. The R1 + R2 reading when the dimmer is bypassed, is a reasonable 1.7 ohms, which is protected by a 6A Type B circuit breaker. However, the lowest reading I obtained whilst testing through the dimmer switch was 86.7 ohms. Is this acceptable?
I can't help thinking that under fault conditions, the protective device will exceed the 0.4 disconnection requirements as laid out in Chapter 41 - so alternatively, should I allow for the increased resistance of the dimmer switch?
Your thoughts and guidance be much appreciated.
Answer: It would appear that the dimmer switch is causing the high resistance value and, as you indicate, this may prevent the protective device from operating under fault conditions.
If your client intends to continue to use the dimmer switch, it may be acceptable to fit a suitably rated RCD to provide automatic disconnection.
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