In Paul Knowles’ letter headlined ‘Lights out time’ (EMC May 2009), which was written in response to my letter a month earlier, he refers to the time when consumer units were available with a residual-current device (RCD) as the main isolator.

I have had this type of consumer unit serving my house for the past 15 years. In all that time, a failing lamp has never tripped the RCD.

Occasionally a lamp has tripped a miniature circuit-breaker (MCB), but never the RCD.

I know the RCD is working OK because I regularly test it with my electrical installation tester, as part of my regular tester calibration checks.

An RCD trips on earth leakage. How can a failing incandescent lamp trip an RCD? The lamp does not have a connection to earth. A failing lamp can trip an MCB because of a momentary short when the filament breaks, and an MCB is designed to trip on short-circuit faults. An RCD is not designed to trip under short-circuit conditions.

I agree with Mr Knowles that the best way to fully comply with the 17th Edition is to use RCBOs (residual-current breaker with over-current protection) to individually protect circuits. However, with RCBOs approximately 10 times the price of MCBs, not everyone can afford this design.

In my opinion, it is wrong to justify the case for RCBOs by arguing that failing incandescent lamps cause RCDs to trip, as this does not happen.

Cliff Drewery, C&J Electrical