I write with reference to Cliff Drewery’s letter about incandescent lamps not taking out residual-current devices (EMC April 2009, p5).

I do not know how old Mr Drewery is but years ago we had consumer units with an isolator for the main switch and MCBs. Then the electrical trade came up with the consumer unit with an RCD as a main switch.

This soon proved to be a wrong decision as the lamps kept taking out the RCD, shutting down the whole house. This made old people very vulnerable and was also a total inconvenience as everything had to be reset.

The trade then came up with the split board, where all the internal lighting circuits go on the non-RCD side – why is that if a lamp never takes out an RCD, as stated by Mr Drewery?

The modern 17th Edition boards are split three times for the same reason. I personally use RCBOs as I believe they are the only way to comply fully.

Paul Knowles, RJ Engineering, Folkestone, Kent