I disagree with a statement made by Colin McAhren in his article on consumer units meeting the 17th Edition requirements (EMC, February 2009, pages 31-32)

McAhren stated: “When an incandescent bulb fails, it almost always trips the residual-current device [RCD].”

This is not the case. A failing lamp often trips a miniature circuit-breaker (MCB) but does not trip an RCD. This is because there is often a short circuit created between the phase and neutral when the lamp fails. RCDs are not designed to trip because of a phase-neutral short circuit. I have never known a failing lamp to trip an RCD.

It is important to raise this as the statement was made to reinforce the argument that it is important that circuits do not share the same RCD device.

In an ideal world, Mr McAhren is right to say that the only way to fully comply with the 17th Edition is to protect every circuit with an individual residual current-breaker with over-current protection (RCBO).

However, in reality this is not viable. It is important for the installation designer to produce a compromise solution to minimise inconvenience by using a mixture of shared RCDs, and using RCBOs for circuits that are most vulnerable to tripping or for those that may cause a hazard or major inconvenience if they were to trip due to a problem on another circuit.

Cliff Drewery, C&J Electrical