Replacing B6 MCBs with C6 on lighting circuits

[Caecilius]

I've got three lighting circuits in my house, all protected by B6 MCBs and all on the non-RCD side of the CU. When a standard bulb fails, it generally "pops" and trips the MCB for that circuit.

I'm considering changing the MCBs from type B to type C, so using C6, to reduce the chance of a popping bulb from tripping them. A couple of questions:

1. Does using C-type MCBs generally prevent blown bulbs from tripping the MCB, or does it just result in a bigger pop?

1. Do I need to worry about the earth loop impedance being low enough so a fault will clear within the required time, or is it always low enough on modern properties? My house was built in 2000.

[Dave Osborne]

Doesn't 'prevent', no. Just makes it less likely. No 'bigger pop' involved.

Earth-fault-loop-impedance vs disconnection time is generally only a consideration/worry for socket circuits (where the max. disconnection time is 0.5 seconds). On a lighting circuit (where the max. disconnection time is 5 seconds) there should be no problem.

Note that disconnection times have changed for the 17th edition of the Wiring Regs., but there's no need for you to upgrade your installation in this respect.

If your lighting circuit is RCD protected, there's definitely nothing to worry about.

[John Rumm]

It should reduce the proportion of times that the whole circuit trips. However different installations seems to get different results in this respect. Some report that the problem is almost unchanged, while for others it is almost a total "cure".

Probably nothing to worry about...

[Caecilius]

Thanks for the answers. I'll get some C6 MCBs and give it a go.

I'll keep the old B6 ones for when the old-style bulbs are finally banned and we're using CFL for everything.