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?

2. 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.

Caecilius wrote:
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?

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


2. 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.

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.

Caecilius wrote:
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?

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".

2. 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.

Probably nothing to worry about...


--
Cheers,

John.

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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.