In message ,
(StephenC) wrote:

My house lighting is a total of 32 lamps, none more than 60 watts,
on 4 circuits, (11, 5, 9 and 7 lights) At the 'MK Sentry' CU, the
circuits are in pairs, to give loads of 16 lights to each 6A MCB.
When any lamp blows, the relevant MCB often trips, which is
annoying, with safety implications - part of house suddenly in
darkness. Any solution? If I split down to 4 seperate 6A MCB's,
will that prevent this? Or, replace the present 6A MCB's with 10A
MCB's? Or type C MCB's, 6A or 10A? Ideally, what I need is a link
for the CU to put a cartidge fuse in, never had this trouble with the
old systems, but I dont think 'MK' do one, although I believe others
may, but won't fit 'MK'CU, Any advice much appreciated.

Interesting system. Have I understood you correctly that there are *two*
wires into the top of each MCB? Four actual circuits running from two
MCBs? This isn't wrong, UNLESS someone has actually wired your lights in
two rings, but you sound pretty confident they are four circuits so
we'll assume that. Your idea of splitting down to four MCBs sounds
sensible for the following reason:

Your current arrangements are too heavily loaded.

First things first. The design of lighting circuits should take account
of the actual load fitted, subject to a minimum of 100W per standard
fitting. Other fittings which only take (for example) low wattage bulbs
can be counted at their rating, but standard bayonet and screw fittings
must be rated at 100W minimum. By this calculation, and assuming all
your fittings are standard, you have a potential 1600W (100W * 16
fittings) on a circuit designed to supply at most 1440W. By splitting
into four, even your most populous circuit will only be 1100W unless
you have something exotic fitted to it.

Granted you say that you don't have anything more than 60W fitted, but
someone might in the future. 60W * 16 fittings is 960W which isn't so
bad, but if everything is on it is a reasonably high load which may
cause problems when bulbs blow.

Certain types or brands of bulb are more prone to blowing the fuse when
they fail than others. Do you have a lot of candle bulbs? Are your bulbs
all from the same manufacturer? Another advantage of splitting into four
is that you'll have fewer bulbs out if the MCB does blow.

To your other points:

Replacing a 6A type-B with a 10A is unlikely to solve the problem unless
it's something which is "only just" happening at the moment. A short is
a short - assuming that is what happens when your bulbs blow - and will
trip any rating of MCB very quickly, subject to loop impedances. The
current rating only really becomes an issue with mild-ish overloads
where (for example) a 6A MCB will eventually trip if presented with a
steady 12A load whereas a 10A probably won't. There are standard curves
for these things in "The Regs" but my copy is in the car and I'm not
going outside for it right now!

On top of that, if your lighting circuits have been wired in 1mmsq
cable, which they probably have, this cable is not rated for 10A load
unless surface mounted IIRC. For this you'd need 1.5mm cable.

A type-C might help, but unless you know what you're doing this is
probably not the best way to avoid the problem. These devices are less
sensitive to very short-term surges (overloads) and designed to overcome
problems with inrush currents to highly inductive devices such as
transformers. This current, although normally enough to trip a B-type
falls far short of a short, IYSWIM. The short circuit which is created
by your blowing bulb probably results in much higher currents, and
delaying the disconnection in any way may damage the cable.

Again, if it is a dead short, you may blow even a cartridge fuse, and
these are slightly more difficult to "fix" than flipping the MCB back
on. When you say you "never had this trouble with the old systems", was
it an old cartridge fuse system, or rewireable fuses? Rewireables have
significantly different characteristics even to cartridge fuses, and
because of this, these days you have to derate cable protected by them.
What I mean is that just because a rewireable didn't blow, don't assume
a cartridge fuse won't either.

There may be other options which others will discuss, but that's me done
for tonight. I need my bed. Bite the bullet and install two more MCBs
(should be done from a design point of view, but may also help your
problem), and ditch all those candle bulbs. Replace them with "low
energy" bulbs and save yourself some money to boot.

If safety is still a concern, give a thought to fitting some emergency
lights - there are some designs these days which are very suitable for
hall & stair use for example.

HTH

Hwyl!

M.

--
Martin Angove: http://www.tridwr.demon.co.uk/
Don't fight technology, live with it: http://www.livtech.co.uk/
.... I hate making predictions; especially about the future!