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.
"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.
That's not satisfactory, although I don't think it is related to your problem. Lighting points should be rated at 100W or actual load if greater, so I'd expect to have your installation divided into 3 or 4 circuits. If you only have 2 MCBs you only have 2 circuits even if they do branch out from the MCB, which is permitted on radial circuits.
| Or, replace the present 6A MCB's with 10A MCB's?
If you have any SES or SBC lampholders you are limited to a 6A maximum fuse/MCB. As there is always the possibility that such may be fitted, it is usual to not have >6A lighting circuits in domestic dwellings.
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.
I had exactly this problem and replaced both 6a MCBs with 10a versions and it hasn't tripped when a bulb failed in 2 years so far. Quick "upgrade" and very efective cure, while still protecting the lighting circuit.
This will not help (except in ensuring that fewer lights fail when it trips). When an incandescent bulb blows, it ionises and goes pratically short circuit and may draw hundreds of amps. It is not related to the number of bulbs on the circuit. The solution is either to use a slower protective device, such as a cartridge fuse carrier (Type C MCB probably still too sensitive), or to use compact fluorescent bulbs.
The second solution is by far the best, as it also reduces environmental pollution and global warming. If you had all 32 lights on at 60W average, you are drawing almost 2kW of power, which is as much as a kettle or fan heater. With compact fluorescents, you'd probably be drawing around 11W per bulb, or 350W.
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