Replacing Fuses with MCBs

[anthony.j.cunningham]

Apologies for the lack of specifics here; I'm just at the stage of finding out if this is feasible or not.

Currenly we have a traditional fusebox (fuses with fuse wire in ceramic cylinder) installed and I wonder if it is possible to replace these hoders with the circuit breaker type where a button pops out? Obviously these things would have to be the same size and have the same fiitings. Has anyone done this successfully or would I need a new fuse box?

My reason for asking is that every time one of the bulbs goes in my kitchen light it takes out the fuse on the lighting circuit with it and out go the downstairs lights. I'm getting a bit fed up of rewiring the fuse.

Thanks

Tony

[mrcheerful]

providing the right type is available, then yes, no problem, I even found some for a really ancient fuse board.

mrcheerful

[John Rumm]

If you have the traditional Wylex type rewireable fuses:

you can get plug in MCBs to replace these:

that they are all "B" type MCBs. These are the "normal" type of MCB that has a quite fast response to fault conditions. Hence they will trip as often (in fact probably more often) in response to bulbs blowing than the rewireable fuses you currently have. Saves replacing fuse wire but still a PITA!

If you replaced the consumer unit with a more modern one then you could use type C MCBs for the lighting circuits. This may reduce the number of times you get spurious trips.

[bill]

> Note that they are all "B" type MCBs. These are the "normal" type of MCB

My fuse box has MEM stamped on the cover, would the Wylex type fit this or is there an alternative.

thanks

Bill

[Alex]

Would such a replacement bring any safety benefit? My "new" (30 years old) has such fuses instead of circuit breakers, and I'm worried about the electrical safety of fuses vs circuit breakers.

Alex

[David Hansen]

On Tue, 1 Nov 2005 10:06:30 -0000 someone who may be "bill" wrote this:-

Probably not.

is the place to start.

[David Hansen]

On Tue, 01 Nov 2005 11:07:42 +0000 someone who may be Alex wrote this:-

You need to split fuses into two, rewirable and cartridge fuses.

Cartridge fuses are moderately expensive, break high currents rapidly and are moderately easy to replace.

Circuit breakers (of the sort we are talking about here) are the most expensive, break moderate currents in time [1] and are easy to reset.

Rewirable fuses are cheap, only break low currents and are fiddly to replace.

For most houses circuit breakers are the most sensible form of protective device. One of the few downsides is that, if the house is near a substation, in some types of fault the circuit breaker will fail to operate properly and the main (cartridge) fuse will operate. Cartridge fuses guard against such problems, but cannot be reset.

[1] for many years now manufacturers have said that some of their circuit breakers will break high currents as rapidly as a cartridge fuse. However, the graphs they use to make these claims stop before the interesting bits.

[John White]

There is no reason to assume that the fuses will be any less safe than circuit breakers, assuming they were correctly installed in the first place and have not been fiddled with.

The inherent problem with rewirable fuses is that you can rewire them with just about anything. It is not uncommon to find that a fuse which keeps blowing is "fixed" by putting in 30A wire instead of 5A. To a lesser extent cartridge fuses can also be abused in this way.

The only case when I would definitely recommend replacing fuses with MCBs on safety grounds is when the property is to be rented out, but in such cases you would probably want to put in a split box anyway.

John

[bill]

Thanks for that - didn't know what MEM meant

Bill

[meow2222]

Rewirable fuses have perfectly satisfactory breaking current ratings.

MCBs are prone to nuisance trips, fuses rarely are. If your fuse is popping when the bulb goes, something isnt right. If you fit an mcb you will have more nuisance trips, not less.

MCBs are generally reckoned to offer a bit more protection than fuses, but its a fair bit less simple than that, and fuses have their plusses too.

Its not usually necessary to go round replacing functioning fuses with MCBs. If you want to worry about risks, getting hung up on trivia like this is barking up the wrong tree. Your stairs are 100 times riskier. Beds, roads, food, kitchens, power tools... not fuseboxes.

If youre using improperly ballotini fused bulbs and cant get decent ones, a simple workaround is to put those lights on their own 3A fuse, or better an mcb, between light fitting and CU. The 3A fuse is easily replaced, or the mcb switched back on. Makes me wonder what kind of bulbs youre using though.

NT

[David Hansen]

On 1 Nov 2005 08:58:06 -0800 someone who may be [e-mail] wrote this:-

That depends on what the short circuit current is at a particular point in the particular installation.

I think you mean someone else, not me.

[Dave Plowman (News)]

Sounds like the circuit is running very close to its limit. Have you calculated the maximum load?

[Rumble]

Midland Electric Manufacturing (or similar). The company has been around for about 90 years.

[John Rumm]

Mains halogen bulbs seem to be the worst culprits here, but any filament bulb can cause a current surge on failure if it manages to ionise the gap between the feed wires at the end of the filament.

Adding RCD protection on appropriate circuits will probably make more contribution to safety than just replacing fuses with MCBs. The MCBs will obviously be more convenient should a trip occur.

[anthony.j.cunningham]

Any idea what might be going on? I reckon I'm not getting the lifetime that I should be out of the bulbs in the fitting.

I was think of something that was easier to reset when the fuse did blow.

It's the ceiling light in the kitchen. I've had two fittings; both contained multiple spot bulbs. The older fitting held the screw-in reflector type, the newer one holds four of the small spots that are more popular now. With each fitting one of the bulbs would blow when the light switched on and take the fuse with it.

Tony

[anthony.j.cunningham]

No but I'd be surprised if it was at it's limit. The most recent failure happened when no other downstairs lights were on. Unless there's something hanging off the circuit that shouldn't be there, of course.

[Norma]

Tried from from Screwfix. Works fine but fuse box cover will not fit

-- MCB too proud.

[John Rumm]

These are usually ok...

These however are famous for poping bulbs and fuses at regular intervals. They are also very sensitive to vibration - so someone walking on the floor in the room above can be enough to shorten the life of the bulbs a fair bit.

(the 12V halogen lights are much better in this respect since the fillament runs at a much higher current and so needs to be thicker and hence stronger. The light produced is also whiter and you get more of it)

[meow2222]

Your mini halogen spots could be sorted by using 12v bulbs, running them off a 12v transformer. No more fuse pops, and cheaper longer lasting bulbs.

Am surprised your R80s take out fuses tho, they dont normally. I'm not clear why its happening.

NT