Replacing fuses with MCB's

[mcbrien410]

My 25 year old MEM consumer fuse box is still in A1 condition and I've only had two fuses blow during that peiod. However I do feel I ought to update and fit some MCB's.

My MEM fuses screw onto the live buss bar and the other side of the fuse takes the wire for the ring main or whatever.

Can I replace these fuses directly with a screw on MCB like say this from ScrewFix... MK 16A SP Type B MCB 6kA

...or do I have to replace the consumer box to one that just has a clip on supporting rail with the supply main wire going in one side of the MCB and the ring main wire into the output side.

T I A Chris.

[John Rumm]

First question is: what problem are you trying to fix? Does your existing CU have rewireable fuses or cartridge fuses?

Most modern CUs have a busbar that connects to the base of all the MCBs rather than a wire as such. The top circuit connections being made to the top of each MCB.

[mcbrien410]

existing CU have rewireable fuses or cartridge fuses?

I do NOT have a problem. I just wish to upgrade from fuses to MCBs. My fuses are rewireable types.

The pictures I've seen of MCBs seem to just clip onto a support bar and have a screw terminal at each side for wires. If they screw to a buss bar I'd only expect to see one screw terminal for the output to the ring or spur or whatever.

Chris.

Chris.

[Grumpy owd man]

You'll be lucky to find a compatible MCB for such an old unit. HAs the unit got a 'smoked grey perspex cover' on it? if so, are the 'prongs' on the fuse carriers oriented side to side or are they top to bottom?

[chris French]

In message , [e-mail] writes

No, one of the screws is for fixing the MCB in place on the bus bar, and presumably ensuring good electrical connection.

[David Hansen]

On 17 Jan 2006 13:54:14 -0800 someone who may be [e-mail] wrote this:-

While there is a copper busbar in consumer units, MCBs have a screw terminal at each side. One terminal is screwed onto the busbar, the other takes the circuit wires. The module clips onto a rail, but the electrical connections are made by screw terminals.

I suspect that a unit that old will not take modern MCBs and MEM don't have any replacements available (check with their web site for what is currently available). Should you wish to replace your existing rewirable fuses, you will need to replace the whole consumer unit. Personally I would not do this unless there were pressing reasons, like a need for extra circuits or something like old age making rewiring fuses more difficult. More importantly, the existing wiring should be tested, using the right equipment, to make sure it is still sound.

[John Rumm]

MCBs do offer greater ease of use, however if you are not having to rewire fuses on a regular basis then there is only a limited amount to gained from the change. MCBs represent slightly better protection in most cases, but not that much in absolute terms assuming the wiring is in a reasonable state (rewireable fuses are still permitted in the current wiring regs). If you are looking to improve safety then you may get more "bang for your buck" making sure you have RCD protection where it will give best advantage.

There are not usually any side connections on MCBs

Take as an example:

bottom screw and terminal is used to secure the MCB to the busbar.

A busbar is usually comb like:

prongs of the bar poke into the bottom terminal of the MCB and the screw is done up. The end prog of the bar pokes into the bottom terminal of an incomer switch or RCD which is fed from the meter tails which fix into the top connection.

The rear of it clips to the DIN rail, and the top screw is used to clamp the wires to the circuit.

[mcbrien410]

The wiring is only 25 years old and done with all the same wire type as today. As I said I've only had two/three fuse blow in 25 years, one self inflicted, one due to a mouse and one due to a light bulb blowing.

The CU does have the two smoked perspex drop down windows that give access to the 12 fuses. About five years ago I did tidy it all up when I inserted a new ring main in a new bedroom and replaced the above mouse bitten cable.

The only reason for replacing the fuses with MCBs was to make it a bit more up to date so that if I did sell up the next person wouldn't have to call out a Lektrishun to replace a fuse.

Chris.

[DJC]

to replace a fuse! and tis uk.D-I-Y

[John Rumm]

I would have thought if you were going to replace it, it would only be worth doing if you were going to upgrade the whole thing so that you could have some form of split load affair to give you RCD protection on sockets etc, and to make sure all the equipotential bonding is up to scratch (since you can almost guarantee that will be a comment on a survey if not to current standards). You make life much simpler with obtaining suitable MCBs etc if you choose a current model of CU rather than sticking with one that uses bespoke parts that may be in short supply.

It would give you the chance to split existing circuits or add new ones where appropriate (like a dedicated ring circuit for the kitchen perhaps)

You could also design in a few extra nice touches like battery maintained emergency light close to the CU that is powered from the same MCB as the lighting circuit that usually illuminates the CU - that way if the circuit trips you can still see what you are doing to reset it.

[David Hansen]

On Wed, 18 Jan 2006 12:17:38 +0000 someone who may be John Rumm wrote this:-

Alternatively just wire a suitable light to another lighting circuit. Other than a major failure, this will allow one to sort out most problems without a torch.

[Geo]

So I can still do /some/ re-wiring affecting the kitchen, bathroom...

Geo

[John Rumm]

Not quite sure how you arrive at that statement from my comment about fuses... ;-)

I take it you are refering to Part Pee rather than the wiring regs (BS7671)...

Regarding part P, you can do any rewiring you like in any location. You can also do it legally if you are prepared to involve building control at the relevant times.

[Geo]

Referring to re-wiring my fuses - there is a wee card with the spare wire wrapped round it on top of the fuse-box. Only used it 30 years ago when some idiot turned on too many lights while the (750w) infrared bathroom dimplex was switched on :-)

Geo

[David Hansen]

On Wed, 18 Jan 2006 16:41:39 GMT someone who may be Geo wrote this:-

The heater should really be on a spur off the ring main. Then the problem would not arise.

[Geo]

It was dumped in '79 when I installed the central heating.

BTW they were also sold as bathroom lamp fittings.

Geo

[David Hansen]

On Wed, 18 Jan 2006 20:22:18 GMT someone who may be Geo wrote this:-

[(750w) infrared bathroom dimplex]

They still have their uses, when it is not worth having central heating on. In spring and autumn for example.

They were but, like heat lamps, should not be wired into lighting circuits. Rather they should be on a (fused) spur from a ring main.

[DavidM]

See below:

I found some MCBs in FADS about a year ago that had bayonets on them and simply plugged in in place of the old rewirable fuse holders. Cost about £15 each and available in a range of ratings. I had to modify the plastic cover on the CU cos they stick out more than the old fuses, but easily done with a small saw and knife. If you have trouble finding them let me know and I'll take one out and get the manufactures name (if it's printed on there). Much more convenient if a bulb blowing causes the fuse to go, as was happening to me. David (remove _notme to email)

[DavidM]

See below:

I found some MCBs in FADS about a year ago that had bayonets on them and simply plugged in in place of the old rewirable fuse holders. Cost about £15 each and available in a range of ratings. I had to modify the plastic cover on the CU cos they stick out more than the old fuses, but easily done with a small saw and knife. If you have trouble finding them let me know and I'll take one out and get the manufactures name (if it's printed on there). Much more convenient if a bulb blowing causes the fuse to go, as was happening to me. David (remove _notme to email)

[norm]

Same problem here with the screwfix ones.