Plug in MCB upgrade

23 Sep 2009
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Isle of Wight
United Kingdom
For many years i've been meaning to replace my wylex fuse box with a new circuit breaker unit.

The present box has the colour coded fuse cartridges:
White 5 amp
Blue 15 amp
Red 30 amp

I have now thought about converting the present box using wylex plug in MCB's which will be cheaper & easier with minimal work.

The mcb's are similarly colour coded with ratings of 6, 16, & 32 amp.

There is a trip switch with a red test button & sliding switch which is located separate from the fuse box.

My question being, is this a good option that will provide the same level of protection as a new circuit breaker box? I do not want to take any risks but conversely to do not want to pay for unnecessary work.

The electrics work fine with no apparent problems.

Many thanks.
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This change will give you the same protection as you have at present, although with the benefit of quicker reaction to a large overload/short circuit. If the test button enabled switch (a RCCD) is before the fuse box then you will have all circuits protected by that unit. This one does not give protection against overload but reduces the danger from electric shocks. Sounds a good idea to make the change; however, one caveat. If you are thinking of changing because there are regular "blown" fuses then the reason needs investigation.

Yes, the RCCD is before the before the fuse box - actually between the meter & box.

Electrics all seem fine. We did have a lighting fuse blow some months ago which was caused it seems by a LED going down. The LED was returned to supplier for testing and a replacement given. Since then no problems and before that I can't recall when the last time a fuse blew.

I do recall this incident prompted an element of bad language due to locating that 'safe place' i had put the fuse wire and then the fiddly bit of connecting the stuff! Not that difficult actually but clearly not as easy and throwing a switch!
they are fairly easy to change, but you must get new shields (which go behind the fuses or MCBs) to match. You will see a small screw in the coloured shield when you pull out the fuse carrier. The MCBs to suit plug in with similar prongs to the fuse carriers. When the shield is removed, the live busbar is exposed.

You must turn off the main switch, but do not need to remove the CU cover (there are live terminals inside even when the main switch is off)

in your case turn off the RCD as well and test for dead.

Change them one at a time (i.e. do not have them all out at the same time)

If you still have any incandescent (filament) light bulbs, then you will probably find the lighting MCB trips when as bulb blows, as they react quicker than fusewire to the momentary surge. Spotlights are especially bad for this.
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Personally I think its a waste of money fitting plug in MCB's, espicially if they are at new prices, unless of course you frequently have a fuse blow, which should be investigated anyway.

Best to have the money towards a new consumer unit.
Probably an all RCBO unit as you have few fuses at present.
Yup, agreed wait until you can afford to buy a new consumer unit and buy some secondhand fuse carriers to sit on top of your current board in case of fault in the meantime. You can then change the wire at your leisure.
Thanks for comments.

Whilst i had been thinking of upgrading for some time I admit the main reason for it now was to re assure my wife. As said previously, we have switched over to led's for our main lounge lighting with 10 units. The one going down actually blew the fuse and knocked out the ground floor lights.

We have had no repeat of this so it seems it was just a one off faulty bulb. My wife was a little concerned however that if i'm out & it happened again then she wouldn't want to change the fuse herself. Not saying she couldn't but you know what i mean!

Paradoxically, i do hope it is a waste of money because that will mean it hasn't happened again!

I am quite confident in changing these myself but with a new box, i'd need an sparks, pushing up the cost significantly.

Further thought required perhaps.
You could just buy one plug-in mcb for the lighting circuit although that will still mean the cover won't fit, or -

buy another fuse and have it ready for a straight swap if it blows again.
My wife was a little concerned however that if i'm out & it happened again then she wouldn't want to change the fuse herself. Not saying she couldn't but you know what i mean!
You could find a friendly electrician (I'm assured they do exist) and ask him nicely if he has any old fuse carriers you could have, then leave one around for your wife to use a a replacement. Make sure it's wired with 5A fuse wire in case she substitutes it for the cooker fuse...
You could just buy one plug-in mcb for the lighting circuit although that will still mean the cover won't fit ....
The fuse covers for at least the later Wylex Standard CUs had knock-outs to enable plug-in MCBs to protrude through the cover.

However, I agree with those who have questioned whether there is any real point in replacing the fuses with MCBs.

Kind Regards, John
it maybe also worth noting that mcb's are more likely to trip annoyly than fuses when lamps fail.

I'm a little surprised an LED unit has caused a fuse to blow.

It maybe worth ensuring you have a lamp with a 13A plug on it, (ready to turn on) so your wife has some light should the fuse or mcb blow.
Thanks again guys, some really good help & advice which i will give some thought to.
I guess the old 'if it aint broke don't try to fix it' applies a bit here.

Re the blown led, it was a bit frustrating tracking it down. Circuit blew one evening without warning so i couldn't tell what had caused it. Replaced fuse & switched back on ...... Circuit blew again! So again replaced fuse to isolate cause which blew again a few times until i switched on the light containing the culprit.......which guess what, was the last one I tried.

Removed offending dead bulb & all ok.
JohnD ........ Thanks for the PM to which I tried to reply but unable to as it says we are 'not friends'!

Anyway, I will reflect on the best way forward before any further action.
Thinking about this further & apols for my lack of knowledge in this area.

My present set up has a RCCB (i think that's what it is) .... Black box with red test button & sliding off/ on switch. This is between the meter & the fuse box.
There is also, before the meter, another black box with a seal on it but no switches - what would this be for?

Also, what extra protection would a new circuit breaker unit give me over staying with the existing box fitted with or without plug in mcb's

Just want to get a correct balance and as my field is accounting, unsurprisingly if I'm going to lay out more cash i like to know the benefits.

I do appreciate your guidance.....thanks.

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