Wylex MCB Plug in Replacement

12 Feb 2005
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United Kingdom
I had an old Wylex Consumer Unit which had wired fuses. (2x 30A and 2x 5A)

I replaced the fuses with plug in MCBs (from Wylex) (2x30A and 2x5A).

However, one circuit tripped when i used a device (eg Vaccuum) even though i have replaced plug in MCBs of the same A rating.

I subsequently replaced this particular plug in with a higher A rated MCB and it works fine.

Again, when i tried the cooker (on another circuit) it tripped the 5A MCB.

I think i have made a mistake while replacing the MCBs and may have replaced lower A MCBs in the wrong places.

Is there any way to check the rating of a circuit and place the correct A MCB in the correct location?
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With the electricity turned off, preferably before the old board if possible.

Take off the fuse carriers that came with the new breakers (one scre on the coloured carrier).

Identify the cable sizes in the board on the brass bar visible.

The two small cables should have the whites, and the two larger of the four cables should have the red. I expect there is more than 1 cable in one of the red mcbs, and possibly more than 1 cable in either of the whites.

If possible, take a photo for us to indentify.

You do not need to remove the entire fuse box cover, you should be able to see with just the carriers removed. Removing the whole cover would expose the live parts coming into the fuseboard, and as you do not really know what you are doing, I would not recommend you take the cover off.
The other way to tell which MCB does which circuit is to turn them on and off and see which circuits go on and off. The 5 amp will quite likely be for 2 lighting circuits eg ground floor and first floor. The 30 amp will be sockets and cooker.
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clam said:
I subsequently replaced this particular plug in with a higher A rated MCB and it works fine.

they trip for a reason. please sya u dont have a 40A on the ring main.
Thanks for this. I did check the circuits and had them mixed up. I now have the 2x 5A ones on the lighting circits and 1x 30A for the cooker and 1x 30A for all the plug points in the house.
im sure you cant swap breakers/fuses to a different rating without also swapping the bases

i think the current plug in mcbs use the same bases as the cartirudge fuse units though so you would have to swap the bases if replacing rewirables.

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