Consumer unit isolator burnt out

[mal165]

Hi, I was changing a bathroom 'light' 'switch ,which was actually a 45 amp pull cord shower switch controlling 4 downlights and the fan.( A separate switch beside the light switch has been installed at some point to feed an electric shower, but only has the supply cable connected). I connected the new switch, powered up and the socket mcb tripped and this has fried the consumer unit main isolator switch. On Investigation there is a loose , and split 1mm cable lying in the ceiling space which I am assuming is the cause of the short. The wiring to the downlighters is really poor quality work and certainly.ly needs to be done properly. No earth wires visible!. I will not have the chance to investigate further until tomorrow but would welcome any suggestions/ ideas. Why would a cable in a ceiling space cause this situation.
Thanks

 

[JohnD]

It wouldn't.

 

[ericmark]

I agree with @JohnD a lighting supply should not be able to draw enough current to burn out a 100 amp isolator, the shower supply maybe, but only if some thing wrong with the isolator to start with.

 

[mal165]

Hi, certainly puzzling. What I don't understand is why the short tripped the 'sockets' mcb and not the mcb for the lights! And also why it fried the isolator.

 

[JohnW2]

Hi, certainly puzzling. What I don't understand is why the short tripped the 'sockets' mcb and not the mcb for the lights! And also why it fried the isolator.

Firstly, to be clear, are you sure that what is tripping is a 'sockets' MCB, rather than an RCBO (the latter with a test button)?

If it is an MCB then, as you and other have said/implied, it sounds as if something(s) is/are very wrong with your wiring. A sockets circuit MCB should only trip as a result of a very high ('excessive') current in the sockets circuit and, if it did trip for that reason, the whole point would be to prevent anything getting 'fried'!

Kind Regards, John

 

[JohnD]

the socket mcb and ... the consumer unit main isolator switch.

it would be interesting to see pics of these components, and the damage you mention.

 

[ericmark]

Note the RCBO and RCD have a test button, and the isolator always has a red lever.

 

[mal165]

Many thanks for your responses. Definately MCB. Consumer unit is a Legrand, possibly a good few years old. As I mentioned the downlighter wiring is a mess . Cables terminated in chock blocks but not contained within an enclosure. I will try to get some pictures tomorrow when I have the chance to investigate further. Thanks again

 

[JohnW2]

Many thanks for your responses. Definately MCB. Consumer unit is a Legrand, possibly a good few years old. As I mentioned the downlighter wiring is a mess . Cables terminated in chock blocks but not contained within an enclosure. I will try to get some pictures tomorrow when I have the chance to investigate further. Thanks again

OK, thanks for confirming.

As I implied, if doing things to the lighting circuit has caused a sockets circuit MCB to trip, and has also damaged a main isolator, then I would say that you need an electrician to look into what could be serious problems with the wiring of your installation.

Do I take it that you are sure that the work you were doing was on a lighting circuit? I presume it must have been since, otherwise, you would porobably have got a shock whilst doing the work!

Kid Regards, John

 

[mal165]

Hi, well what did today uncover? Someone at some point appears to have added a second ring main wired in 1.5mm cable and for some reason has lead a spur from one of the sockets into the bathroom ceiling space. This was the wire which had shorted out the sockets mcb. The downlighter wiring is appalling, as is the fan wiring. Anyway had a quote today for a new consumer unit and the guy is recommending an MK 10 way rcd unit. What are your thoughts on MK? My preference would have been Hager.

 

[flameport]

MK 10 way rcd unit. What are your thoughts on MK?

Obsolete since last year, contains the wrong type of RCDs, surge protection not an option and only chosen because it's on clearance special at Screwfix. Pretty much the worst possible choice.

Hager with RCBOs. Or same from Wylex, Crabtree, Schneider.

 

[mal165]

Many thanks, pretty much what I had reckoned. Neat wiring, eh.

 

[plugwash]

MK seemingly decided that all the new surge protection/RCD types/arc fault protection that is coming in was too hard and decided to get out of the CU market. Anyone fitting one of their CUs now for anything other than a short term temporary installation is either ignorant or doesn't care about their customers.