What are these MCB's doing in my CU??

[HappyHippyDad]

Hello all,

I decided to have a look inside my CU to see if I could work out which MCB was supplying what. I thought it would be fairly straight forward but its not! :rolleyes4:

My aim is to fit a new up to date CU when I complete my Domestic Installers course as I want to practise for a while until I feel confident and competent in others' houses.

I have worked out the shower/cooker/lights and Ring main but there are 2 MCB's left which do not seem to be doing anything:

1. 15A MCB with a 2.5mm cable (I had an immersion heater removed when gas central heating went in, perhaps this is what it is? I also had night storage heaters removed)

2. 5A MCB with 1mm cable which appears to be doing nothing

3. Also there are 3 wires going into the 30A MCB which feeds the Ring Main, what could the third wire be?

Thanks guys...

 

[Welshsparky]

3rd wire is a radial more than likely to 1 or 2 sockets, 15 amp will be the immersion, Consumer unit change is notifiable by the way

 

[Darkwood]

A guess as im not there but ......

yes highly likely a redundant immersion heater supply.
leave the 5amp off and time will tell, (alarm, doorbell, o/s light)
3 wires possibly a ring and a spur which is acceptable but you could fit it on its own circuit.

 

[telectrix]

so he will complete the certificate for the CU change just before his assessment for elecsa, napit, or the other one.

 

[Adam W]

1. You could practice testing the circuits you think these breakers control - presumably there's still a spur or some kind of connection where the immersion heater/storage heaters used to be.

2. Think about what might be running off a 5A breaker that wouldn't be on the lighting circuit. Again you could practice testing to make sure.

3. Have a look at the standard circuit arrangements in the back of the regs book.


... Or you could cheat and someone on a forum could give you the answers and you could learn nothing.

 

[SirKit Breaker]

Turn them all OFF, then turn them back on one at a time, see what works and what doesn't. When you turn one on, turn the other one off so you only have one at a time on, this way you will find out if anything is fed from more than one source, not uncommon especially if DIY Charlie has had a go.

Cheers..........Howard

 

[telectrix]

and, if proved that the 2.5mm on it's own is indeed a redundant immersion heater supply, use that to firt a couple of extra sockets on a radial, separate RCD from the other socket outlets. that will give you a bit of circuit separation in case of a fault taking out 1 RCD.

 

[sedgy34]

if you gonna practise do it on a wooden board and install a lighting circuit or small ring main dont start messing with your "LIVE" Consumer unit unless your confident to do so :nono:

 

[AIM]

I sometimes use old immersion circuit to provide loft socket/lights. You can then work on main lighting or other circuits in the loft [de-energised] without resorting to head-torch/hand lamp in the loft.

 

[Kenny.]

The old immersion supply in my house is now a dedicated freezer circuit on RCBO

 

[telectrix]

daren't tell what mine is. suffice it to say that 1. i didn't do it, and 2. it's on the list for improvement.

 

[HappyHippyDad]

Thanks all.

I have left the 5amp MCB off for a while now. I think it is redundant as its not the door bell, outside light, smokes etc etc. Cant think what it may have been. Just as a bit extra to this thread, how do I get the redundant cable out or do I just chop him off and make him safe?

The only other thing that I have had taken out is the night storage heaters but I'm assuming they must have had 2.5mm?

Dont worry Sedgy, i have absolutely no intention of doing absolutely anything to it until completely confident, competent and qualified!! It even felt strange opening it up and breathing on it!

 

[Adam W]

Thanks all.

I have left the 5amp MCB off for a while now. I think it is redundant as its not the door bell, outside light, smokes etc etc. Cant think what it may have been. Just as a bit extra to this thread, how do I get the redundant cable out or do I just chop him off and make him safe?

Burglar alarm? This 1mm cable is presumably going somewhere ie to an accessory; it might be an idea to trace it back to find out what it is/was doing. Any redundant cables should be removed completely if possible so as not to cause confusion and potential danger - like I said this cable coming out of the 5A breaker should be terminated properly somewhere and not just hanging around in the wall waiting to give someone a shock.
You can either remove the cables the same way they would have been put in, or if capping has been used you may be able to just pull it out from behind the capping.

 

[Octopus]

One of the pitfalls of doing CU changes in houses is getting a good understanding of what the MCB's do. Unless you take the cover off, prior to doing the work and find out, you may as well be pxxxing in the wind.

I recently did a board change where neither I nor the homeowner could identify one circuit, so I left it switched off in the new CU and went away. About 3 weeks later he called, the plinth type heater in the kitchen, (which wasn't obvious at all) wasn't working - so I went back, tested the said circuit and issued a revised EIC.

Every day is a learning day!

 

[i=p/u]

aye i was working today and was removing old cables because i dont like to leave them in like many of yous.

so was taking away old time clock for heating system as was moved within new works. this was live and the light in utility was taken from time clock but the board was closer.. beggers belief..