MCB replacement question

[LoriT71]

I moved house a year ago and the cooker socket has never worked properly since then. The fuse in the consumer unit blows any time the oven is used or the kettle is boiled (using that socket). It doesn't blow instantly - maybe after a minute or so (sometimes more, sometimes less). I thought (based on advice from a friend) that the socket needed replaced. So I arranged that - but the MCB is still tripping.

So, I took a closer look at the consumer unit - and discovered that the MCB for the cooker socket is only 6amps (the same as the MCB for the doorbell!!). The others are either 32amp or 16amp. I don't know how this happened (the consumer unit was fitted by a qualified, but retired, electrician). But at least I know now why the cooker and kettle were overloading the socket.

I've now bought a replacement MCB (Wylex) from Screwfix and wondered how difficult it would be for me to fit it myself? I'm not the kind of person (usually, anyway) to just blunder ahead and potentially get it disastrously wrong. So I've been reading, researching and scouring YouTube for examples of someone replacing an MCB. So far, I've got sidetracked watching another billion electrical-related videos (cos I'm just geeky that way). But I've not actually managed to find exactly what I need.

Can anyone advise me as to whether it's technically feasible for me to attempt this myself? And if so, can you point me in the direction of some basic instructions? Either YouTube or written instructions would be great. If your advice is that this is too technical and/or dangerous (or illegal?) to attempt then I'll definitely get someone in to do it. But I'd like to be able to learn to do as much of this stuff as I can (within reason).

Thanks in advance.

Lori
x

 

[LoriT71]

PS And yes, I'm ashamed of the fact that I'm only getting around to resolving this problem now. And I've essentially existed without an oven for almost a year! LOL! A goddess in the kitchen I am not! Thank goodness for a gas hob and a microwave! LOL!

 

[endecotp]

It's not difficult, but first you need to be certain that it wasn't done like that for a reason, e.g. the cable is too thin.

 

[Taylortwocities]

Hold on there fella. (Whoops, not fella I see)
That MCB may be 6amps for a reason!

Not many DIYers appreciate this (some so-called electricians too) but the MCB is there to protect the cable itself. So firstly you need to confirm the size of the cable for the cooker circuit. Without that, you cannot decide what size MCB to buy - so questions for you

1. What size cable do you have for the cooker circuit?
2. Does this cable go from the consumer unit to the cooker, and nowhere else?
a rough guide to cable csas can be found in the WIKI here //www.diynot.com/wiki/Electrics:flatpvccables
3. (as it looks like you have already bought the MCB) what size is the MCB?

 

[ban-all-sheds]

4. What is the loop impedance of that circuit, and will it allow the use of an MCB of that rating? //www.diynot.com/wiki/Electrics:MCB2

And while you are there, re TTC's comments/questions about cable size: //www.diynot.com/wiki/Electrics:MCBtoCable

 

[LoriT71]

Sorry for the delay replying. I was down taking photos. I'll have a look at the links posted above and will come back here with more info.

Thanks once again! x

 

[LoriT71]

I forgot to say, the new MCB I've bought is the same as the one that is currently to the left of the cooker MCB in the photo (code number NHXB32). x

 

[Taylortwocities]

What a complete horlicks. Whoever installed that was not an electrician!

 

[LoriT71]

My brain is slightly objecting to new words like "loop impedance" at this time of night. But looking at the wires, I'm thinking that the cable is more than able to cope with the new 32amp MCB. Yes?

I don't think I have any way of checking the answer to your question about whether the cable goes ONLY to the cooker socket. Or do I? I'm just assuming that since it's labelled as cooker on the consumer unit, and there is only one red coated sheath of wires going into the MCB, that it confirms that it's only going to the cooker socket. I hate sounding so vague and "assumey/guessy". If it turns out that more info (that I'm unable to figure out) is needed before you could all advise me then I'll admit defeat (honest! LOL!). But I'm hoping that the photos will reveal all that's needed for me to move forward with fitting the new MCB?

Lori
x

PS Please don't worry about replying just now. I'm off to bed now, so I'll check again for any replies tomorrow.

 

[securespark]

Hi Lori,

One way to check what that circuit feeds is to turn all the other MCB's off and check what is left working. If all that is working is the cooker and associated socket (if present) it is fairly safe to assume there is nothing else on that circuit. It does look like 6mm², which will be happy on a 32A breaker. I'm not sure by "cooker socket" whether you mean:

https://www.tlc-direct.co.uk/Images/Products/size_3/MKK2747.JPG with a single oven plugged in

or: https://www.tlc-direct.co.uk/Images/Products/size_3/BG870.JPG with an oven hard-wired to it (either directly or via one of these: https://www.tlc-direct.co.uk/Technical/DataSheets/BG_Nexus/BG879.pdf)

or: https://www.tlc-direct.co.uk/Images/Products/size_3/BG874.JPG, or: https://www.tlc-direct.co.uk/Images/Products/size_3/BG872.JPG with an oven hard-wired to it (either directly or via one of these: https://www.tlc-direct.co.uk/Technical/DataSheets/BG_Nexus/BG879.pdf)


Either way, it would be good to see pictures inside the accessories connected to that cooker cable.

What is the total possible load for the oven? This can be found on a ratings plate or sticker usually inside the door or sometimes on the back.

Loop impedance is a measurement of resistance. It is checked to make sure resistance in the circuit at the point of testing is low enough to ensure timely disconnection of the supply in the event of a fault. The loop impedance really needs a bit of pro kit to check, but it could be hired.

As has been said, the connections in your CU are not made off in the neatest, most secure fashion.

Nor is it compliant with regulations dating back as far as 1991, having no RCD protection.

It also has 2 non-Wylex MCB's fitted, which could mean a poor connection to the busbar or distortion as they are not designed to fit the CU.

It also appears to be missing a busbar cover (piece of plastic that attaches below the MCB's to cover the bare copper strip) and the hinged flap attached to the front cover that covers the MCB's.

The current standard is to fit a metal CU. While it is not imperative to keep up with standards, it would be desirable to at least fit RCD protection and one way to do this would be to replace the MCB's with RCBO's.
These would give RCD protection to each individual circuit, although, depending on the age of the rest of the installation, you may have an issue with putting the lighting circuits on different RCBO's.

It may be best, given what we can see from the way the CU has been installed, to get an electrical check on the whole installation and bite the bullet and fit a CU to current standards.

 

[Taylortwocities]

How long ago was that consumer unit installed??. there is no RCD protection there, at all, and that has been a safety requirement for many years.
It looks to me that he just installed anything that he had lying around in his shed...

The Clipsal MCBs are also wrong They should be Wylex and not another make.

Given the shambles of the installation, the unknown of the reasons for the cooker MCB being only 6amp and the above factors, I seriously advise you to get an electrician in to review what is going on there.

As SS has suggested, I would recommend that you get a new consumer unit installed that meets today's safety standards.

EDIT......
Looking at the dates of installation. You have NHXB Wylex MCBs there. IIRC the NHXB range were introduced in about 2010 to replace the NSB range (which had a problem called catchingfire!!).
So I estimate that your installation is only about 5-6 years old and your retired electrician has installed something that does not meet the requirement then. You should have had a consumer unit with RCD safety built in. What he has done is apalling.
Also, I will bet that you never got a notification of completion from the local authority (anybody buying your house will want this).

So all in all, get a registered electrician to check everything and make it safe for you.

 

[ban-all-sheds]

retired electrician has installed something that does not meet the requirement then.

He probably neither knew nor cared.

 

[ban-all-sheds]

How long ago was that consumer unit installed??.

I moved house a year ago...

So either Lori had it installed within the last year (which makes the "electrician"'s lack of care even worse), or it was done sometime in the preceding 5-6 years.

And in that case....

I will bet that you never got a notification of completion from the local authority (anybody buying your house will want this).

Lori bought it without one....

 

[winston1]

Changing that MCB is actually very simple. It would appear to have the right cable. We don't know about an RCD, there could be one feeding the whole Comsumer unit. We also don't know why the wrong MCB was fitted.

So. Switch off the big red switch to the right. Off is down. Bear in mind the two wires feeding it at the top will still be live and are partly exposed.
Using the correct pozidrive screwdriver loosen the top screw of the MCB and pull the wire out.
Loosen the bottom screw and lift the MCB out.
Reverse the above process to fit the new MCB.

Regarding the two Clipsal MCBs on the lighting circuits. Lighting circuits usually are on 6A MCBs which is normally adequate unless you have loads of halogen spotlights. You could replace one of them with the removed 6A MCB, and get a new 6A MCB to replace the other.

If you are at all worried about any aspect of this get a sparks to do it. No more than 30 minutes work.

 

[bernardgreen]

Make sure the jaw in the terminal at the bottom of the MCB is fully open so the the bus bar can get into the jaw BEFORE you fit the MCB onto the bus bar. If the jaw is not fully open it can happen that the bus bar goes behind the jaw and is not clamped when the jaw is screwed shut

You could replace one of them with the removed 6A MCB,

NO do NOT do that, that MCB has been repeatedly over loaded and tripped many times. It may no longer operate as a 6 amp MCB