Electrikery - Scary Stuff

[puffernutter]

We had a bit of a smell in the hall on Saturday morning. Burning plastic. Tried a number of places - eventually traced it to one of my consumer units (installed by a Part P qualified electrician) a few years ago.

Here's what I found (same MCB, different views)

Here's the interesting bit! I did a little bit of forensic analysis and I think I know why - and it may be worth sharing.

This is the busbar side of the mcb. The preformed copper busbar has the first "tooth" intentionally bent to allow for the position of the main breaker terminal. However, this breaker is slightly (not by much, only 1mm or so) deeper than the rest of the mcbs and when I put the busbar in to the replacement mcb first, it was kept from making a really firm contact by this extra depth, In fact you could tighten it as much as you liked, but it would still pull out.

So if you did the breaker first, it would hold it tight and you could get a false impression of the security of the immediately next mcb, giving a slight resistance, giving heat etc.

The other mcbs will all tighten OK as they are that much further away and the angle means more of the copper is in the connection. This can re-inforce the false view as they will hold the busbar firm!

A salutory tale, lucky we were around to smell it!

Cheers

PN

 

[Welderpaul]

Had the same at work on the outgoing side of the MCB - loose terminal....Luckily during the day!

 

[123hotchef]

lucky escape, was that a mcb that came with the CU

 

[WorkshopChris]

All to common unfortunately, luckily the geniuses in charge decided the solution is now to stick it in a nice metal box to contain the fire, rather than the proper solution to have high quality terminals with two independent screws on the outgoing and a standard heavy arrangement on the incoming.

 

[brightspark]

if they are correctly fitted and checked in the 1st place it wouldn't happen. 2 many so called electricians fitting them incorrectly I see it regular the busbar isn't inside the the clamp but pressed up outside of it

 

[puffernutter]

if they are correctly fitted and checked in the 1st place it wouldn't happen. 2 many so called electricians fitting them incorrectly I see it regular the busbar isn't inside the the clamp but pressed up outside of it

This is my point.

It depends how you fit it. If you do the terminal into the breaker first (which is logical for most working right to left) then you have no real way of testing the first mcb in is actually clamped on the copper strip, or the resistance you feel is due to the clamping action of the breaker. The best way to do it is to do the first mcb first (if you see what I mean) then you know it's clamped, but that is neither intuitive or logical.

The solution is to have breakers and mcbs to identical dimensions lengthwise to prevent this problem.

PN

 

[brightspark]

what u will find is fit all the breakers and unscrew them for the bar to be fitted . what u will find is your screwdrivers are slightly magnetic and will pull the clamps forward and wont engage when u slip the bar in what I do is maker sure the screws are pushed fully back using a wooden pencil then slip the busbar in and tighten one end up to the other starting from the rcd 1st .

 

[puffernutter]

Good advice. Worth double checking that the screws are tight and more importantly, being effective!

 

[dannyp]

number of times i've plugged big 3ph kit in in someone elses workshop and a phase suddenly dissapears, tighten a few screws or refit the a breacker or 8 and all is good

 

[Turbo]

What I like to do if wiring a consumer unit is to install the busbar then unclip the mcb's & isolator from the din rail then do a tug test like you do with wires to make sure the bus bar is securely clamped in the mcb's!

 

[MCKDAVID]

agree with bright spark, I did all my own rewiring / consumer unit (signed off by the official inspector) but you have to check that it's on the right side of the connector, our inspector certainly did.

Other than the initial installation, although I can't say I do it yearly, or periodically by date, but I'm in the consumer units fairly frequently for one thing or another, and by force of habit from my maintenance days, after I've isolated everything, I give them all a little tug on the core and a quick check with the screw driver to see if they're still tight... my mate used to laugh at me, until he had a MCB do exactly the same as above, funnily enough he does the same checks as me now... takes minutes, but good piece of mind.

 

[dobbslc]

Having read your post twice is sounds like the mcb was not fitted right in the first place and the busbar was on the wrong side of the clamp (very easy to do ) its got hot and melted. If you have replaced it with the correct make of mcb it should fit in ok, I'm wondering if the busbar has deformed with the heat?
What make board is it?

 

[8ob]

Had something similar happen and the sparky blamed it on dust from recent building works, he changed the whole board.

Bob