Earth bar melting CU enclosure!

Adam Smith · May 21, 2012

A friend rang me today to ask if I could come and look at his melting CU. He said that all the circuits were holding and everything operating correctly, he only noticed something was wrong when he saw smoke coming from under the stairs!!

His land lord sent round a 'Multi trade' lad to 'fix' it, which involved just turning the shower circuit off and leaving.

I had a look today expecting to find a loose neutral melting away in there but instead found the earth bar to be the one melting!

Does anyone know what faults could cause the earth bar to overheat?

The only thing I could think of was an earth fault leading to overcurrent and a loose earth at the CU. There is no RCD and all circuits are on MCBs. IR tested the shower which was apparently the problem but all seemed fine, cables terminated correctly and all shiny and clean. Tightened all connections in CU and checked Ze which was fine also.

Cheers in advance

Darkwood · May 21, 2012

when you say earth bar was melting do you mean it got hot and melted its plastic holdings, with due respect the earth bar has a large csa, are there any signs of cable overheating at termination points, is it possible th multi-trade guy reconnected a neutral or main earth before he left, it sounds more like lost neutral and a earth/neutral fault carried all the neutral current down the earth path. Could have been external DNO issue.

Darkwood · May 21, 2012

In fact the more i think the more likely it was what i said a missing incoming neutral and a E/N fault on shower or its cable had the whole neutral load coming down the shower earth, i bet the earth in the shower cable has melted into the other cores. I would be slightly worried about not knowing why the neutral was lost, enquire with neighbours and also the main switch could have poor neutral contact so check it out.
The other possibilty is a poor neutral on shower circuit alone and a E/N short again let the current return down the lower csa earth wire getting it hot and conduction the heat into the earth bar, again the earth core of shower cable would have signs it overheated.

Dcf · May 22, 2012

in about the last year i have come across 3xshowers connected by plumbers that have had the earth connected to the neutral terminal in the shower and the neutral to where the earth should be.End result is 38-40A down a 2.5 or 4.0mm earth and if its not rcd protected (which it should be ) then its not gonna trip any fuse/mcb
Mira shower seem the worse culprit due to the terminal block layout.just thought

Adam Smith · May 22, 2012

Cheers for your reply Darkwood. It was the surrounding plastic that had melted an not the the actual bar (Guess there would be more cause for concern if that were the case!) The cabling on the shower circuit looks fine throughout but it was never establish if that was the problem circuit.

Could the lack of RCD be allowing an E/N fault to occur? I guess the breaker would pick it up eventually when the current gets high enough but there's quite a difference in current capacities between 2.5 and 6mm! (mainly guess work there)

My first port of call was to check the neutrals were nice and tight which they weren't overly, but have seen looser ones working fine. Do you think a loss of the neutral would be most likely as the fault has only occurred once and for no apparent reason? Would a TNCS system still work with the loss of incoming neutral or am I being a silly billy!

I apologise for my limited technical knowledge, still learning!

Darkwood · May 22, 2012

If current is going through one load then returning down an earth path then it wont get any higher than a standard load current and wont be detected by mcbs/fuses if the earth wire got hot enough it could melt into the live and create a short which would be detected, TNCS been combined N/E would be rare to lose as it has multiple tappings to earth along its route to ensure it always give a path as the consequences of it failing are dangerous in the least, you may see the PME sticker (protective multiple earthing) and this is expressing the N/E is grounded at every available point. Losing this would lose both incoming N an E and a N/E fault may direct the current down your internal earthing to incoming gas or water pipes to ground but as i said rare (cable theft has been known to cause this in sub-stations but other issues will probably have caused alot more damage in this instance). I would be inclined to think its a consumer side issues but do a visual on the tails as well to see they are showing no signs of heat damage, the shower cable is looking suspect though but been 2nd man on site it makes it really difficult to solve without knowing what he found and did, you may not have the full story.