Negative Wire Fault In Consumer Unit.

[Daffy D]

So all the electric went out. Thought it was just another power cut but as we were the only ones effected it must be something else.

Checked the consumer unit and all the cut outs were in the "on" position, nothing had tripped. Then checked where the power comes into the house and both the main knife breaker and the 30amp breaker were both OK.

So as nothing had tripped why was there no electricity in the house?

Obviously needed further investigation so opened up the front of the consumer unit and found that the NEGATIVE wire from the main breaker had burned/melted away from the connection bar.

As you can see from the photo the right side of the bar, where the offending wire was attached is blackened and burnt but the other end of the bar is relatively OK. This makes me think there was a problem with the connection of the main wire to the bar. I'm guessing the connection was not very tight and over time arching caused it to melt away.

Just to get us through the rest of the day I cleaned up the offending wire and connected it to one of the other points on the clean end of the bar.

This is a bit worrying as the thing got hot enough to melt the copper cable and probably hot enough to start a fire so why did non of the switches trip off?

Was it because it was on the Negative side, in which case whats the point of having any fuses at all?

Would a "SafetyCut" unit have prevented this from happening?

[sirineou]

The only thing that I can think of is that the wire was loos and there was arcing between the wire and the buss bar

[Litlos]

Correct answer is there is no negative, though probably a neutral which should be attached to the neutral bar where the cooked wire used to be attached to. Though in Thailand it may be the active attached to the neutral bar. Same result though, unless you assume the neutral is actually a neutral and touch it, short answer on this assume nothing and do not touch anything unless sure it is isolated.

The damage is probably caused by a loose connection to the bar, this causes heat and generally burns something out. Had trouble with electric motors/devices burning out recently, this probably had something to do with it? This would be caused by the bad connection and subsequent heat inducing high resistance which lowers voltage to devices, and at the same time increases amps, till eventually the bad point fails due to excessive heat load at the connection point.

Would a Safety-Cut help here, probably not as the purpose of this device is primarily to detect faults to earth and secondary is to provide protection for overload, which would not happen with this type of fault.

Best protection for this type of fault is to ensure that the electricians that do the connections only after having the morning noodles and have a bit of "Power":behind the screwdriver. A keen sense of smell as you walk past the box and if in doubt a hand near the box to detect heat are the backup.

Cheers

[Crossy]

Yup to all the above, almost certainly a loose neutral connection.

For some unknown reason loose neutrals seem to go undetected for ages before finally doing something catastrophic as in the case of our OP.

[T_Dog]

Had the same thing happen during construction where the neutral line going into the main breaker got hot and melted the breaker assembly. A welder had tapped into it and then not tightened it back up. A loose connection has resistance so it will heat up as current flows through it.

It is always a good idea to turn off the mains and have an electrician tighten down all the connections after a month of living in a house. Did that here and although I didn't find anything super loose, I was able to torque them down a bit better.

[electau]

The main neutral to an electrical installation should be terminated under a tunnel type connector terminal with 2 screws or by a bolted lug to the neutral bar. Copper to copper or copper to brass. With Aluminium conductors (16sqmm and over) use the correct termination lugs. Metering screws should also be 2 screw type.

With an MEN installation this can cause a potentially hazardous situation, the main earth and electrode ( with earth bonding ) maintain the touch voltage to less than 50VAC.

With a TT system this problem does not occur one justs loses supply.

No, an RCBO/RCD will not disconnect the consumers mains.

The problem is that many neutral links and earth bars (on DSBs) have only single screw terminals.

[Litlos]

Just to highlight a potential new problem it looks like the 2 pole breaker on the left is the incomer from the supply. If this is the case the size of the link wire between the breaker and the neutral bar does not look to be very large. From the photo appears to be 2.5 or maybe 4mm2 which I think is either to small or right on the limit, though for some reason I thought the link was supposed to be 10mm2, where is that dammed book of Thai electrical specs

Maybe Crossy or Electau can chime in on this one, my experience is industrial rather than domestic and all our switchboards had pre-wired link bars.

Cheers

[electau]

Just to highlight a potential new problem it looks like the 2 pole breaker on the left is the incomer from the supply. If this is the case the size of the link wire between the breaker and the neutral bar does not look to be very large. From the photo appears to be 2.5 or maybe 4mm2 which I think is either to small or right on the limit, though for some reason I thought the link was supposed to be 10mm2, where is that dammed book of Thai electrical specs

Maybe Crossy or Electau can chime in on this one, my experience is industrial rather than domestic and all our switchboards had pre-wired link bars.

Cheers

The link from the neutral bar to the terminal of the MCB should be the same size in sqmm as the incoming neutral.

Edited November 23, 2012 by electau

[Daffy D]

Yea! I figured it was a loose connection but as I did the installation myself can't actually blame anyone .

It's a bit odd as I normally tend to check things several times and always give the screw just that little extra twist, though in mitigation there was a lot going on at the time and it was done in a bit of a rush.

Always intended to tidy it up but somehow never managed to get round to it.

"Litlos" you're right the link wire between the main breaker and the neutral bar is 2.5 which at the time was sufficient but with additions over the years it is now definitely on or under the limits.

Have been planning to improve the system with a new "SafteyCut" consumer box and upgrade the main cable to 4mm or even higher, guess that job has been moved up now to the top of the list.

I realize the correct term these days is "Neutral" but in my day we called it "Negative". You know where you are with Positive and Negative, Neutral sounds so lame like it sort of does nothing.

[Litlos]

Sounds like the OP is on track to upgrade to a safe installation. The SafeTcut or similar is an easy way to upgrade your electrics and provide some extra protection for your family for active to earth type electrocution which is probably the most common type of electrical fatality, or in the case of Thailand TT system even a semi earthed neutral to earth will kill, but still protected by the SafeTcut type type device.

Offshore it is pretty much SOP to check all the terminals on switchboard using AC on AC duty every 5 years as they seem to come loose by themselves. Big stuff is done by regular IR heat surveys on the switchboards.

Sounds like the OP has a background in DC auto electrics from the references to negative. Most DC systems now refer to what used to be negative as 0VDC to accommodate +and - VE voltages found in a lot of systems, only automotive seems to keep the negative reference.

Cheers

[doglover]

The main neutral to an electrical installation should be terminated under a tunnel type connector terminal with 2 screws or by a bolted lug to the neutral bar. Copper to copper or copper to brass. With Aluminium conductors (16sqmm and over) use the correct termination lugs. Metering screws should also be 2 screw type.

With an MEN installation this can cause a potentially hazardous situation, the main earth and electrode ( with earth bonding ) maintain the touch voltage to less than 50VAC.

With a TT system this problem does not occur one justs loses supply.

No, an RCBO/RCD will not disconnect the consumers mains.

The problem is that many neutral links and earth bars (on DSBs) have only single screw terminals.

Rather than modify the neutral bar in the consumer unit to meet the 2 screw requirement here is a thought (space permitting). Strip 3, 4 or 5 times the insulation off the wire as would normally be required for termination. Poke the wire through the first hole on the neutral bar and then work the wire into a hook and poke the end up from the bottom into the neighbouring hole on the neutral bar. It would be desired to keep the loop to the minimum required and refrain from damaging the wire. Cut off any excess wire poking out of the second hole to keep the installation clean.