Loose Cable On The Neutral Bar

[Rimmer]

I had an interesting event happen a few days back. Early one morning the air con and fan in the bedroom went off, the other air cons and fans and such were still on though. On investigation I found that the neutral cable had either worked loose from the terminal on the neutral bar, or it had never been tightened or had the threads stripped by over tightening and bodged up. Either way it then heated up the neutral terminal to such a degree that it completely blew up a 32amp breaker on one phase, it did not clear the main breaker on the board which looks like a 125amp MCB. (it is not an MCCB) It also did clear any of the 63amp fuses in the main switch by my 50KVA transformer but it did clear one phase fuse on the transformer.

I have since replaced all the 32 amp breakers with 20a and I guess I should downsize that 125am main MCB and the 63amp main switch fuses. Seems like the electrician felt big is better, that's all very well but their is no discrimination between them. I did not check the protection and discrimination when they were installed either.

I had to purchase a completely new TP board for Baht2700 just to take out the neutral bar as a neutral bar on its own is not available, in hindsight I could have used a crimp terminal on the neutral cable and bolted the cable directly to the bar.

Would be interested in any thoughts about discrimination between the breakers and fuses, possible fault levels to cause such a catastrophic blow out and what I can do to ensure a closer protection than what was previously installed.

I have not heard of this happening before but the guy in the electrical shop where I bought a new board said it happens all the time in Thailand.

Cost me a new TP board and 5 photo switches so I guess it could have been worse.

[Crossy]

Nasty, lucky it didn't cause more damage.

Loose neutrals can have all sorts of interesting and seemingly illogical effects that deny explanation particularly in a domestic 3-phase situation with a reasonable balance.

What you've done will certainly do no harm although whether it will prevent a recurrence.

As to why it occurred, who knows, that's not an aluminium cable is it??

[Rimmer]

Nope its a four core NYY copper BUT horror of horrors the terminal on the neutral bar IS aluminum and it melted, looks almost like the threads were stripped. One thing is for sure though I will have the covers off once a month and just check all those terminals.

What do you think about taking out that 125a MCB main and replacing it with a smaller size? I calculate the full load if turn everything on will not exceed 56amps.

One school of thought back then was that those auto main switches only protect the solid copper bars in the board!! Any thoughts on that? Despite that I feel like putting in maybe a 63a or 80amp breaker What say you?

Maybe I should also replace the 63amp fuses in the Square D Main switch at the transformer with a lower rating so I don't have to call out the PEA just to change one of my transformer fuses. What do you think? Each phase cant be taking more than 20 amps IF they balanced the phases correctly.

Also I seem to remember a Q1 fuse can clear faster than an MCB under some fault conditions so where would that leave me?

EDIT

I don't have much faith in those so called automatic main switches they sell in DBs unless they are a proper MCCB

[electau]

An auto main switch is a standard MCB. An RCD is a not an MCB, An RCBO is a combination RCD and MCB.

[T_Dog]

Rimmer... What caused the high current that caused the transformer fuse to blow? Was it an imbalance of the 3 phases, some other type of yet-identified fault, or did the terminal block melt enough to short out? Seems unusual to have such high currents caused just from an open neutral. Would be good to see a diagram of the loops and nodes to see how this really occurred.

Also... Electricians failing to tighten down terminals is a fairly common thing in my experience. Eagerness to get the job done prevails and not many double-check their work.

[Rimmer]

Rimmer... What caused the high current that caused the transformer fuse to blow? Was it an imbalance of the 3 phases, some other type of yet-identified fault, or did the terminal block melt enough to short out? Seems unusual to have such high currents caused just from an open neutral. Would be good to see a diagram of the loops and nodes to see how this really occurred.

Also... Electricians failing to tighten down terminals is a fairly common thing in my experience. Eagerness to get the job done prevails and not many double-check their work.

I really have no idea why that happened, I was hoping someone here would have some ideas. The only load on that phase was a 3KW aircon. Their was no other load on the entire board at the time.

The system worked well for four years so maybe the neutral just worked it self loose.

I think I am going to downsize that main MCB in the board if I can find a smaller rated unit in the same frame size, 125amps is way too much.

[InterestedObserver]

Rimmer... What caused the high current that caused the transformer fuse to blow? Was it an imbalance of the 3 phases, some other type of yet-identified fault, or did the terminal block melt enough to short out? Seems unusual to have such high currents caused just from an open neutral. Would be good to see a diagram of the loops and nodes to see how this really occurred.

Also... Electricians failing to tighten down terminals is a fairly common thing in my experience. Eagerness to get the job done prevails and not many double-check their work.

Obviously a lack of protection coordination. We have a 125MCB backed-up by 63A fused main disconnect switch backed-up by unspecified transformer fuses, with the transformer fuses clearing the fault.

[Rimmer]

Rimmer... What caused the high current that caused the transformer fuse to blow? Was it an imbalance of the 3 phases, some other type of yet-identified fault, or did the terminal block melt enough to short out? Seems unusual to have such high currents caused just from an open neutral. Would be good to see a diagram of the loops and nodes to see how this really occurred.

Also... Electricians failing to tighten down terminals is a fairly common thing in my experience. Eagerness to get the job done prevails and not many double-check their work.

Obviously a lack of protection coordination. We have a 125MCB backed-up by 63A fused main disconnect switch backed-up by unspecified transformer fuses, with the transformer fuses clearing the fault.

Right and I should know better but never checked the protection levels assuming incorrectly that the sparks knew how to do it. What say you to a 63a MCB main in place of that 125 amp, assuming one is available in the same frame size.

[bankruatsteve]

Just to toss in a thought... a loose neutral can sit there and spark/burn and not set off the breaker. (lots of fires start from that) The important thing about the breaker is to size to the wire it's protecting. Downsizing doesn't really make anything safer.