Lost neutral

[Tim]

Hi guys,

Today i went to a hotel that had lost its neutral at the main incommer, and has been presumed out for around a year. The neutral link had been undone during a previous refurb and tightened up in the undone position. The problem has only been found after a small fuse board from the blue phase caught fire. Most down light transformers/florries/coffee machines etc have been fried, though it has only really affected things on the blue phase.

My questions are how does everything keep working, as it has been for a year, without a neutral, im thinking either the neutral has only recently dropped out (but then why was it done up so tightly in the undone position) or the earth has been a return path for a year or so.

My other question is why is it that almost all of the equipment affected is fed from blue phase? Could it just be that blue phase just happens to have almost all of the electronic equipment on it.

I do not have a lot of details for the job, i expect i will be back there in a week or so, we only went down today for a few hours to rig up some tempoary lights to get them back up and running.

I do not know the earthing arrangement, and by all means correct me if my assumptions are wrong.

Here are a couple of photos of stuff i found whist investigating.

If the breaker doesnt fit? cut up the board.

Breaker doesnt fit? Bend the board

Some lovely melted cables

Getting a bit hot?

Oooh pretty colours

Cable doesnt fit? Give it a haircut.

Cheers,
Tim

 

[MDJ]

Sounds like the earth did the JOB haha

 

[La Poste]

This is a very interesting subject and I am looking forward to reading peoples replies.

 

[Tim]

Follow up question:

There was an AC board that was really buzzin, we took the cover off and measured the current on each phase.

Red 100A
Yellow 2A
Blue 2A

I turned off all the breakers on the red phase and 1 by 1 the buzzin got quieter (each breaker was pulling approx 15A) then once they were all off the buzzin stopped. I reset them and all was fine, no buzzin.

We ran out of time to investigate further, but my only guess is that the pcbs in the ac units had shorted enough to allow more current to flow and turning them off and on stopped the arcing.

Correct me if im wrong.

Cheers
Tim

 

[telectrix]

crying out for a rip it out and start from scratch.

 

[Tim]

Sounds like the earth did the JOB haha

I am with you on that, but unless its a tncs system (i think unlikly on a building this size but not impossible) where is the n-e link? In the electronics maybe?

 

[Tim]

crying out for a rip it out and start from scratch.

Im working for someone else atm and thats what ive advised, but i know they wont pay for it as the bodged refurb was only a year ago.

Ive counted over £1000 of led lamps blown already lol

 

[Guest55]

what a mess lol !

the biggest surprise is that an installation as crappy as that actually has anything as "hi-tech" as rcbos.

as for still working without a neutral for a year , well its still tracking back to the origin somewhere , not difficult really if its tnc-s.

those incomer tails must be huge lol , as a 125a switch is good for 35mm and half the cores have been trimmed just to get it in.

not surprised youve lost a phase with all that heat damage , single phase voltage can rise to over 300v when the neutral drops out , which will will fry any electronics plugged in at the time.

its this exact reason why i never use a 4 pole main switch for the neutral on a commercial 3 phase db , it makes a complete mess if the neutral contacts fail - bolt direct to terminal bar , always ;-)

 

[Silly Sausage]

Ah, Lost Neutral, interesting stuff!

I predict the company who did last year's refurb will be receiving some interesting communications in the near future! :smilielol5:

 

[Tim]

what a mess lol !

the biggest surprise is that an installation as crappy as that actually has anything as "hi-tech" as rcbos.

as for still working without a neutral for a year , well its still tracking back to the origin somewhere , not difficult really if its tnc-s.

those incomer tails must be huge lol , as a 125a switch is good for 35mm and half the cores have been trimmed just to get it in.

not surprised youve lost a phase with all that heat damage , single phase voltage can rise to over 300v when the neutral drops out , which will will fry any electronics plugged in at the time.

its this exact reason why i never use a 4 pole main switch for the neutral on a commercial 3 phase db , it makes a complete mess if the neutral contacts fail - bolt direct to terminal bar , always ;-)

Hi biff,

The rcbos were installed with the new db (the one with the dodgy tails) in the refurb a year ago.

I guessed it would be returning through the earth, but what if it is not a tncs system? (just trying to cover all bases untill i know more about the job)

What would cause the voltage to rise differently on different phases? As most the damage is on the blue phase, why were not all phases damaged equally?

Have a look at the pic with the tails, the red phase has heated up so much that the mccb has started to burn.

Cheers,
Tim

 

[Bobster]

All to do with vectors and balanced loads on 3phase systems.

If the load was higher on the red and yellow phase with no neutral to tie them together the blue would float causing the voltage to rise. (In a nutshell)

 

[Tim]

All to do with vectors and balanced loads on 3phase systems.

If the load was higher on the red and yellow phase with no neutral to tie them together the blue would float causing the voltage to rise. (In a nutshell)

Cheers rob, I get the rough idea of what your saying but what do you mean by tie them together?


Does anyone have any idea about my question on post 4?


Cheers,
Tim
Cheers,
Tim

 

[Stroppy]

There is good information about lost neutrals on this thread: http://www.electriciansforums.net/e...309-suppliers-neutral-break-2.html#post674305 I had a lost neutral at a hotel a while ago and had the same problems of over-voltage frying lots of things.

Edit: Tony's post has the clearest explanation IMO: http://www.electriciansforums.net/electrical-wiring-theories-electrical-regulations/69309-suppliers-neutral-break.html#post670565

To answer your questions, if the loads on all phases were roughly the same then their voltages would stay reasonably close to your nominal 230v, and bearing in mind a lot of kit is designed to work within a range of, say, 200-250 volts or even 100-250v no-one would be any the wiser that the voltages on the phases were varying with load. Only when the voltages go outside the tolerances of the appliances (too high or low) would you notice they stop working, or catch fire.

 

[Tim]

Those diagrams are good, now i can visualise it.

Cheers stroppy

 

[Engineer54]

what a mess lol !

the biggest surprise is that an installation as crappy as that actually has anything as "hi-tech" as rcbos.

as for still working without a neutral for a year , well its still tracking back to the origin somewhere , not difficult really if its tnc-s.

those incomer tails must be huge lol , as a 125a switch is good for 35mm and half the cores have been trimmed just to get it in.

not surprised youve lost a phase with all that heat damage , single phase voltage can rise to over 300v when the neutral drops out , which will will fry any electronics plugged in at the time.

its this exact reason why i never use a 4 pole main switch for the neutral on a commercial 3 phase db , it makes a complete mess if the neutral contacts fail - bolt direct to terminal bar , always ;-)

All incoming breakers on a main switchboard, especially if the switchboard has 2 incoming breakers and a bus coupler, will Always be 4 pole!! Can't remember the last time we've used a 3 pole isolator/breaker on any sub-main Distribution boards either. The whole idea is to be able to isolate ALL live conductors from an installation, not just the phase conductors....

I agree with you on those cables feeding that 3 pole isolator (they look like shaped conductors too). If there was a need for that sized cable, for whatever reason, the isolator should have been provided with spreader extension bars of a suitable size to accommodate those cables. Or some other means being provided, to accommodate those cables before connection to the isolator....