3 Phase 125 a fused isolator, neutrals all melted!

[topquark]

Thre things I'd be looking at, loose connections, undersuzed neutral conductor then un balanced phases (can we still call them phases when refering to 3 phase?).

Phases is awite as a generic term, if you refer to an individual core (allegedly) it's a line ... go figure

 

[Inteificio]

Seems a complex problem.

I know I do not have the experience to answer, but would like to know what happened as a learning opportunity.
So here is how I would look at the situation, please advise (have not been doing this long)

First is to check is the size of the neutral.
If someone has used the 'over half' rule and put in a tiny neutral it could be as simple as massively unbalanced lighting resolved by client education of balancing loads (or putting in a decent N).

If the neutral is equal size with conductors then my guess would be harmonics causing positive interference on the neutral conductor and ramping the current.

That would be a lot harder to fix and short of just putting a bigger neutral in I am not sure what to do.

I would like to know what the relevant current draws are on each line. a few clamp meters and get them to go through their lighting sequence seeing how it changes.

So anyone feel like helping a noob learn?

 

[Knobhead]

Seems a complex problem.I know I do not have the experience to answer, but would like to know what happened as a learning opportunity.So here is how I would look at the situation, please advise (have not been doing this long)First is to check is the size of the neutral.If someone has used the 'over half' rule and put in a tiny neutral it could be as simple as massively unbalanced lighting resolved by client education of balancing loads (or putting in a decent N).If the neutral is equal size with conductors then my guess would be harmonics causing positive interference on the neutral conductor and ramping the current.That would be a lot harder to fix and short of just putting a bigger neutral in I am not sure what to do.I would like to know what the relevant current draws are on each line. a few clamp meters and get them to go through their lighting sequence seeing how it changes.So anyone feel like helping a noob learn?

Have a look for "neutral harmonics" on Google, some of it is heavy going.

 

[greengap]

Hi Guys

I have been looking at the above thread to try and figure out my problem which is similar.
I work on a very big construction site and we deal with a lot of substations supplying different parts of the site.
At our main office block I have seperated our offices into smaller sections each supplied by an office DB (This is all temp electrical installations i.e. the orange boards) Today I was called out as a lot of PC's and air cons had popped! Upon opening the DB the main incoming neutral had completely burnt away as in the lugs connected onto the neutral stud were gone! I have an earth leakage relay that was protecting the secondary side but for some reason it had not operated. This is a 3 phases installation and a TN-S earth arrangement. My biggest question is why did the PC's and aircon's (220V) blow up. All that happened was that the neutral was gone therefore the circuit was incomplete and the appliances would stop working not blow up as if they were receiving 380V? Really stumped on this.

 

[malcolmsanford]

When you lost your neutral you effectively put 400volt across the LIVES on your single phase system, you had what is referred to as a floating neutral.

You have to remember that each phase is at 120 degrees to each other and so lags each other at a potential of 400volts. The neutral ties this down as to be Uo-230, if you lost that neutral then you are back at 400 volts.

The earth leakage will not have operated as you never had an earth fault.

 

[greengap]

So just to confirm even though my neutral was missing the one live wire in the single phase circuit was now carrying 400V into the socket outlets and then into the aircons etc?

 

[malcolmsanford]

Yes you LINE/NEUTRAL of your single phase system becomes 2 phases and therefore 400volts as near as dam it

 

[greengap]

Wow thanks so much for your help I really appreciate it as I have to give an answer to the big bosses as their PC's blew up which is never a good thing. Is there any possible solution to prevent this from happening again apart from making sure the connections are tight? Thanks again

 

[malcolmsanford]

How many terminations have you on site ? Terminations fail with age and load, all you can do really is introduce a good PPM system

I'm surprised that part of your insurance agreement does not insist on yearly thermal checks on your panel boards to detect hot spots.

Fluke actually do an hand held infrared thermometers that you simply point at the terminations or DB and it will detect hot spots, using this with a good PPM schedule will highlight problems before they happen

Portable Thermometers | Fluke 60 Series Handheld Infrared Thermometers

 

[greengap]

Have had a look at the instruments and management have agreed to buy this Fluke so as to prevent this. I am not sure what the insurance policy states as I was transfered here in July but will check that up (thanks again for that). Do you think surge arresters would have helped in this case? Just a thought? As you can probably see I have to fix up a lot of issues left behind! I know one sparky shouldn't wrong another one but one cannot help think what they were thinking

 

[malcolmsanford]

It would have depended where you fitted these SPD. They will come in 3 classes Type I II and III the type I will normally be fitted at the origin of the supply mainly for transient voltages due to lightening strikes and are not that common in the UK.

Type II would be at local distribution boards if they are 3 phase then the SPDs would not have been much use for you as the 400volt would not be deemed a surge, if they were single phase boards then yes they would have protected the system. These type are quite common in installations that have high switching due to inductive loads such as motors, elevators or HF lighting.

The Class III are very local and are often incorporated into a BS 1363 Socket outlet to protect appliances or equipment as a singular option.

These items are not cheap and it really would be a costing exercise. Personally I would be advising a good PPM schedule with the infra red technology, and perhaps even if the insurance don't insist on it having a contracting in once a year to do thermal imaging