unusual voltages on 3 phase

[thomas123]

Hi,

today i tested the voltage on a 3 phase board and discovered something unusual, when the boards main switch was closed i tested between L1 and N and had 400v, L2 and N 230v, L3 and N 110v.

But when the main switch was open, all of them where at 230v.

Could this be something to do with the load being unbalanced?

 

[Sidewinder]

From what you are have written I would say the board is wired incorrectly.

What were the readings wrt earth?

Is this fed from a std. 3ph 400V supply direct from the DNO?

 

[NozSpark]

UH OH!!

I think I'd be getting back there, isolating and re checking the incoming connections and phase rotation... ASAP!

 

[batty]

Not sure what youy mean by open and closed probably easier to say off or on. Voltage between phases should be roughly 415 volts and from phases to neutral it should be roughly 240 volts if you are getting something different you have something seriously wrong and should not leave it like that.

 

[kerching]

What are you testing it with, as it may make a difference. I used to have a cr4ppy tester that read 110 if V was over 50 and less than 240. It also read 240 if V was over 110 and less than 415. Sent it back to suppliers after 10 minutes use.

Trying to fathom out if it could be a mix up of old new colours and a phase and neutral has got transposed somewhere. But I have just bottled 25 litres of Wine for my 1st Grand daughters 1st birthday party tomorrow and felt compelled to sample a random shelectshion, hic!

...and yes, I know 1 is a bit early to start drinking but,... :coat

 

[Sidewinder]

kerching,

Good point.

When measuring voltage I use a voltage measuring meter, never a voltage indicator!

I have a Fluke 87 DMM that lives in my toolbox, always use that first for fault finding, never a voltage indicator, though may move on to other instruments later.

Sounds like you had a good day!

 

[kerching]

Depends on what I am testing...Fluke T100 for the mundane stuff, next stage is a T5 if I need an 'actual value' then Fluke 113 RMS if I am getting serious...or sometimes just the Drummond which now must be 30 years old and on its 3rd lamp.

Horses for courses really, as what I do is so varied...even over the course of a day. For example, one day last week

Shut down Leisure centre at 0600 to fit MCCB

2nd fix kitchen extension

Callout to loss of power at customers 120 mile round trip North

Callout to Mains failure at customers 110 mile round trip South

back home at 2100h

................. :coat

 

[Sidewinder]

Bit like my work Kerching, though mine goes through mechanical too!

Mind 3 days this week 3 jobs 1 per day coz I was not sure if they would let me out to book another job after I finished.

They may have just kept me there and given me full room and board at Her Majesty's Expense!!!

 

[thomas123]

N-E was 0v,

 

[kerching]

Bit like my work Kerching, though mine goes through mechanical too!Mind 3 days this week 3 jobs 1 per day coz I was not sure if they would let me out to book another job after I finished.

They may have just kept me there and given me full room and board at Her Majesty's Expense!!!

I did a bit of work in one a few years ago . All bad lads, Warder watching my back and another Warder watching his. [No idea who was watching number 2s back though]

Never forget the smell, ever!... :coat

 

[batty]

N-E was 0v,

Well at least something is right then.

 

[Sidewinder]

I am in the works dept mate, well away from the cons!

Have an escort though in 2 of the 3 the last is Butlins with green jogging bottoms, i.e. and open one!

I cover 4, 2 cat b, with some cat a, there the security is tight.

They look after you though.

 

[thomas123]

i'll have to get a clamp meter on the phases next week, i think its something to do with the inbalance.

"There are several causes of voltage unbalance. 1) Variations in single phase loading cause the current in the three-phase conductos to be different producing different voltage drops causing the phase voltages to become unbalanced; 2) Blown fuses or overcurrent protective devices; 3) Unbalance single phase loadings; or 4) fault on one of the phases."

 

[Sidewinder]

thomas,

I have seen 3ph systems with 2x the load on 1 ph that was on the other 2, I still had representative mains voltages,of around 240, and we are talking about a ca 196A, 96A, 98A, type of imbalance!

I suspect this is more than a little imbalance.

To get 400V L1:N is concerning me somewhat.

Do you have phase to earth voltages?

 

[ProDave]

Hi,today i tested the voltage on a 3 phase board and discovered something unusual, when the boards main switch was closed i tested between L1 and N and had 400v, L2 and N 230v, L3 and N 110v.

But when the main switch was open, all of them where at 230v.

Could this be something to do with the load being unbalanced?

I would say you have a floating, or high impedance neutral.

Switch off, no load, all is balanced and voltages are correct.

Loads on, neutral is pulled off voltage by the imbalanced loads.

Normally the neutral carries the imbalance current and maintains it's voltage, but if it's floating, or a high impedance, then it will get pulled one way with an imbalanced load.

This is a 3 phase & N supply (star) not just a 3 phase (delta) supply isn't it?

 

[kerching]

Thinking about it a bit more now that the wine tasting has ended...I reckon you have got a loose main neutral either in the board or upstream :run

this information is worth

what you have paid for it... :coat

 

[Sidewinder]

Could be kerching, also could be as prodave suggests.

I am carp at these fault finding from posts!

 

[Evans Electric]

I was wondering about the neutral not being connected properly .

I don't think the imbalance would cause that , most TP boards with SP circuits carry different currents on each phase , can't be helped.

What is the neutral cable colour ? Blue or black ?

 

[ProDave]

I was wondering about the neutral not being connected properly . I don't think the imbalance would cause that , most TP boards with SP circuits carry different currents on each phase , can't be helped.

What is the neutral cable colour ? Blue or black ?

If the 3 phase loads are balanced, then there will be zero current flowing in the neutral.

But if the loads are imbalanced, as they are when a 3 phase feed is feeding single phase loads, then the neutral will carry the imbalance load back to the star point of the substation transformer. The neutral never normally carries as much current as each of the phases, which is why you will sometimes find a 3 phase supply where the neutral is a smaller CSA than the phases.

I am 99% sure in this case, for some reason, the neutral is high impedance, so these imbalance loads are not getting back to the transformer. So what happens is the neutral voltage pulls one way to the point where the loads become balanced if that makes sense.

Try measuring neutral WRT earth with no load, then again with a good selection of imbalanced loads connected. It should remain at virtually the same voltage, but I'm wiling to bet it will be a long way from earth under load in this case.

Do this test, then come back and tell us the result.

 

[thomas123]

I would say you have a floating, or high impedance neutral.Switch off, no load, all is balanced and voltages are correct.

Loads on, neutral is pulled off voltage by the imbalanced loads.

Normally the neutral carries the imbalance current and maintains it's voltage, but if it's floating, or a high impedance, then it will get pulled one way with an imbalanced load.

This is a 3 phase & N supply (star) not just a 3 phase (delta) supply isn't it?

yeah it is, and like you said, it goes back to normal when there is no load. how can i check the impedance of the neutral? the mains cannot be shut down under any circumstances, can only do live tests

edit, sorry i didnt realise there was another page of posts after this one..