Fourth pole on isolator

[JohnW2]

Imagine the scenario of a 10KW load on ph1, 10W halogen bulb on ph2 and nothing on ph3 and the star point loses the neutral:
View attachment 284207
I won't bother doing the Ohms law calculations but I'm sure it becomes apparent quite quickly the pd across the bulb will be almost the ph to ph voltage, I'll go so far as predicting within 99.9%

Indeed. Thanks.

What I was 'missing' was a scenario such as you depict, with single-phase (phase-N) loads supplied by more than one phase -I was thinking (and hence 'talking about') only of the situation in which there were single-phase loads only supplied by one phase.

Kind Regards, John

 

[JohnW2]

The neutrals of the loads may be disconnected from the supply, but they are still connected to each other!

Indeed. As I've just written to Sunray ...

"... What I was 'missing' was a scenario such as you depict, with single-phase (phase-N) loads supplied by more than one phase -I was thinking (and hence 'talking about') only of the situation in which there were single-phase loads only supplied by one phase."

Kind Regards, John

 

[SUNRAY]

Indeed. Thanks.

What I was 'missing' was a scenario such as you depict, with single-phase (phase-N) loads supplied by more than one phase -I was thinking (and hence 'talking about') only of the situation in which there were single-phase loads only supplied by one phase.

Kind Regards, John

It's quite amazing how 3ph can cause so much confusion when something unexpected has happened or has gone wrong.

On a redeveloped floor of a building we had exactly the problem when a chiller plant shorted 2 phases together and the connexion feeding the neutral bar in the DB blew. virtually every bit of kit in the AV rack I'd been commissioning and many tens of PC emitted vast quantities of the wrong coloured smoke.

I had a situation once where one phase was 30° out and after calling in dno/supplier it was a real struggle to convince them of the fault and the fact my oscilloscope was uncalibrated was irrelevant. Even when he did accept it he didn't understand why 3ph motors didn't run correctly and pulled excessive current.

 

[JohnW2]

I had a situation once where one phase was 30° out ....

Interesting. How on earth can that arise?

Kind Regards, John

 

[SUNRAY]

I have no idea, the symptoms were simply poor motor performance and current, then checking voltages; ph to N were good but ph to ph were wrong. Speaking to DNO/supplier they tried to say it wasn't their problem. Following day I took a scope in and it was obvious, they did come out and their big reaction was calibration although the different timing was easy to see measured on the 2 beams.
It did get corrected but of course we never found out what happened.

Interesting. How on earth can that arise?

Kind Regards, John

The second guy arrived with a radio controlled tester which showed our red, yellow, blue was out of step with national standard although rotation was correct.

 

[333rocky333]

Fair enough, but I'm still confused about the way in which this all started, when you wrote ...

Any 'internal 240V equipment' would have to be connected between neutral and one of the phases. If the neutral 'breaks early' then any such internal equipment would be 'disconnected from the supply', such that there would be no PD between the L and N feed to that equipment (other than due to stray capacitance etc.).

What am I missing?

Kind Regards, John

Sorry for the confusion, it was more directed to the Ops first post which asked whether that 4th pole was for the N on a 415 circuit.
To which I said I would choose one with a dedicated N
However the op is using single phase, so my response did not really affect his situation

 

[bernardgreen]

I had a situation once where one phase was 30° out

I suspect ( but cannot do the maths to prove it ) that the fault was on the 3 phase HV supply to the 3 phase HV to LV transformer.

 

[JohnW2]

I have no idea, the symptoms were simply poor motor performance and current, ...

It does sound very odd.

.... then checking voltages; ph to N were good but ph to ph were wrong.

I can't really see how that would be possible. If the waveforms of all three phases relative to a common refence waveform were 'as expected', then surely the phase-to-phase would also necessarily be 'as expected', wouldn't it?

I'm also somewhat struggling to imagine how just one of the P-P phases could be wrong.

Speaking to DNO/supplier they tried to say it wasn't their problem. Following day I took a scope in and it was obvious, they did come out and their big reaction was calibration although the different timing was easy to see measured on the 2 beams.

That seems to be a rather silly ("rubber stamp'/'knee jerk') reaction on their part. Determination if the relative phases of two waveforms is essentially qualitative, such that 'calibration' is irrelevant!

Kind Regards, John

 

[JohnW2]

I suspect ( but cannot do the maths to prove it ) that the fault was on the 3 phase HV supply to the 3 phase HV to LV transformer.

Although I could probably do the maths, if I knew which maths to do, this is a field beyond my experience.

However, I thought that a 3-phase transformer could be viewed as (indeed, could be simulated by) three single-phase transformers, hence with the phase of the output of each of those notional transformers corresponding to the phase of its input. If that is the case, there could only be 'incorrect relative phases' in the LV supply if there was a corresponding 'error' in the input to the HV-LV transformer - which, in turn, would reflect the phase differences coming out of the generator (which I would have thought was determined by the physical construction of the generator).

However, Sunray has now told us that the Phase-neutral phases were 'good' in the scenario he's mentioned. which I take to mean that the relative phases of the three P-N waveforms were 'as expected' - in which case I really don't understand his story at all!

Kind Regards, John

 

[333rocky333]

Once had a call out where the 3 phase motors were playing up, all 3 phases to N were ok, but phase to phase weren't.
Turned out 2 phases blew in the road, but for 2 phases the cables had welded together, left a situation where we had just Two phases and N .
Two of the original phases both now being the same phase.
I quess if it weren't for the 3 phase fridges, it could have remained in that state without anyone even noticing.

 

[JohnW2]

Once had a call out where the 3 phase motors were playing up, all 3 phases to N were ok, but phase to phase weren't.
Turned out 2 phases blew in the road, but for 2 phases the cables had welded together, left a situation where we had just Two phases and N .
Two of the original phases both now being the same phase. I quess if it weren't for the 3 phase fridges, it could have remained in that state without anyone even noticing.

That's a very specific (and presumably very unusual) combination of circumstances./faults. but I can understand that.

However, whilst I can conceive of circumstances (such as you describe) in which two phases would end up in-phase with each other and, of course, of situations (like 'normal') when two phases were 135 degrees apart), I really can't (so far) see how they could end up with a phase difference which was 30 degrees different from what it should be.

Kind Regards, John

 

[SUNRAY]

I have no idea, the symptoms were simply poor motor performance and current, then checking voltages; ph to N were good but ph to ph were wrong. Speaking to DNO/supplier they tried to say it wasn't their problem. Following day I took a scope in and it was obvious, they did come out and their big reaction was calibration although the different timing was easy to see measured on the 2 beams.
It did get corrected but of course we never found out what happened.

I too have absolutely no idea how how a phase could be in the wrong place but I hope this helps the result.

Not very technical but a quick knock-up on paint:

Drew the 3 phases then added the 3 3 ph-ph lines and did rough calculations from measured coordinates

There will be errors but not too bad.

All 1ph supplies are fine but 2 of the phases are well above and below that expected.
3ph motors ran slow and shaking and pulling much higher current, making the motor overloads trip.

 

[SUNRAY]

I too have absolutely no idea how how a phase could be in the wrong place but I hope this helps the result.

Not very technical but a quick knock-up on paint:
View attachment 284278
Drew the 3 phases then added the 3 3 ph-ph lines and did rough calculations from measured coordinates

There will be errors but not too bad.

All 1ph supplies are fine but 2 of the phases are well above and below that expected.
3ph motors ran slow and shaking and pulling much higher current, making the motor overloads trip.

I've had this in my media for nearly a year:

the green is the result of adding black and red together and lags red by 30°, equally it leads blue by 90°. So the phase relationship is available at a push. Again I say I have no idea how such a confusion of the supply could arise.

 

[JohnW2]

I too have absolutely no idea how how a phase could be in the wrong place but I hope this helps the result. ... Not very technical but a quick knock-up on paint:
View attachment 284278
Drew the 3 phases then added the 3 3 ph-ph lines and did rough calculations from measured coordinates .... There will be errors but not too bad. .... All 1ph supplies are fine but 2 of the phases are well above and below that expected. .... 3ph motors ran slow and shaking and pulling much higher current, making the motor overloads trip.

Thanks. I need to think about this (maybe 'for ever' ), but I have other calls on my time at the moment. Watch this space!

Kind Regards, John