Split phase

[jimmer]

Could I have as much info regarding split phase installation complications to a new farm dairy unit I am going to be starting soon.
what I've heard so far:-
you should not switch neutrals on split phase circuits
how is volt drop calculated on 480v supplies for sub mains
can you have a 4 core sub main with 2 phases, neutral and earth of equal sizes or should I double the neutral?
are there any issues using a 2 pole rcd main switches on 2x single phase boards?
there will be no split phase circuits apart from the sub main which will be swa

all help much appreciated

 

[Pete999]

Your profile doesn't give out much info jimmer, are you an Electrician?

 

[jimmer]

Your profile doesn't give out much info jimmer, are you an Electrician?

yes
(niceic approved)

 

[Anthony S]

yes
(niceic approved)

Don't the nic offer a technical help line for their members ?

 

[davesparks]

Why would you want to switch the neutral? Or do you mean you shouldn't use Switches which break both L and N?

Volt drop is calculated the same way it is for any other circuit, ohms law doesn't change!

What makes you think you need to double the neutral? Do you treble the neutral for 3 phase supplies?

 

[jimmer]

Don't the nic offer a technical help line for their members ?

thanks for the help

 

[jimmer]

Why would you want to switch the neutral? Or do you mean you shouldn't use Switches which break both L and N?

Volt drop is calculated the same way it is for any other circuit, ohms law doesn't change!

What makes you think you need to double the neutral? Do you treble the neutral for 3 phase supplies?

It is to do with the star point on the transformer disconnecting first and voltage rising I think, not a great deal of info on this topic

yes I know of ohms law and your saying I should use 480v?

i suspected one neutral of an equal size to phase, as phases are 180 degrees apart

 

[davesparks]

It is to do with the star point on the transformer disconnecting first and voltage rising I think, not a great deal of info on this topic

yes I know of ohms law and your saying I should use 480v?

i suspected one neutral of an equal size to phase, as phases are 180 degrees apart

If the phases are 180degrees apart where did you get 480V from? It'll be 0V if they are at 180 degrees, you get 480V from two 240V phases which are in phase (at 0degrees)

Tony has posted a very good thread on the subject of transformers somewhere around this forum, reading it will help you understand the theory of this.

 

[JD6400]

Don't forget to use equipment rated to the higher voltage

 

[Marvo]

By the sounds of it you're only going to have 240v final circuits so you can 'split' the supply at source into 2 x separate single phase supplies into 2x single phase consumer units. This would be preferable for me because you avoid the need to use MCB's and RCD's rated to 530 volts or higher.

 

[Knobhead]

The supply should rightly be called “centre tapped”, 240V→0→240V = 480V, or in simpler terms L1→N→L2.

And before anyone starts, the two legs aren’t at 180° phase angles, they are “in phase” IE at 0°.
I[SUB]n[/SUB]=I[SUB]1[/SUB]-l[SUB]2[/SUB] or I[SUB]n[/SUB]=I[SUB]2[/SUB]-l[SUB]1[/SUB], whichever give a +ve value so the neutral can be of equal size to the line. Therefore the neutral current will never be greater than either line.

If they were at 180° then:
L[SUB]1[/SUB]→N=240V
L[SUB]2[/SUB]→N=240V.
L[SUB]1[/SUB]→L[SUB]2[/SUB]=0V
I[SUB]n[/SUB]=L[SUB]1[/SUB]+L[SUB]2[/SUB]

If only single phase circuits are to be used then the only >400V equipment needed is the OCPD at the origin, BS88 fuses cover that easily.
If you intend to install two boards by the supply intake you can dispense with the separate OCPD. Downstream of the intake it’s treated as two independent supplies.

In reality it’s no different to a centre tapped site transformer but in this instance the centre tap is used as neutral as well as being connected to earth.

I have drawings to illustrate this system but I’m sorry I’ll not post them here.

Don’t they teach theory anymore? This is basic apprentice stuff.



For interest:
The last true 480V load I came across was in my grandfathers milking parlour, 40+ years back. That was just for the two motors for the vacuum header, every thing else was bog standard 240V.

 

[Engineer54]

The supply should rightly be called “centre tapped”, 240V→0→240V = 480V, or in simpler terms L1→N→L2.

And before anyone starts, the two legs aren’t at 180° phase angles, they are “in phase” IE at 0°.
I[SUB]n[/SUB]=I[SUB]1[/SUB]-l[SUB]2[/SUB] or I[SUB]n[/SUB]=I[SUB]2[/SUB]-l[SUB]1[/SUB], whichever give a +ve value so the neutral can be of equal size to the line. Therefore the neutral current will never be greater than either line.

If they were at 180° then:
L[SUB]1[/SUB]→N=240V
L[SUB]2[/SUB]→N=240V.
L[SUB]1[/SUB]→L[SUB]2[/SUB]=0V
I[SUB]n[/SUB]=L[SUB]1[/SUB]+L[SUB]2[/SUB]

If only single phase circuits are to be used then the only >400V equipment needed is the OCPD at the origin, BS88 fuses cover that easily.
If you intend to install two boards by the supply intake you can dispense with the separate OCPD. Downstream of the intake it’s treated as two independent supplies.

In reality it’s no different to a centre tapped site transformer but in this instance the centre tap is used as neutral as well as being connected to earth.

I have drawings to illustrate this system but I’m sorry I’ll not post them here.

Don’t they teach theory anymore? This is basic apprentice stuff.



For interest:
The last true 480V load I came across was in my grandfathers milking parlour, 40+ years back. That was just for the two motors for the vacuum header, every thing else was bog standard 240V.

I can't say that i've ever seen such a Centre Tapped 240v-0-240v /480 volt set-up in the field, ...so to speak!! lol!! Must be a rural overhead supply set-up, seen a few old wierd and wonderful rural TX supplies when i was working with my uncle after school/weekends....

 

[JD6400]

The supply should rightly be called “centre tapped”, 240V→0→240V = 480V, or in simpler terms L1→N→L2.

And before anyone starts, the two legs aren’t at 180° phase angles, they are “in phase” IE at 0°.
I[SUB]n[/SUB]=I[SUB]1[/SUB]-l[SUB]2[/SUB] or I[SUB]n[/SUB]=I[SUB]2[/SUB]-l[SUB]1[/SUB], whichever give a +ve value so the neutral can be of equal size to the line. Therefore the neutral current will never be greater than either line.

If they were at 180° then:
L[SUB]1[/SUB]→N=240V
L[SUB]2[/SUB]→N=240V.
L[SUB]1[/SUB]→L[SUB]2[/SUB]=0V
I[SUB]n[/SUB]=L[SUB]1[/SUB]+L[SUB]2[/SUB]

If only single phase circuits are to be used then the only >400V equipment needed is the OCPD at the origin, BS88 fuses cover that easily.
If you intend to install two boards by the supply intake you can dispense with the separate OCPD. Downstream of the intake it’s treated as two independent supplies.

In reality it’s no different to a centre tapped site transformer but in this instance the centre tap is used as neutral as well as being connected to earth.

I have drawings to illustrate this system but I’m sorry I’ll not post them here.

Don’t they teach theory anymore? This is basic apprentice stuff.



For interest:
The last true 480V load I came across was in my grandfathers milking parlour, 40+ years back. That was just for the two motors for the vacuum header, every thing else was bog standard 240V.

Hi Tony , thinking about it , if it is on a new dairy unit ? It may still need the upfront s-type to be rated for the higher voltage , depending on how he plans to provide a point of main isolation

 

[Marvo]

If the phases are 180degrees apart where did you get 480V from? It'll be 0V if they are at 180 degrees, you get 480V from two 240V phases which are in phase (at 0degrees)...........

I think the confusion comes because if you have a tester that shows you the voltage waveform as a graph and you connect firstly with your red test lead on the L1 and the black test lead on N it will show you a waveform that's 180 degrees apart from the same test with the red lead on L2 and the black lead on N.

As you correctly say though the waveforms are actually in sync hence you don't get zero volts L1-L2.

 

[Knobhead]

Hi Tony , thinking about it , if it is on a new dairy unit ? It may still need the upfront s-type to be rated for the higher voltage , depending on how he plans to provide a point of main isolation

S rating is a time function of the operating characteristics of the RCD. Nothing to do with the maximum operational voltage.

To add an S rated RCD is purely the decision of the designer when bearing in mind the downstream equipment and its location, etc.

If you treat the 480V supply as purely two supplies then it will be no different to any other installation.

If like my grandfathers farm there are motors operating at 480V then the easiest way is to have a sub board solely for that one purpose fed by Henley blocks off the tails.

In effect you will have three boards, two at 230V and one at 480V. You can use a multi phase board BUT this adds the problem of 2 pole switching while the majority of the load is Sp+N

Board 1 480V L1→L2
Board 2 240V L1→N
Board 3 230V N→L2

 

[JD6400]

Hi Tony , it was not so much it being an s-type ( although prudent for discrimination ) , it was more the fact that it being a dairy unit he will need an upfront rcd of 300mA or less to protect the whole installation .
And as it will almost undoubtedly need to be TT ( as I have never yet come across a holding that has had the designed considerations for it not to be ) , I would expect to see it double as a point of main isolation as well .

Sorry I should have explained myself better the first time

 

[GEOFF S]

This is quite simple, its classed as a single phase three wire system. In effect two phases 180 degree apart, the neutral point being at the center between the two phases.

They cant be in phase, draw out two vectors with the same direction and magnitude.

Cheers

 

[Knobhead]

I’ve been avoiding posting any of my drawings here.

 

[GEOFF S]

There are 2 sine waves, The positive of teh first sine is in phase with the negative of the second sine.

Cheers

 

[Knobhead]

This is quite simple, its classed as a single phase three wire system. In effect two phases 180 degree apart, the neutral point being at the center between the two phases.

They cant be in phase, draw out two vectors with the same direction and magnitude.

Cheers

Read up on transformer phase and vector angles and then tell me I'm wrong.

 

[ElectroChem]

I’ve been avoiding posting any of my drawings here.

I knew I hadn't quite grasped the point of distinction Tony. The two live wires are IN PHASE (they are driven by the same winding and reach their maximum values simultaneously) but of OPPOSITE POLARITY (the center tap means that relative to earth one will be +ve and the other -ve). Have I got it?

 

[GEOFF S]

I knew I hadn't quite grasped the point of distinction Tony. The two live wires are IN PHASE (they are driven by the same winding and reach their maximum values simultaneously) but of OPPOSITE POLARITY (the center tap means that relative to earth one will be +ve and the other -ve). Have I got it?

Yes you have it, two sine waves, both pass through 0 at the same time, the polarity is 180 degrees out of phase.

If you look on a oscillascope you would see the positive of one sine reach its maximum at teh same time as the other meets its negative max. The two sine ways give you the voltage. If they were in phase the two sine ways would be superimposed, the area of the sine would not change.

Cheers

 

[GEOFF S]

Read up on transformer phase and vector angles and then tell me I'm wrong.

Do we need to consider the HV/LV displacement of a Tx to understand what voltage and current will exist in a LV polyphase system?


Cheers

 

[GEOFF S]

I knew I hadn't quite grasped the point of distinction Tony. The two live wires are IN PHASE (they are driven by the same winding and reach their maximum values simultaneously) but of OPPOSITE POLARITY (the center tap means that relative to earth one will be +ve and the other -ve). Have I got it?

Yes that is correct, if you looked on a oscillascope you would see the two sinewaves, the positive of one meets its maximum at the same time the other sine meets its maximum negative.

Cheers

 

[Knobhead]

Do we need to consider the HV/LV displacement of a Tx to understand what voltage and current will exist in a LV polyphase system?


Cheers

Yes, the primary makes a difference to phase shift for poly phase transformers. I’ve written an article about all kinds of transformers on another site.

The commonest type of 3 phase unit is vector group Dyn11 where the secondary leads the primary by 30°. The 11 comes from the clock position.
Vector groups are based on the physical angle not the output waveform. Hence the centre tapped being 0°.

 

[GEOFF S]

Yes, the primary makes a difference to phase shift for poly phase transformers. I’ve written an article about all kinds of transformers on another site.

The commonest type of 3 phase unit is vector group Dyn11 where the secondary leads the primary by 30°. The 11 comes from the clock position.
Vector groups are based on the physical angle not the output waveform. Hence the centre tapped being 0°.

Hi Tony

Out of intrest, your diagram uses kirchoffs cxurrent law to determine the neutral current, im not aware of the having to condider the primary/secondary winding connections, equally when calculating voltages within the LV system i have never taken winding displacments into account.

Could you explain how you take into account the phase displacment in your calculations.

Paralell Tx require phasing(circulating currents), and 30 degree phase shift nulifies third harmonics, im interested into how your calculations include primary/secondary phase displacement.

Cheers

 

[Knobhead]

PM sent

 

[Knobhead]

Yes you have it, two sine waves, both pass through 0 at the same time, the polarity is 180 degrees out of phase.

If you look on a oscillascope you would see the positive of one sine reach its maximum at teh same time as the other meets its negative max. The two sine ways give you the voltage. If they were in phase the two sine ways would be superimposed, the area of the sine would not change.

Cheers

To play devils advocate, what do you get if you place the oscilloscope on L[SUB]1[/SUB]→L[SUB]2[/SUB]?

 

[Barry White]

 

[GEOFF S]

Thats exactly what you will see.

Cheers

 

[Silly Sausage]

Thats exactly what you will see.

Cheers

On two channels.
I think Tony means L1 & L2 on ONE channel!

 

[Knobhead]

Thats exactly what you will see.

Cheers

Wrong!

Stop playing the smart arse. Learn about transformer phase and vector diagrams.

 

[davesparks]

Well isn't that pretty! I think it's going to peak at a bit more than 240V though.
You could easily create that trace on an oscilloscope screen, but it's not an accurate representation of what is going on.

 

[Knobhead]

I think √2 comes in to it somewhere doesn’t it Dave?

I don’t think we’re wrong, but you never know.

240V RMS = 339.41V peak→base line.

 

[ElectroChem]

240V RMS = 339.41V peak→peak.

Wouldn't the peak-to-peak be twice that Tony?

 

[Barry White]

Well isn't that pretty! I think it's going to peak at a bit more than 240V though.
You could easily create that trace on an oscilloscope screen, but it's not an accurate representation of what is going on.

was a rough representation so I used RMS voltages it was my representation of what I thought I would see would you like to draw it please

 

[Knobhead]

Wouldn't the peak-to-peak be twice that Tony?

I was altering it while you posted

 

[Knobhead]

was a rough representation so I used RMS voltages it was my representation of what I thought I would see would you like to draw it please

You used peak→base line which would give 169V RMS.

I did the drawing years ago and it's on photobucket for use elsewhere.

 

[ElectroChem]

I was altering it while you posted

Gotta be quick around here, there's always some smart-arse apprentice looking to score a point off the boss!

 

[Knobhead]

Gotta be quick around here, there's always some smart-arse apprentice looking to score a point off the boss!

True, many years ago I was the apprentice. I also got my arse kicked when I got things wrong.

I worked with some real old barstewards, learnt a lot from them though.
Up until about ten years ago I still phoned my old foreman for a chat. At 85+ he told me where to look for a fault in a 2000HP motor he’d never even seen. He was spot on the mark.

Can’t phone him now, R.I.P. Ted

 

[GEOFF S]

I think √2 comes in to it somewhere doesn’t it Dave?

I don’t think we’re wrong, but you never know.

240V RMS = 339.41V peak→base line.

Its called the crest factor, and it takes you from peak to RMS, although under certain condtions(Fault) the crest value can be greater.

Stop playing the smart arse. Learn about transformer phase and vector diagrams.

Okay, if the vectors are in phase, or 180 Degrees apart you get the same answer, though from reading J & P transformer book and other publications they state 180 degree, some were 90 degree.


Cheers

 

[Barry White]

FFs its centre tapped there would be a sine wave above and below the base line

take 2 comming in a mo

 

[GEOFF S]

Take three

For the "in phase vectors" to add we need a series connection within the Tx, this isnt the case, and it the vectors were 180 degree apart and wired in series they would subtract.

The Tx is center tapped, extract.

Center-tap transformer. The tapping point is in the exact center of the secondary winding providing a common connection for two equal but opposite secondary voltages. With the center-tap grounded, the output

V[SUB]A[/SUB]​

will be positive in nature with respect to the ground, while the voltage at the other secondary,

V[SUB]B[/SUB]​

will be negative and opposite in nature, that is they are 180[SUP]o[/SUP] electrical degrees out-of-phase with each other.

Now VA= NA/NP * VP
VB= NB/NP* VP

230 at 0 Degrees plus 230 at 180 degree = 460 Volts

 

[GEOFF S]

Take four

If we have a winding that delivers 480 volts, we earth one end(0 V and reference) and tap 1/4, 1/2 and end we would measure 120 v, 240 v and 480 v all in phase from that reference.

If we center tap the winding and earth(0 v and reference) what we would see is at the end of one winding 240 + and the other end 240 -.

So the two phases from the center tap reference are 180 degrees apart.

Cheers

 

[Knobhead]

Your just not getting this are you. Convention with transformers is governed by angular orientation of the windings not the output waveform.

I’m not continuing this further on here due to constraints imposed on me by the moderators.

some were 90 degree.

90° phase displacement. Explain for the other members.

 

[Leyland30]

This is quite simple, its classed as a single phase three wire system. In effect two phases 180 degree apart, the neutral point being at the center between the two phases.

They cant be in phase, draw out two vectors with the same direction and magnitude.

Cheers

Agree, the wave forms are 180 degree apart.

 

[Leyland30]

Jimmer, did you get it sorted.