Quick question - Reg number prohibiting SP RCBOs on TT

[JohnW2]

You're suggesting that at point X on the diagram there are two currents which are out of phase with each other. How can that be when there is only a single current path around the circuit?

I think he's just created a self-fulfilling (but rather surprising) definition, just as he did with his two speakers, with their terminals marked '+' and '-'. In other words, I think he is regarding the 'middle ends' of the two resistors (i.e. those joined at point X) as being some sort of reference (just as the two '+' terminals of the two speakers), and is observing that the current is coming out of one such end and going into the other such end - and is then calling that 'out of phase'.

Kind Regards, John.

 

[ban-all-sheds]

I don't know if the IEE/IET has any sort of official rules, but it has been accepted as a basic fact of electrical engineering for about the last 120+ years.

So do you simply refuse to accept this different term because of the 120 years of history, or do you claim to have more expertise than the IET & BSI and that they are simply wrong?

Simple question - when you look at the currents flowing in R1 and R2, with respect to their common point, are they out of phase?

No.

Apologies in advance to any others, but I'm going to have to use some large letters here, as you seem to keep missing them -


So when looking at the currents WITH RESPECT TO THE CENTRE POINT they always flow in the same direction, i.e. at any point in time they are both flowing away from the centre point, or both flowing towards it, never one going one way and one the other?

Wrong.

He will see cars coming toward him, then pass and go away from him. Just as the current at the mid point between the two halves of the load must approach that mid point from one side and leave via the other.

Exactly - different directions. Not the same direction. Not in phase. Out of phase.

To transpose your analogy to a railway, if you were standing on the platform at Rugby as the London-to-Liverpool express went through, then obviously the train would be approaching you from London direction, pass you at Rugby, then head away from you toward Liverpool.

But where did the train change direction?

It changed direction as it passed me.

BECAUSE I AM THE POINT OF REFERENCE. EVERYTHING ELSE HAS ITS ONLY EXISTENCE WITH REFERENCE TO ME.

Before it got to Rugby it was travelling from London to Liverpool. After it passed you at Rugby it was still travelling from London to Liverpool. It didn't suddenly turn around and start going back towards London, did it?

Before it reached me it was travelling towards me. After it passed me it was travelling away from me.

So it's still going in the same direction.

Towards.

Away.

Two different directions.

Not the same direction.

And that's exactly what's happening with the current at the mid point of R1 & R2.

You are failing to understand where the point of reference is. Or you are being wilfully obtuse because you want to appear as if you don't understand.

You are not, ever, moving with the current and not experiencing a change of direction.

You are not, ever, observing the current from outside the frame of reference.

You must, always, for ever, and only, look at the potential differences, and therefore the currents WITH RESPECT TO THE CENTRE POINT.

You're suggesting that at point X on the diagram there are two currents which are out of phase with each other. How can that be when there is only a single current path around the circuit?

If you took the situation of a normal 3-phase supply, and had only two points of reference in it, being the ends of two of the phases, what you would see would be a simple 50Hz AC supply with an RMS voltage of 398v.

Putting aside what you know about our normal 3-phase supply, there would be absolutely no way you could know, or tell, or detect, or determine anything beyond "I have a 398V supply".

If you connected a load between those two points you'd see a single alternating current which at any time was only going in one direction.

It's only when you introduce more points of reference between which to measure, or to have one fixed point which you relate all the others to, that you would be able to see a 3-phase supply.

You agree that with R1=R2 no current flows through the neutral wire which has been added?

Because they are 180° out of phase, so they cancel each other out, the balance is zero.

That seems to be the problem. This new description doesn't make sense.

It makes perfect sense - it's just a new way of looking at it. Look at it in the right way and it's fine.

Now - whether it makes sense as in is there any point to it is a completely different question, but that isn't the one we are discussing.

Not just me. Every electrical engineer for over a century, and many more today who will continue to refer to it as a single phase system because that's what it is.

So the stance you are taking is that if you had some unified way of ranking every electrical engineer in the world, or even who has ever existed, the combined rating of JPEL/64 would be at the very bottom.

I see.

 

[ban-all-sheds]

I think he's just created a self-fulfilling (but rather surprising) definition.

It's not a self-fulfilling definition.

It's simply looking at the term "2-phase 3-wire" (which was NOT of my creation), and thinking "Now in what way does that make sense?"

 

[JohnW2]

You agree that with R1=R2 no current flows through the neutral wire which has been added?

Because they are 180° out of phase, so they cancel each other out, the balance is zero.

That one got me thinking a bit. However, a simpler way of looking at it is to say that there is never, at any point during the cycle, any pd between the CT of the transformer and the junction of R1 and R2 - which is an adequate way of explaining the absence of any current through a conductor connecting those two points, without any need to postulate cancelling currents.

Kind Regards, John.

 

[bernardgreen]

Where is Kirchoff when you need him ?

http://www.electronics-tutorials.ws/dccircuits/dcp_4.html

 

[JohnW2]

Where is Kirchoff when you need him ?

Even he may have a bit of a problem here. Two cancelling antiphase currents, resullting in no net current in the conductor and no pd across the conductor is, to my mind, never going to be distinguishable from simply 'no current' - so invoking it as an explanatory concept is perhaps not all that helpful.

Kind Regards, John

 

[Tech99]

So do you simply refuse to accept this different term because of the 120 years of history, or do you claim to have more expertise than the IET & BSI and that they are simply wrong?

If you want to put it that way, both. I think the 120-odd years history backs up the fact that most of us recognise a single phase system for what it is. Just because some committee decides to adopt a changed terminology doesn't necessarily mean that it's right.

Has any justification for this peculiar "2 phase 3 wire" description been given? If so it might give us some insight as to the committee's thought process.

So when looking at the currents WITH RESPECT TO THE CENTRE POINT they always flow in the same direction, i.e. at any point in time they are both flowing away from the centre point, or both flowing towards it, never one going one way and one the other?

How is that a different direction? Of course at point X on my diagram you can't have an instantaneous current flowing down from R1 and simultaneously up from R2, because it would mean that the currents would combine at point X and then go where exactly? Into a black hole? As linked above, this is basic theory as outlined by Mr. Kirchoff with his first law quite a long time ago.

Exactly - different directions. Not the same direction. Not in phase. Out of phase.

So a car which is travelling from north to south as it approches you will not still be travelling from north to south, i.e. in the same direction, as it passes you and disappears into the distance?

To transpose your analogy to a railway, if you were standing on the platform at Rugby as the London-to-Liverpool express went through, then obviously the train would be approaching you from London direction, pass you at Rugby, then head away from you toward Liverpool.

But where did the train change direction?

It changed direction as it passed me.

No it didn't. If you could stand in London and observe the train for the whole of its journey you'd see it travelling in one direction, away from you, all the way to Liverpool. If you could stand in Liverpool and see it travel all the way from London, you'd see it coming towards you for the whole journey - One direction. Just because you're standing at Rugby and the train appears, to you, to be coming towards you at first and then going away from you doesn't mean it's changed direction. It's no different with the current at point X on the diagram.

You are failing to understand where the point of reference is. Or you are being wilfully obtuse because you want to appear as if you don't understand.

I think the problem is that you're trying to apply some fixed point of reference which isn't applicable to the situation.

I don't dispute your voltage waveforms with reference to point X. Yes, the voltages at A & B are 180 out of phase with each other with respect to point X. But now you seem to be trying to insist that what I sketched above is a 2 phase system even though I didn't include a c.t. on the xfmr and a neutral connection.

If you really think that because the current is flowing towards point X from R1 and away from it to R2 with the xfmr polarity as indicated that makes it a 2 phase system, how do you think that a single phase system can exist at all?

Here's about as simple as it gets:

What is happening with the current at any of the points A - E marked on the diagram?

If you took the situation of a normal 3-phase supply, and had only two points of reference in it, being the ends of two of the phases, what you would see would be a simple 50Hz AC supply with an RMS voltage of 398v.

Putting aside what you know about our normal 3-phase supply, there would be absolutely no way you could know, or tell, or detect, or determine anything beyond "I have a 398V supply".

If you connected a load between those two points you'd see a single alternating current which at any time was only going in one direction.

I agree with all of that. But according to you, if my load consisted of two series resistors across that supply and I then use the mid point of those resistors as a reference, I have two currents which are out of phase. So if I put a single 1 kilohm resistance across the terminals its single phase but if I split it into two 500 ohm resistances it's suddenly become two phases?????

It makes perfect sense - it's just a new way of looking at it. Look at it in the right way and it's fine.

Now - whether it makes sense as in is there any point to it is a completely different question, but that isn't the one we are discussing.

But surely it is? If you have to come up with convoluted ways to try and make the new description make sense, ways which go against long-established definitions and which then give rise to other inconsistencies, doesn't that make you think "Hang on a moment, what's this all about?"

 

[JohnW2]

I was one of those who, originally, felt that "2-phase 3-wire" made sense (for the 180° case), and I still think that it does in some ways. Whether it is the most reasonable terminology, or whether it made any sense to tamper with seemingly long-established terminology, is a different matter.

If (as probably sometimes happens) a premises is supplied with a neutral and two phases of a 3-phase supply, then it would seem (to me) entirely appropriate to regard those premises as having a 2-phase supply. Loads could be connected to between each of the phases and N, and the resulting currents through those loads would, I think/hope be accepted as out-of-phase.

If one accepts that, then there is surely nothing magic about 120° phase difference. The argument would surely still apply with 130°, 150°, 179° or, unltimately, 180°, wouldn't it?

Indeed, as I've said before, the 'offending' diagram in the current edition of the regs actually has three variants of "2-phase 3-wire", with phase angles of 180°, 120° and 90°? Is there as much argument/disagreement about the inappropriateness of the terminology when the phase angle is 90° or 120° as when it is 180°?

Kind Regards, John.

 

[ban-all-sheds]

If you want to put it that way, both. I think the 120-odd years history backs up the fact that most of us recognise a single phase system for what it is. Just because some committee decides to adopt a changed terminology doesn't necessarily mean that it's right.

Different doesn't mean wrong. Changed doesn't mean wrong.

Still - as you know more than they do, are a greater recognised expert etc....

How is that a different direction?

One is towards, one is away.

Towards and away are not the same.

If you stand on a slope, facing sideways, to the left of you is up, to the right is down.

Up and down are not the same.

Of course at point X on my diagram you can't have an instantaneous current flowing down from R1 and simultaneously up from R2, because it would mean that the currents would combine at point X and then go where exactly? Into a black hole? As linked above, this is basic theory as outlined by Mr. Kirchoff with his first law quite a long time ago.

Of course you can't, and nowhere have I said that you can.

All along I have said that you never get both currents flowing towards point X, or both of them away from point X, it's always a case of one going towards it and one going away from it.

Towards.

Away.

Different directions.


Just like, (and because), from point X, the potential difference to one end of the winding is positive, and to the other it is negative.

They are never both positive or both negative.

They are always 1 positive and 1 negative.

Positive.

Negative.

Different directions.

So a car which is travelling from north to south as it approches you will not still be travelling from north to south, i.e. in the same direction, as it passes you and disappears into the distance?

Neither, and this is why you are failing to understand it.

It's not travelling from north to south, it is travelling towards me and then it is travelling away from me.

There is no other frame of reference. No north, no south, just me.

Every motion is relative to me.

I can see nothing external.

There is nothing external.

The car comes towards me and then it goes away from me.

Towards and then away - a change of direction RELATIVE TO ME.

No it didn't.

Yes it did, and again this is the problem of you not looking at the frame of reference properly.

If you could stand in London and observe the train for the whole of its journey you'd see it travelling in one direction, away from you, all the way to Liverpool. If you could stand in Liverpool and see it travel all the way from London, you'd see it coming towards you for the whole journey - One direction. Just because you're standing at Rugby and the train appears, to you, to be coming towards you at first and then going away from you doesn't mean it's changed direction.

But I'm not in London, nor am I in liverpool.

I cannot see London.

I cannot see Liverpool.

I have no knowledge of London or Liverpool. Crucially I have no knowledge of the relationship between London and Liverpool.

I am in Rugby.

The train is coming towards Rugby, and then it is going away from Rugby.

Towards.

Away.

Different directions WITH RESPECT TO RUGBY.

It's no different with the current at point X on the diagram.

Indeed it is not.

I think the problem is that you're trying to apply some fixed point of reference which isn't applicable to the situation.

It is applicable.

It is the only thing which is applicable.

It must be applicable.

I don't dispute your voltage waveforms with reference to point X. Yes, the voltages at A & B are 180 out of phase with each other with respect to point X. But now you seem to be trying to insist that what I sketched above is a 2 phase system even though I didn't include a c.t. on the xfmr and a neutral connection.

I was showing you the voltages.

If you want to take issue with the fact that your diagram did not have the centre tap then I'll just dismiss the diagram as irrelevant because the system we are talking about has a centre tap, so there's not much point you trying to explain how something doesn't work by using a diagram which is not of the something.

If you really think that because the current is flowing towards point X from R1 and away from it to R2 with the xfmr polarity as indicated that makes it a 2 phase system, how do you think that a single phase system can exist at all?

By not using all of the points available.

Just like I can power a 230V SP load from a 3-phase supply by choosing the right pair of points to connect to.

Or a 398v SP load by choosing a different pair of points to connect to.

If I only connect to two, I will only ever see a single phase supply. To see more than one I would need to see more than two points.

Here's about as simple as it gets:

Dismissed - that's not a drawing of a 2-phase 3-wire system.

I agree with all of that. But according to you, if my load consisted of two series resistors across that supply and I then use the mid point of those resistors as a reference, I have two currents which are out of phase. So if I put a single 1 kilohm resistance across the terminals its single phase but if I split it into two 500 ohm resistances it's suddenly become two phases?????

Remember that you would need to connect the point where you split it to the supply, i.e. add the 3rd wire which takes it from 2-wire to 3-wire.

Do that and what do you have?

If you have to come up with convoluted ways to try and make the new description make sense, ways which go against long-established definitions and which then give rise to other inconsistencies, doesn't that make you think "Hang on a moment, what's this all about?"

It's not particularly convoluted - all you do is to observe the system from the point of view of the centre point, note that the voltages are out of phase, say "OK - that's what they mean by a 2-phase 3-wire system", and move on.

It doesn't give rise to other inconsistencies, and to adopt the position that once something has been thought of in a particular way for more than x years we must never think of it in any new ways, and anyone who does is wrong and knows nothing is dangerous and arrogant.

Yes - it did make me think "Hang on a moment, what's this all about?".

And my conclusion from my thinking was "Ah - that's how to look at it", not "JPEL/64 know nothing about electrical engineering, they are wrong".

 

[Tech99]

If (as probably sometimes happens) a premises is supplied with a neutral and two phases of a 3-phase supply, then it would seem (to me) entirely appropriate to regard those premises as having a 2-phase supply. Loads could be connected to between each of the phases and N, and the resulting currents through those loads would, I think/hope be accepted as out-of-phase.

Certainly. With a 120° phase difference the currents through each load so connected would have to be out of phase.

If one accepts that, then there is surely nothing magic about 120° phase difference. The argument would surely still apply with 130°, 150°, 179° or, unltimately, 180°, wouldn't it?

I see the angle you're coming from, but 180° represents something of a special case because it's simply a reversal of the current compared to 0°.

If you want to argue that the currents through the two loads R1 & R2 aren't in phase with a single-phase 3-wire arrangement because you've arbitrarily declared that at any given instant the current must be flowing from end A to end B of each load and you've connected them so that when current is flowing A to B in R1 it's flowing from B to A in R2, then the solution is simple: Swap the A & B connections to one of the loads. You're back to the speaker example of whether they're wired +ve to -ve at the centre point or -ve to -ve.

You can't do that with the 120° phase angle you get with the same two loads connected phase to neutral on 2 phases of a conventional 3-phase supply. Whichever ends of each load you care to designate A & B (or however you care to swap the connections around in an attempt to get current flowing in phase from end A to end B of each load), you simply can't do it.

Look at how that might apply to a motor. With multiple, identical field windings on the stator and without using any features like shaded poles, capacitors to introduce phase shifts etc. then with any multi-phase supply you should be able to generate a rotating magnetic field which will start the motor and guarantee the direction in which it will run every time.

If you have, say 6 field windings, you can connect them to a 3-phase supply to generate such a field easily. If the motor always starts in the wrong direction, it's easily corrected by swapping two of the phases.

You can do the same with a genuine 2-phase supply by the appropriate use of field windings. Many early motors were designed for the 2-phase system with a 90° phase angle.

But try and do it with what the IEE/IET now seems to want to call a 2 phase supply which is really only single phase. You can't. By connecting the (identical, remember) field windings in various permutations all you can do is reverse the current flow in one relative to another. You can't generate a 90° or a 120° difference in order to get a rotating field which will guarantee the direction in which the motor will start. All you can get is 0° or 180° in any given winding relative to the current flow in any other. So certainly you can wire the motor so that when pole piece 1A is N and 1B is S then pole piece 2A is N and 2B is S, or 2A is S and 2B is N. But maximum magnetic force in pole pieces 1A/1B and 2A/2B will always coincide because there's no phase angle between them. You could get the motor to start, but without something else involved either to give the rotor a kick in the right direction or to introduce some sort of phase shift on a starting winding, you couldn't guarantee which way it will turn.

 

[JohnW2]

Certainly. With a 120° phase difference the currents through each load so connected would have to be out of phase.

If one accepts that, then there is surely nothing magic about 120° phase difference. The argument would surely still apply with 130°, 150°, 179° or, unltimately, 180°, wouldn't it?

I see the angle you're coming from, but 180° represents something of a special case because it's simply a reversal of the current compared to 0°.

It's certainly a case which has special properties/consequences - but so, in some (different) senses are phase differences like 90° and 120°. However, 180° is also part of a general continuum, being the angle which is midway between 179° and 181° (both of which you would probably accept as justifying a '2-phase' description).

No-one has directly answered my question as to whether they are happy with the third (120°) variant of the current regs' diagrams of "Two-phase 3-wire" justifies the "Two-phase 3-wire" description. If, as I would personally think, they do accept this as a reasonable description, then we can at least dismiss the assertion that "Two-phase 3-wire does not exist (under any circumstances)".

Kind Regards, John

 

[Tech99]

Different doesn't mean wrong. Changed doesn't mean wrong.

It doesn't necessarily mean right either. Besides, whichever you think is right, it can't be both one phase and two. If the committee coming up with this nonsense is sure that it's right now by calling it 2 phases, then it must be saying that it's been wrong all these years by calling it 1 phase before.

And before this apparent sudden change, what did you call it? Single phase or two phase?

How is that a different direction?
One is towards, one is away.

Towards and away are not the same.

If you stand on a slope, facing sideways, to the left of you is up, to the right is down.

Up and down are not the same.

No, but if you release a ball at the top of the hill and let it roll down until it stops at the bottom, it's only ever moved in one direction - Downward.

Just because you see it moving toward you then away from you from your vantage point halfway up the hill doesn't change that.

All along I have said that you never get both currents flowing towards point X, or both of them away from point X, it's always a case of one going towards it and one going away from it.

And that does not make the currents out of phase. At ANY point on a conductor the current at any moment must be flowing towards that point from one side and away from it on the other.

It's not travelling from north to south, it is travelling towards me and then it is travelling away from me.

But if you see that car coming towards you from the north, passing you, then going away from you to the south, it is travelling north to south. It's travelling north to south when it's approaching you and it's travelling north to south when it's going away from you. It hasn't changed direction, it's been going in one direction the whole time. There is nothing special about the point you're standing where at that moment the car "changes" from moving towards you to moving away from you. Put somebody else a couple of hundred yards away from you and what happens?

First the car is moving towards both of you. Then it passes the other person and is moving away from him but is still moving towards you. Then it passes you and is moving away from both of you. Have there been two changes of direction of the vehicle then, from towards/towards, to towards/away, to away/away? There's nothing special about your point of observation, nor about the point of observation of the other person some distance away. The vehicle has been travelling in the same direction the whole time.

I was showing you the voltages.

I've never disputed that the voltages at opposite ends of the winding are out of phase relative to the centre tap. That by itself does not make it 2 phases.

Dismissed - that's not a drawing of a 2-phase 3-wire system.

But it's a drawing which illustrates that at any of the points I've marked the current will be flowing towards that point and away from it at any given instant (as it must, since it has nowhere else to go). And that seems to be the crux of your argument about 2 phases. But you're agreeing that what I drew there isn't 2 phases.

You can't have it both ways: Does the current flowing both towards and away from any given point at the same instant in time make it 2 phases or doesn't it?

Remember that you would need to connect the point where you split it to the supply, i.e. add the 3rd wire which takes it from 2-wire to 3-wire.

Do that and what do you have?

You have an extra wire connecting the xfmr c.t. to the mid point of R1 & R2. The instantaneous direction of I1 doesn't change. The instantaneous direction of I2 doesn't change. The direction of I1 compared to that of I2 at any given instant doesn't change.
If R1=R2 so that no current flows in the third wire, not even the magnitude of I1, I2 or I1 relative to I2 change. If no currents have changed their direction or relationship to each other in any way, how can you have introduced a second phase of current?

It's not particularly convoluted - all you do is to observe the system from the point of view of the centre point, note that the voltages are out of phase, say "OK - that's what they mean by a 2-phase 3-wire system", and move on.

It doesn't give rise to other inconsistencies

But it does. If you're saying it's OK to call it 2 phase because you can see two voltages out of phase with each other relative to some other point, then you need to apply that consistently. Go to point B in the diagram above and use it as your reference. What will be the phase relationship of the voltages you see at points A & C, relative to B?

And what about a 3 phase system where you have only three supply lines, such as a delta configuration? How are you going to select a common reference point and see three voltages relative to that point which are all out of phase with each other? You'll only be able to see two. So do you want to call that a 2 phase system now as well? If not, why not?

 

[Tech99]

No-one has directly answered my question as to whether they are happy with the third (120°) variant of the current regs' diagrams of "Two-phase 3-wire" justifies the "Two-phase 3-wire" description.

Sorry, not having a copy I don't know what it's depicting. Could you explain what it's showing?

 

[JohnW2]

No-one has directly answered my question as to whether they are happy with the third (120°) variant of the current regs' diagrams of "Two-phase 3-wire" justifies the "Two-phase 3-wire" description.

Sorry, not having a copy I don't know what it's depicting. Could you explain what it's showing?

Untitled

• JohnW2
• 3 Jul 2012
• 1

Kind Regards, John.

 

[ban-all-sheds]

Whether it is the most reasonable terminology, or whether it made any sense to tamper with seemingly long-established terminology, is a different matter.

I agree, and I'm not sure what problem(s) it was designed to address. It may be that they were wrong to introduce the term, but that doesn't make the term itself wrong.

If (as probably sometimes happens) a premises is supplied with a neutral and two phases of a 3-phase supply, then it would seem (to me) entirely appropriate to regard those premises as having a 2-phase supply. Loads could be connected to between each of the phases and N, and the resulting currents through those loads would, I think/hope be accepted as out-of-phase.

And what if you supplied the premises with just the two phases and no neutral? Would it still be a 2-phase supply?

If one accepts that, then there is surely nothing magic about 120° phase difference. The argument would surely still apply with 130°, 150°, 179° or, unltimately, 180°, wouldn't it? ... Is there as much argument/disagreement about the inappropriateness of the terminology when the phase angle is 90° or 120° as when it is 180°?

That was a question I asked early on, but I've not seen a definitive answer, and certainly nobody has been able to point to any official definitions which exclude any particular angle(s).