Ah that's called getting info from our super digital records system that was set up by a company in Asia. It is not quite correct, nor does it in places reflect what was on our original system!
Ah that's called getting info from our super digital records system that was set up by a company in Asia. It is not quite correct, nor does it in places reflect what was on our original system!
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And I thought Unseen University (UU) was a mythical invention of Terry Pratchett. Next I will find there is really a Disc World!
Exactly - and that's Fig 3.3 in the BGB. In fact, it's the first of three variants in that Figure - "Two-phase 3-wire (180 degrees)". The other two variants are "Two phase 3-wire (90 degrees)" and "Two phase 3-wire (120 degrees)".
Agreed.
Again, agreed. As you might imagine, 312.1.1 of the BGB does not have this arrangement.
I'm a bit lost here. Moving the reference point (neutral/earth) from one end to center surely does change the situation from one-phase to two-phase. With the reference point at the end, the two L's are in-phase, relative to the reference; with the reference in the centre, the two L's are 180 degrees out of phase relative to the reference. I don't understand why you are suggesting otherwise.
I agree, but that's Fig. 3.2 in BGB ("Single-phase 3-wire (0 degrees)"). To be fair to the BGB, 312.1.1 is not necessarily saying that all of these arrangements are actually used - it merely indicates what they would be called if they were used.
Kind Regards, John.
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P
Paul_C
And that bears comparison with the corner-grounded delta arrangement which is also still found in North America. Both are a basic 240V delta system and both are 3-phase systems. The variously named high-leg/wild-leg/red-leg/bastard-leg arrangement just moves the ground to a midpoint on one of the windings and extends that point to the supplied installation so that 120V loads can be connected - Effectively that same "split phase" arrangement but applied to one side of a 3-phase delta system.
And while looking at North American systems, remember that the 1-phase 3-wire arrangement is the norm for residential and light commercial supplies throughout the country, just at 120/240V instead of the 240/480V found in Britain.
I don't know if the change in BS7671 is entirely home-grown or has been prompted by some "harmonized"/CENELEC/"Euro-Norm" idea which will be mirrored in other European standards, but either way, it's going to create a conflict with the rest of the world which is going to continue to refer to single-phase 3-wire systems. More scope for confusion.
I'm not suggesting otherwise, since clearly the voltages at the two ends of the winding are 180 degrees out of phase with each other as referenced to the center tap. What I'm saying is that that fact by itself does not make it a 2-phase system.
Consider the relative phases of the currents flowing in each half of the winding with various 240 & 480V loads applied. They're in phase with each other (assuming purely resistive loads to keep things simple, of course). The phase relationship of those currents will be exactly the same whether the earth is at the midpoint, at one end of the winding to give a 240V live and a 480V live as described above, or if there was no earth on the system at all.
Indeed, and it's presumably worse than that - since, as I've been explaining, BS761 now does 'define' a "single-phase 3-wire system" - but with a meaning different from that which you are talking about.
That's what I don't get, assuming that the centre tap was connected to the third supply wire ('neutral'). Relative to that 'neutral', the two lines (180 degrees) out-of-phase, and I have to say that makes it sound like a 2-phase system to me.
I suppose one test is to see what happens if one touches together L1 and L2. With the "one-phase 3-wire system" described in BS7671, nothing (or little) would happen. With what you're talking about, there would presumably be a big bang! Another test is what is happening to current in the 'neutral' - with one-phase, one would expect the currents from the two 'circuits' to be additive; with two-phase one would expect them to be (at least partially) 'subtracttive'.
Only with respect to one end of the whole winding. As agreed, relative to the centre point, they're 180 degrees out of phase.
As above, I don't see this. You keep talking about 'earth', but it's really the 'neutral' (third wire of the 3-wire system) that matters - and if that's connected to the midpoint, it still looks to me (in my 'innocence'!!) as if you've got a 2-phase system.
Kind Regards,John.
P
Paul_C
At risk of creating a circular argument, not with the conventional 1-phase 3-wire arrangement. Tapping a line from the midpoint of the winding is what makes that line a neutral* conductor, with the result that the voltages at opposite ends of the winding are out of phase with respect to that common neutral point, but with the currents through each part of the winding still being in phase.
One could say that about any point along any winding though. If you had just a simple 240V secondary with no taps, at any random point you care to select along that winding the current will be flowing toward that point from one direction and away from it in the other - Mr. Kirchhoff's First Law at its most basic. The current is flowing in the same direction at any point you select on that simple winding. Putting a center tap on that winding doesn't alter that - The magnitude of the two currents in each half of winding will be different if any neutral current is flowing, but the current is still flowing in the same direction (and in phase) at any point on that winding you care to select, either side of the midpoint.
* I was referring to earth as the reference because I was trying to use the term neutral for its true meaning, as the conductor of the system which carries any imbalance current. Although common usage talks about a neutral even when it's just the earthed conductor of a simple 2-wire system, and although that rather misleading application of the term has been endorsed by the Wiring Regs. for many decades, such a conductor is not a neutral in the true electrical sense of the term.
In that new BS7671 "1-phase 3-wire" system you described, the earthed conductor which is common to both live feeds would indeed be carrying the sum of the individual currents, and is thus not a neutral, whether earthed or not.
With the conventional 3-wire system, it's by being connected to the center-point of the winding that one of those conductors becomes the neutral, since it carries the imbalance current from the two live poles. Where an earth connection is made to the circuit isn't going to change that, nor will it change the direction of currents relative to each other in the winding.
That presumably is the crux of this confusion - you appear to be majoring on the currents in the transformer winding. As far as the 'supply' is concerned, what surely matters is the voltages provided to the consumers- if those voltages are out-of-phase (hence the consumers' currents also out of phase), then I would call that a two-phase supply, regardless of what was going on within the transformer.
That may also be causing some of the confusion. Whilst you might be strictly true, common usage (and BS7671) uses 'neutral' to refer to the conductor providing a return path for consumers' current, whether the system is earthed anywhere or not. In the present context, the 'third wire' of a 3-wire system is therefore generally called 'neutral'. by most people and also by BS671 - even when (as in their Fig 3.2) it is carrying the sum of in-phase return currents relating to both the live feeds.
No argument with that. Your only problem is that BS7671 calls that "two-phase 3-wire". I personally would have thought that the very fact that the (out-of phase) consumer currents were largely 'cancelling', such that the neutral would only carry the imbalance current would confirm that it was a two-phase supply. Indeed, I would even suggest that one way of defining a multi-phase supply (as compared with a single-phase one) would be that it resulted in only imbalance currents flowing in the neutral conductor.
Where is everyone else? Am I alone in finding it difficult gto understand your argument (even if it is enshrined in longstanding convention)?
Kind Regards, John.
I have always thought that a single phase supply was incapable of itself creating a rotating magnetic field, whereas a 2-phase 90° or 3-phase 120° could. Indeed, to make a single phase motor start, it's necessary to create a second phase with a capacitor or a lagging magnetic field with a short circuit.
Of course there is logic in calling what was a single phase 3-wire system 2-phase, the problem arises because of a change from the past and ambiguity. All could be resolved if, in future, reference was made to the angular phase difference between phases.
Of course there is logic in calling what was a single phase 3-wire system 2-phase, the problem arises because of a change from the past and ambiguity. All could be resolved if, in future, reference was made to the angular phase difference between phases.
A curious advantage claimed by that article for the high legover system is that it requires two transformers instead of the three required by a normal three phase system!
I'm sure that's correct. I'm less certain about 2-phase 180° (Paul's "1-phase 3-wire") - but, at the least, I'm pretty sure that it couldn't create a magnetic field which rotated in a predictable direction.
I don't actually think anyone (and certainly not Paul, who regards it as single-phase) is suggesting that a 3-wire 180° supply (whether one calls it single-phase or 2-phase) would be usable as a polyphase supply. However, that's not a problem. Even straightforward 3-phase 120° supplies are, of course, very commonly used to provide single-phase supplies to domestic installations.
It will have become clear that such is my belief - and, conversely, I see very little logic in calling a 3-wire 180° supply a 'single-phase' one, even if that has historically been the case. As I see it, the source of the supply (be it a transformer, generator, inverter or whatever) is simply a 'black box' with two line and one 'neutral' (common) conductors emerging from it - and if the two line conductors bear voltages which are out-of-phase (relative to common), then it seems to naive me that it's a two-phase system. Paul's argument for it being single-phase is based on what happens withing the 'black box', and I personally don't see why that should affect the way in which the nature of the supply is viewed.
So it seems, and it appears (to me) that 'the past' has been a bit illogical - but maybe (as so often!) that's just me! However, as you say, this apparent change in terminology could be a big problem.
To be fair, that's almost exactly what the BGB does - as I've said, its Fig. 3.3 depicts 3 variants of "two-phase 3-wire" - 'subtitled' 180°, 90° and 120°. If people describe the arrangement we've been discussing as "two-phase 3-wire 180°", I would have thought that their meaning should be totally non-ambiguous, even if the arrangement used to be called 'single-phase'.
Kind Regards, John.
P
Paul_C
The voltage and current relationships at the load end of the system mirror those at the generating end though. The voltages on each live pole are 180 degrees out of phase with each other as referenced to the neutral, but the direction and phase relationship of the currents through each side of the load are no different than they would be without the neutral being connected, so they aren't actually out of phase with each other.
Start by visualizing a simple 480V transformer winding with just a 2-wire connection to an installation, at which two 240V 60W lamps are connected in series across the supply. Obviously the currents through each lamp must be identical and must be in phase.
Now add the neutral conductor between the transformer winding's center tap and the mid point between the two lamps. You know that if the loads are perfectly balanced on each side of the supply there will be no neutral current. The instantaneous direction, phase, and magnitude of the currents through each lamp will not change in any way.
If you now connect a second 60W lamp in parallel with one of the existing lamps, current will flow in the neutral due to the imbalance. The magnitude of the current in one outer of the supply compared to the other will now be different, but the instantaneous direction and phase relationship between them will be no different than it was when you had just the 2-wire connection at 480V.
Well, BS7671 has now decreed that to be so, but that's the whole point of the argument - Up until now BS7671 has correctly recognized it as a single-phase system, as did the Wiring Regs. for decades before they became BS7671.
One problem with that definition is that there are multi-phase systems which don't have a neutral, such as 3-phase delta or the 2-phase 4-wire system mentioned earlier.
Also, consider the old 3-wire d.c. distribution systems which also had a neutral. Being d.c., you couldn't call those multi-phase, could you? The single-phase 3-wire system under debate here is just the a.c. version of the same basic arrangement.
Hmmm. There is obviously no such thing as absolute voltage, so when we talk of 'voltage' (and hence phase) we are referring to a potential difference relative to some reference point. ....
I agree that adding the neutral changes nothing, but whether the currents (and voltages) are in phase (with or without an N connected) again depends upon what one uses as the reference.
Imagine that, as is very commonly the case, the loads have N and L markings on their input terminals. Particularly given that there is usually an expectation that N will be close to earth potential, it then becomes conventional to measure voltages relative to N. If you connect the two loads in series across the 480 supply with the L terminal of one connected to the N terminal of the other, then I agree that conventionally measured currents will be in phase in the two loads. However, if, as would be the case in the arrangement we're actually talking about, one connected the two N terminals together, then, relative to that reference point, the currents in the two loads would be 180 degrees out of phase.
The fact that, with most systems, N is close to earth potential makes this particularly relevant, since will will then have an interest (safety-wise, for example) to talk in terms of voltages relative to earth - and that is what really matters to a consumer getting a 2-wire 240v feed from the system we're discussing. As I've said before, consider two adjacent properties with 2-wire feeds, their L's coming from different sides of the transformer. Now connect a 'widow-maker' between sockets in their two houses and it will become apparent (at least to me!) that the two supplies are out-of phase! If they were truly in-phase, in a 'perfect' situation plugging in the widow-maker would have no effect.
It might be easier to think about this in terms of a DC supply - with N connected to the joining point of two batteries in series. It is then very clear that, realtive to the 'common'('N') conductor, one L is providing a negative supply and the other a positive supply. Again, connect the L of a 'postive' property with the L of a 'negative' one and you'll get a bang!
Indeed. I've agreed several times that the apparent change could result in some serious confusions and misunderstandings - although, as has been said (and as the BGB does) one always includes the phase angle in the description, any ambiguity actually vanishes.
See above. One obviously wouldn't normally use the word 'phase', but would talk of polarity. However, if you're talking of the direct analogy of the a.c. system we're discussing then, yes, as above, I would call it a 'two polarity' (c.f. 2-phase) system - since, relative the their 'common' (which would normally be more-or-less at earth potential), one L would be providing a positive supply and the other a negative supply.
Please note that I'm going to be a bit out of circulation for the next day or two.
Kind Regards, John.
P
Paul_C
If you're comparing the "L-N" instantaneous flow through each 240V load, then exactly the same could be said for the two loads connected in series across a 2-wire 480V supply as in the first part of my two-lamp example above. In fact the same could be said for the two loads in parallel across a simple 2-wire 240V supply if you happen to swap the connections to one of them. Does merely swapping the two connections to a simple 2-wire load constitute a change of phase?
Again, it depends upon one's reference point. Relative to the neutral, and to earth assuming that the neutral is earthed, then clearly the voltages appearing in each house are 180 degrees out of phase. But ignoring any small voltage difference between the neutral at each end of that cord, the neutral doesn't play any part in the bang which will result when you switch on. That's caused by the fact that you've just applied a short of very low resistance directly across the 480V secondary of the transformer.
Certainly, but again the neutral would not play any major part in that bang. It would happen if the neutral were not even connected, since you're shorting out the whole battery across the two outers.
But the comparison with two 240V loads across a 480V supply, then adding a neutral, then changing the loads, and none of that affecting the direction of the currents through each part of the load is similar. The only difference with the a.c. version is that those currents are reversing 100 times per second (for a 50Hz supply, of course).
Not due to experimenting with connections between opposite poles in adjacent houses, I hope!
So a site transformer creates a 2-phase supply?
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