Single phase, 2 phase, 3 phase etc

[PBC_1966]

However, as I said, I nevertheless personally found that original terminology confusing and illogical, but you have not yet responded to that comment.

With 3-wire single-phase you still have just a single secondary winding on the transformer, just with the addition of a center tap. No matter whether you put a balanced load across the three wires or load one side to the neutral more heavily than the other, the current through one half of the secondary is always in phase with the current flowing through the other half. Hence it's still single phase.

I've always thought of these things from the POV of an installation/user, which/who receives a 3-wire supply from a 'black box', the inner workings of which I know nothing about. On that basis, I find it just as illogical to call it "single phase" when two of the wires carry voltages which are 180° out-of-phase (wrt. the third wire)

But that will depend upon which of the three wires you select as your common reference point. If you happen to choose the middle wire from the transformer as your reference, then yes, the voltage waveforms you observe on each of the other two wires will be 180 degrees out of phase with each other. If you happen to choose one of the two outer wires as your common point of reference, then the voltage waveforms on the other two wires will be in phase with each other (just at different amplitudes).

 

[JohnW2]

With 3-wire single-phase you still have just a single secondary winding on the transformer, just with the addition of a center tap. No matter whether you put a balanced load across the three wires or load one side to the neutral more heavily than the other, the current through one half of the secondary is always in phase with the current flowing through the other half. Hence it's still single phase.

Yes - but, again, you're talking about what happens within the 'black box'. What surely matters about the supply is what the installation/user 'sees', regardless of where it comes from, and how is was generated?

I've always thought of these things from the POV of an installation/user, which/who receives a 3-wire supply from a 'black box', the inner workings of which I know nothing about. On that basis, I find it just as illogical to call it "single phase" when two of the wires carry voltages which are 180° out-of-phase (wrt. the third wire)

But that will depend upon which of the three wires you select as your common reference point. If you happen to choose the middle wire from the transformer as your reference, then yes, the voltage waveforms you observe on each of the other two wires will be 180 degrees out of phase with each other. If you happen to choose one of the two outer wires as your common point of reference, then the voltage waveforms on the other two wires will be in phase with each other (just at different amplitudes).

Of course - but the neutral will come identified, and used, as such. Could one not make corresponding comments about a 3-wire "90°" supply, or a 4-wire 3-phase ("120°") supply, if one didn't know which of the wires was the neutral?

You still haven't told me whether you regard 3-wire 90° or 120° supplies as "single-phase" or "2-phase".

Kind Regards, John

 

[PBC_1966]

Yes - but, again, you're talking about what happens within the 'black box'. What surely matters about the supply is what the installation/user 'sees', regardless of where it comes from, and how is was generated?

But it's a similar situation at the load end.

Take two load resistances, R1 & R2, and connect them in series across the two ends of the supply transformer winding. The current flowing through R1 must be in phase with that through R2, since they are in series with no other available path. I assume you would agree that this can't be anything but a single-phase supply, since all you have is two wires from source to load?

Now run a third wire from the center-tap of the transformer to the mid-point of R1 & R2. Is the current flowing through R1 now out of phase with that through R2 or is it, at any given instant of the cycle, still in the same direction and still in phase?

Of course - but the neutral will come identified, and used, as such. Could one not make corresponding comments about a 3-wire "90°" supply, or a 4-wire 3-phase ("120°") supply, if one didn't know which of the wires was the neutral?

No, because in those cases whichever wire you select as your reference, you'll see voltage waveforms on the others which are out of phase with each other with respect to your chosen reference wire.

You still haven't told me whether you regard 3-wire 90° or 120° supplies as "single-phase" or "2-phase"

They can be called 2-phase legitimately, since in either case you have currents at the generation end and currents at the load end which are not in phase with each other. You can't do that with a single-phase 3-wire system. Adding the neutral in the middle might alter the value of currents flowing through R1 & R2 (indeed it must do so if R1 & R2 are not equal), but it doesn't change the phase relationship of the currents flowing in any given part of the circuit.

For comparison with the above, assume a supply to a building of two phases plus the neutral of a standard 240/415V U.K. 3-phase system. Connect your load resistances R1 & R2 in series once again across the two phases, and obviously since it's a simple 2-wire connection the currents in R1 & R2 must be in phase. Now connect the neutral to the mid-point of R1 & R2. Have the phases of the currents through R1 & R2 now changed in relation to each other? There's the difference.

 

[JohnW2]

But it's a similar situation at the load end. ... Take two load resistances, R1 & R2, and connect them in series across the two ends of the supply transformer winding. The current flowing through R1 must be in phase with that through R2, since they are in series with no other available path. I assume you would agree that this can't be anything but a single-phase supply, since all you have is two wires from source to load?

Of course. AS I have already said, 'phase' is meaningless with a 2-wire supply. All one has is two conductors with a pd between them.

Now run a third wire from the center-tap of the transformer to the mid-point of R1 & R2. Is the current flowing through R1 now out of phase with that through R2 or is it, at any given instant of the cycle, still in the same direction and still in phase?

This is where we totally disagree, since our answers to that question will differ. Relative to the 'neutral' (centre tap), the currents through R1 and R2 are 180° out of phase. When current is travelling towards the neutral through R1, then current will be travelling away from neutral through R2. Relative to neutral, that is a phase difference of 180°.

Look at it in terms of Kirchoff's Law if you want. For simplicity, consider the situation in which R1=R2, hence no current through the 'neutral'. Per K's Law, the currents at the point where the resistors join must sum to zero. hence, if (at any point in the cycle), the current through R1 (relative to neutral) is X amps, then the current through R2 (relative to neutral) must be -X amps. If the current through R2 (relative to neutral) is always -1 times the current through R1, that is, by definition, a phase difference (relative to neutral) of 180°.

Your argument would only 'work' if one measured phases relative to something other than 'neutral' (e.g. one of the 'live' supply conductors), but I can't see much sense in doing that.

You still haven't told me whether you regard 3-wire 90° or 120° supplies as "single-phase" or "2-phase".

They can be called 2-phase legitimately, since in either case you have currents at the generation end and currents at the load end which are not in phase with each other. You can't do that with a single-phase 3-wire system.

As above, you can do that, in terms of phases relative to neutral.

Adding the neutral in the middle might alter the value of currents flowing through R1 & R2 (indeed it must do so if R1 & R2 are not equal), but it doesn't change the phase relationship of the currents flowing in any given part of the circuit.

I'm not sure what you mean by "change the phase relationships" since, as we have agreed, when there are only two wires (and no other reference), phase is meaningless. However, again as above, when one adds a neutral, the currents through R1 and R2 relative to that neutral will be 180° out of phase.

For comparison with the above, assume a supply to a building of two phases plus the neutral of a standard 240/415V U.K. 3-phase system. Connect your load resistances R1 & R2 in series once again across the two phases, and obviously since it's a simple 2-wire connection the currents in R1 & R2 must be in phase. Now connect the neutral to the mid-point of R1 & R2. Have the phases of the currents through R1 & R2 now changed in relation to each other? There's the difference.

As I've said, I see no difference.

If you look at this mathematically, the expressions one can derive are totally general. The case in which the phases are 180° degrees apart is not a special case, any more than is the case with 90°, 120° or any other supply phase difference.

Kind Regards, John

 

[PBC_1966]

This is where we totally disagree, since our answers to that question will differ. Relative to the 'neutral' (centre tap), the currents through R1 and R2 are 180° out of phase. When current is travelling towards the neutral through R1, then current will be travelling away from neutral through R2. Relative to neutral, that is a phase difference of 180°.

But that's no different than the situation which exists with the simple 2-wire connection before we added the neutral, and you've already agreed that such cannot be anything but single phase.

Go back to that basic 2-wire connection and examine the currents at the midpoint between R1 & R2. Current flowing toward that point from R1 is flowing away from that point into R2 (it couldn't do anything else, obviously, as there's nowhere else for it to go - Kirchoff's First Law at its most basic). Adding the neutral doesn't change that, it just provides an alternate path for the current to flow if R1 & R2 are unequal.

Look at it in terms of Kirchoff's Law if you want. For simplicity, consider the situation in which R1=R2, hence no current through the 'neutral'. Per K's Law, the currents at the point where the resistors join must sum to zero. hence, if (at any point in the cycle), the current through R1 (relative to neutral) is X amps, then the current through R2 (relative to neutral) must be -X amps. If the current through R2 (relative to neutral) is always -1 times the current through R1, that is, by definition, a phase difference (relative to neutral) of 180°.

Again, if you want to consider a current flowing toward any given point as X amps and current flowing away from that point as -X amps, the same applies to the basic 2-wire circuit. It has to apply to any arbitrary point you care to select in any simple series circuit.

As you say, if R1 & R2 are equal, adding the neutral changes nothing. The instantaneous value and direction of current in both R1 & R2 at any given moment are exactly the same as they would be without the neutral, so with absolutely no change in the currents, how can a single-phase system have become a 2-phase system?

Your argument would only 'work' if one measured phases relative to something other than 'neutral' (e.g. one of the 'live' supply conductors), but I can't see much sense in doing that.

I think that's where the problem lies; you seem to want to look at everything (relative voltages and currents) with respect to the neutral as soon as the latter is introduced. Why? There's nothing special about it in respect to the number of phases; it's just there to carry any imbalance current if the loads on each pole or phase are unequal. How would you regard a 3-phase delta arrangement where you don't have a neutral?

I'm not sure what you mean by "change the phase relationships" since, as we have agreed, when there are only two wires (and no other reference), phase is meaningless. However, again as above, when one adds a neutral, the currents through R1 and R2 relative to that neutral will be 180° out of phase.

I mean the phase of the current through R1 relative to the phase of the current through R2. In the simple 2-wire connection, current starting at zero in both will increase sinusoidally in both until it reaches its peak in both at the same instant, then it will decrease and pass through zero again at the same instant, reach its peak in the opposite direction at the same instant, and so on. The current through the two resistances is in phase. It can't not be (disregarding tiny amounts of stray capacitance and inductance which might introduce a tiny phase shift, but obviously we've been assuming that all along).

Adding the neutral connection in the 3-wire system doesn't alter that in any way: The instantaneous current will still be zero in both R1 & R2 at the same moment, still reach its peak at the same instant, etc. In other words, adding the neutral results in absolutely no change in the phase relationship between the currents in R1 & R2. In the balanced load situation where R1 & R2 are equal, connecting and disconnecting the neutral has absolutely no effect on anything, even the value of currents, so how can connecting it create a second phase?

If you do the same thing with 2 phases of a 3-phase wye supply, you will see a phase shift in the currents through R1 & R2 when you add the neutral connection to their mid-point.

 

[JohnW2]

Go back to that basic 2-wire connection and examine the currents at the midpoint between R1 & R2. Current flowing toward that point from R1 is flowing away from that point into R2 (it couldn't do anything else, obviously, as there's nowhere else for it to go - Kirchoff's First Law at its most basic). Adding the neutral doesn't change that, it just provides an alternate path for the current to flow if R1 & R2 are unequal.

I can't disagree with any of that, but I'm not sure of its relevance.

I suppose my point is best illustrated when (for any type of supply) loads are connected between 'phases' and neutral. For illustration, imagine that the frequency was reduced to, say, 1 Hz and that an LED (in series with an appropriate resistor!) was connected between each 'phase' and neutral, with the LEDs all pointing 'in the same direction' (e.g. relative to neutral). The LEDs would then each 'light up' once per second. If it were a (4-wire) 3-phase supply, the three LEDs would reach peak brightness 1/3 of a second (i.e. 120°) apart. If it were a (7-wire) 6-phase system, the six LEDs would reach peak brightness 1/6 of a second (60°) apart. If it were a 3-wire 90° or 120° supply (which you accept as 2-phase), the two LEDs would freach peak brightnness 1/4 of a second 990) or 1/3 second (120) apart. If it were a 3-wire system such as we are discussing, the two LEDs would reach peak brightness 1/2 second (180°) apart. That sounds like "2-phase" to me, just as the others sound like 3-phase, 6-phase and 2-phase!

... or, if you prefer, use a multi-channel oscilloscope. Connect the neutral to the common input, and each of the 'phases' to one of the the other inputs. With 3-phase you would see 3 sine waves, 120° apart from one another. With 6-phase, you would see 6 sine waves, 60° apart from one another. With 3-wire 90° or 120° supplies (which you accept as 2-phase), you would see 2 sine waves, 90° or 120° apart. With the 3-wire setup we're discussing, you would see two sine waves, 180° apart. Again that sounds like "2-phase" to me.

In other words, adding the neutral results in absolutely no change in the phase relationship between the currents in R1 & R2. In the balanced load situation where R1 & R2 are equal, connecting and disconnecting the neutral has absolutely no effect on anything, even the value of currents, so how can connecting it create a second phase? ... If you do the same thing with 2 phases of a 3-phase wye supply, you will see a phase shift in the currents through R1 & R2 when you add the neutral connection to their mid-point.

That's a bit unfair, since by adding the neutral you would be creating an unbalanced load, by connecting two of the three phases to neutral through a resistor! If you had R1=R2=R3 the phase relationship between currents in the three resistors (i.e. 120° apart) would be the same whether the resistors were connected between-phases or between each of the phases and neutral, wouldn't it?

Kind Regards, John

 

[PBC_1966]

I can't disagree with any of that, but I'm not sure of its relevance.

It's relevant because the situation is the same whether you have that third wire in the picture or not. Adding it or removing it in no way alters the phase relationship between the current flowing in R1 and that flowing in R2.

I suppose my point is best illustrated when (for any type of supply) loads are connected between 'phases' and neutral. For illustration, imagine that the frequency was reduced to, say, 1 Hz and that an LED (in series with an appropriate resistor!) was connected between each 'phase' and neutral, with the LEDs all pointing 'in the same direction' (e.g. relative to neutral).
The LEDs would then each 'light up' once per second. If it were a (4-wire) 3-phase supply, the three LEDs would reach peak brightness 1/3 of a second (i.e. 120°) apart. {.....} If it were a 3-wire system such as we are discussing, the two LEDs would reach peak brightness 1/2 second (180°) apart.

Indeed they would if connected as you describe with both anodes or both cathodes connected to the neutral, but by using LED indicators you've now introduced rectification into the picture, since an LED will light only on alternate half-cycles. Swap the connections to one on the single-phase 3-wire supply so that one LED has anode to neutral and the other cathode to neutral, and they'll be going on and off together. Just, in fact, as they would go on and off in phase if you removed the neutral entirely. Disconnect the neutral with your arrangement and they'd never light up at all. Your two phases of LED lighting are because you've introduced rectification and each LED is lighting up on its own half cycle, not because it's a 2-phase supply.

Try the same experiments with small filament lamps and what will you see? Each lamp will pulsate on and off twice per second, but with the 3-wire single-phase arrangement the two lamps will always be exactly in phase. And they'll remain in phase if you remove the neutral.

. or, if you prefer, use a multi-channel oscilloscope. Connect the neutral to the common input, and each of the 'phases' to one of the the other inputs. {.....} With the 3-wire setup we're discussing, you would see two sine waves, 180° apart.

Of course, because you're using a mid-point on the transformer winding as your reference point. If you took a typical U.K. 110V CTE building site transformer and hooked up each side of the output to the two channels of your 'scope with the common to earth you'd also see two waveforms 180 degrees out of phase. But it's not a 2-phase supply, is it?

That's a bit unfair, since by adding the neutral you would be creating an unbalanced load, by connecting two of the three phases to neutral through a resistor!

It doesn't matter whether it's a balanced load across all three phases or not. Leave the third phase disconnected or add another resistor R3 from it to neutral to balance the load, it doesn't alter the fact that with R1 & R2 simply in series across two phases the currents within them will be in phase, while when you add a neutral connection to the mid-point of R1 & R2 the currents within those two resistors will be out of phase with each other.

If you had R1=R2=R3 the phase relationship between currents in the three resistors (i.e. 120° apart) would be the same whether the resistors were connected between-phases or between each of the phases and neutral, wouldn't it?

Delta or wye configuration you'll get the 120 degree phase difference between each of the three, yes. But that doesn't change the fact with R1 & R2 across two phases of a 3-phase supply adding the neutral changes the phase relationship of the currents in R1 & R2, while when R1 & R2 are across a single-phase 3-wire supply adding the neutral connection does not.

 

[JohnW2]

Indeed they would if connected as you describe with both anodes or both cathodes connected to the neutral, but by using LED indicators you've now introduced rectification into the picture, since an LED will light only on alternate half-cycles.

Indeed - but that rectification is essential for one to be able to distinguish between pds which are 180 apart. As you say, without rectification ....

Try the same experiments with small filament lamps and what will you see? Each lamp will pulsate on and off twice per second, but with the 3-wire single-phase arrangement the two lamps will always be exactly in phase.

Quite - because a filament lamp cannot distinguish between two currents 180° degrees apart. If you feel that the rectification is 'cheating', substitute an analogue ammeter (and rapidly-acting eyes!) for the LEDs and you will still see the 180° phase difference (provided you connect the corresponding input of both meters to the neutral).

If you took a typical U.K. 110V CTE building site transformer and hooked up each side of the output to the two channels of your 'scope with the common to earth you'd also see two waveforms 180 degrees out of phase. But it's not a 2-phase supply, is it?

Not if only the two ends if the winding were made available to the user. However, if the CT were made available, with the option to connect loads between it and one of the "Ls", then, yes, that would obviously be identical to the 3-wire system we're discussing (which I, and now the IET, regard as 2-phase ).

Delta or wye configuration you'll get the 120 degree phase difference between each of the three, yes. But that doesn't change the fact with R1 & R2 across two phases of a 3-phase supply adding the neutral changes the phase relationship of the currents in R1 & R2, while when R1 & R2 are across a single-phase 3-wire supply adding the neutral connection does not.

I don't disagree with any of the things you are saying about the effects of connecting (or not connecting) neutrals, but I don't really see why that should be regarded as a means of determining how many 'phases' a supply has.

Given a black box with N wires coming out of it, one can determine 'how many phases' are present.

Kind Regards, John

 

[PBC_1966]

Indeed - but that rectification is essential for one to be able to distinguish between pds which are 180 apart. As you say, without rectification ....

O.K., so take your two LED's, wire them in parallel but "reversed" anode-to-cathode and cathode-to-anode, then connect them (with suitable series resistor) across a 2-wire single-phase 1Hz supply. You'll still see them lighting alternately, but it's not a 2-phase supply. You're just seeing the effects of rectification with one lighting on the positive half-cycles and the other on the negative.

Quite - because a filament lamp cannot distinguish between two currents 180° degrees apart. If you feel that the rectification is 'cheating', substitute an analogue ammeter (and rapidly-acting eyes!) for the LEDs and you will still see the 180° phase difference (provided you connect the corresponding input of both meters to the neutral).

And what would happen to the ammeters if you disconnected the neutral?

I don't disagree with any of the things you are saying about the effects of connecting (or not connecting) neutrals, but I don't really see why that should be regarded as a means of determining how many 'phases' a supply has.

But connecting/disconnecting the neutral in the single-phase 3-wire example doesn't change the relationship of the currents in the loads. Connecting/disconnecting the neutral when it's 2 phases of a conventional 3-phase system does. In the latter case you can get current waveforms though the two parts of the load which are out of phase; in former case you cannot. And it's current which "does the work" in providing power.

Your 1Hz supply with simple lamps might be a good way to visualize the situation. Take a 3-phase supply, or 2-phases & neutral from a 3-phase supply, and connect the lamps in all possible permutations. There is no way you will able to get all the lamps pulsating on and off in phase. Now connect your lamps to a single-phase 3-wire supply, including one live-to-live if you wish. They'll all be pulsating exactly in phase with each other.

 

[JohnW2]

O.K., so take your two LED's, wire them in parallel but "reversed" anode-to-cathode and cathode-to-anode, then connect them (with suitable series resistor) across a 2-wire single-phase 1Hz supply. You'll still see them lighting alternately, but it's not a 2-phase supply. You're just seeing the effects of rectification with one lighting on the positive half-cycles and the other on the negative.

Sure, but that's a different situation. If the rectification worries you, take my example and have pairs of red and green LEDs in parallel 'reversed' relative to one another, with 'the same end' of each pair connected to neutral. You would then see one going red when the other went green, demonstrating that the currents going through the two resistors+LED pairs were 180 out-of -phase wrt the neutral. One is merely using the rectification inherent in an LED as a means of indicating direction of current flow.

And what would happen to the ammeters if you disconnected the neutral?

Nothing, any more than it would with my LEDs or 'LED pairs'. However, as I've said, I don't see that is an indication that the supply is not '2-phase'.

As with the CTE building site transformer, any single phase supply can be regarded, and used, as a 2-phase one if one has access to a connection to a low-impedance path to a 'mid-point' of the supply.

I presume that it cannot 'just be me' given that the IET (or, at least JPEL/64) had made this recent change in their terminology!

Kind Regards, John

 

[PBC_1966]

Sure, but that's a different situation. If the rectification worries you, take my example and have pairs of red and green LEDs in parallel 'reversed' relative to one another, with 'the same end' of each pair connected to neutral. You would then see one going red when the other went green, demonstrating that the currents going through the two resistors+LED pairs were 180 out-of -phase wrt the neutral.

And again, if you disconnected the neutral, that would not change, and you acknowledge that a simple 2-wire connection cannot be anything but single phase. Or if you left the neutral connected but swapped the connections to one of the pairs of LED's you have them going red together and green together. That's just a consequence of which way the current is flowing through the LED pair as you said yourself.

Nothing, any more than it would with my LEDs or 'LED pairs'. However, as I've said, I don't see that is an indication that the supply is not '2-phase'.

But, as I understand your argument, you're saying that the fact that one ammeter is reading positive and the other negative at any given instant shows that it's a 2-phase supply. I'm pointing out that you can have exactly the same thing happening with a simple 2-wire single-phase supply if you connect one ammeter the opposite way round to the other (ditto for your LED pairs). And conversely, if you swap the connections to one over you could have both moving exactly in sync with each other with the 3-wire arrangement.

Either way, both ammeters will have corresponding peaks and nulls; it's only a consequence of which way round you connect the terminals as to whether they both move the same direction on the scale at any given moment or opposite directions.

Connect your loads wye configuration (or delta for that matter) to a 3-phase supply with ammeters in each load section, and you'll never be able to get the peaks and nulls to correspond, regardless of which way round you connect each meter.

As with the CTE building site transformer, any single phase supply can be regarded, and used, as a 2-phase one if one has access to a connection to a low-impedance path to a 'mid-point' of the supply.

I presume that it cannot 'just be me' given that the IET (or, at least JPEL/64) had made this recent change in their terminology!

I wonder why they came up with this arbitrary change though. Thinking about it now, I think I'm beginning to get some suspicions as to the real reason.

 

[Mattatooi]

This is slightly off topic but thought this would be interesting...

I work primarily as a live sound engineer with large production companies, using large PA systems, and 3 phase is standard. For audio, its generally always a 63a3ph supply. For distribution the distros we use are a little different. All control (mixing desks, radio racks, 1ph amp racks (for monitors), backline (instruments, amps) etc are ALL on the same phase (all the single phase outlets are on the same phase on the distros) and generally all the larger amps for PA are then wired evenly across all 3 phases. (most of the distros have a ton of 13/16/32a 1ph and then 4 or so 32a3ph and usually 63a1ph x 3 on each phase and also usually a 63a3ph through) So as a result you end up with a little more load on the first phase. The reason being, which I have been told, is that if there was a fault between an instrument and the sound equipment on different phases you would then have a potential difference 208v.

Many a times I have had sparks comment on this unbalanced load. Now I am not an electrician, but do find it interesting. So getting slightly back on topic, I was wondering how this would effect things in a domestic enviroment. If you have a 3phase supply, what prevents potential differences? Perhaps modern day MCBs etc prevent this, and its become just habit for some of the longer lasting companies around.

Oh one other thing, even thought its a 63a3ph supply, we are nearly never ever anywhere near it. Only thing that could use most of the load are lots and lots of subs being driven really hard! because really a sound system doesnt generate much heat! Everything is always well well below its max load with loads of headroom, because as you could imagine you don't want a show to stop !

 

[JohnW2]

But, as I understand your argument, you're saying that the fact that one ammeter is reading positive and the other negative at any given instant shows that it's a 2-phase supply. I'm pointing out that you can have exactly the same thing happening with a simple 2-wire single-phase supply if you connect one ammeter the opposite way round to the other (ditto for your LED pairs). And conversely, if you swap the connections to one over you could have both moving exactly in sync with each other with the 3-wire arrangement.

All true, but what one cannot do is swap the connections to the meter (or the load) 100 times a second!

Connect your loads wye configuration (or delta for that matter) to a 3-phase supply with ammeters in each load section, and you'll never be able to get the peaks and nulls to correspond, regardless of which way round you connect each meter.

Sure, but that's at least partially because 120° is not half of 360°, whereas 180° is. I wonder what would happen with a hypothetical "4-phase" supply?

I wonder why they came up with this arbitrary change though. Thinking about it now, I think I'm beginning to get some suspicions as to the real reason.

I can't believe that it was "arbitrary". Despite some of the the things that get into the regs, these people are far from daft, either individually or collectively. They must have realised that they were considering a change that was potentially confusing, given the 100+ years of history of the 'established terminology, so they surely must have had what they believed to be a 'good reason' for the change. I'd be interested to hear of your 'suspicions', which I guess are probably 'political' in some sense!

Kind Regards, John

 

[JohnW2]

...So as a result you end up with a little more load on the first phase. The reason being, which I have been told, is that if there was a fault between an instrument and the sound equipment on different phases you would then have a potential difference 208v.

I don't really understand that. A potential difference between what and what?

Many a times I have had sparks comment on this unbalanced load. Now I am not an electrician, but do find it interesting. So getting slightly back on topic, I was wondering how this would effect things in a domestic enviroment. If you have a 3phase supply, what prevents potential differences?

Even serious imbalances between phases won't have any appreciable effect on voltages (beyond the voltage drops one would get, anyway, even with single-phases systems). The main effect of imbalance between phases is that the neutral current increases. With perfect balance, neutral current will be zero, but with the worst case of imbalance (all load on one of the three phases) the neutral current will be the same as in the used phase.

Kind Regards, John

 

[PBC_1966]

All true, but what one cannot do is swap the connections to the meter (or the load) 100 times a second!

Now you've lost me - What does swapping the connections every half cycle (I'm assuming that's what you mean, assuming a 50Hz supply) have to do with the point I made? We were talking about your hypothetical 1Hz supply and with your suggested connections how the ammeters would peak and null at the same times, but when one is indicating positive the other would be indicating negative. All I said was that if you permanently swapped the connections at the terminals of one ammeter, you have them both reading positive and negative at the same time.

Sure, but that's at least partially because 120° is not half of 360°, whereas 180° is. I wonder what would happen with a hypothetical "4-phase" supply?

Hmmm.... That's an interesting one because if you were talking about evenly spaced currents around a phase vector diagram (i.e. 90 degrees between), I think that effectively you'd end up with a 2-phase system.

I'd be interested to hear of your 'suspicions', which I guess are probably 'political' in some sense!

Upon reflection, I wonder if it was done in an attempt somehow to try and "simplify" things for those with a good knowledge of what BS7671 says but without a real grasp of fundamental electrical principles.

The single-phase 3-wire system is not especially common within the consumer's installation in Britain. It exists in many rural areas within the local distribution network (I can think of at least a handful of examples within a few miles of where I used to live in rural England), but within the parts of systems covered by BS7671 it's relatively uncommon, since the majority of installations are either single-phase 2-wire 240V or 3-phase 4-wire 240/415V.

There's the occasional service which takes 2 phases from a normal 3-phase network, and in most cases that second phase is introduced only to distribute the load more evenly, not because there's anything which needs 415V. The same could be said for the relatively rare properties which have the full 3-wire 240/480V service (although there used to be some 480V-range motors on some old farms, but they must be getting pretty scarce now, so almost all loads are straight 240V).

In the case of an installation which takes a 3-wire 240/480V service or 2 phases from a 240/415V 3-phase network, the basic requirements with regard to switching, fusing etc. are similar - Single-pole switching must be in the live poles only, fuses must be in the live poles/phases and not in the neutral, etc. If you do happen to have a L-L load at 415 or 480V, in either case it will require double-pole fusing and double-pole isolation.

So has this been done so that the electrician doesn't actually have to think too hard about what single-phase means in relation to a 3-wire supply? Just tell him it's a 2-phase supply, since for most purposes it will have to be wired the same as if it were 2 phases from a 240/415V 3-phase supply anyway.
Of course, there are exceptions (switchgear which should be rated at more than the normal 415V one finds these days, for example), but otherwise it's very similar.

Maybe I'm way off the mark, but it's just a suggestion.