Quick question - Reg number prohibiting SP RCBOs on TT

[ban-all-sheds]

The argument does work, because you are using the loudspeaker to measure the current with respect to the mid-point, so if you deliberately connect one of your measuring instruments the wrong way round so that it inverts the reading then you can't turn round and say you've proved that the two readings are always the same.

I'd be interested to know if you, or Tech99 actually believe that a 2-phase 3-wire system with a 180° angle can actually exist.

Forget the 100 years history of split-phase, do you admit the possibility of a 2-phase 3-wire system with a 180° angle?

 

[Tech99]

If the currents are going in the same direction then why are the diaphragms moving in opposite directions?

Because that depends upon the polarity of the magnetic field generated by the coil on each half cycle, which in turn depends upon which way the current is flowing through that coil on that half cycle. And which way the current flows through the speaker coil on any half cycle can be changed by simply swapping the connections over, as I described above.

What if you connected those two speakers in parallel across a single transformer winding supplying a suitably low voltage at an appropriate frequency? No centre tap connections to worry about. You wouldn't try to argue that the source is anything but single phase, would you?

But just as with the series connection, you could have the speakers moving in-phase or anti-phase, depending upon which way round they're connected in parallel relative to each other. Connect +ve of each speaker to one side of the supply and -ve of each to the other, and they'll move in the same direction on each half cycle - The normal arrangement we would want when using multiple speakers. Swap the connections to one of the speakers, and you'll have them moving in opposite directions on each half cycle. But that's all still from a single phase source.

You still keep failing to look at the two voltages, and two currents, with respect to the common point, i.e. the mid point.

I think maybe the problem is that that's all you seem to be looking at. With no neutral current, at the xfmr mid point the current flowing towards the centre tap from one half of the winding is matched by that flowing away from it to the other half. How could it do anything else?

Select any random point along the conductor in a circuit in which current's flowing and the current must be flowing toward that point from one direction and away from it in the other. It has to. where else could it go?

No, because you are observing the currents flowing in the lamps with respect to the mid point,

Why? The absolute direction of the current through each lamp at any given moment hasn't changed just because you've added the neutral connection. And if you didn't have that third wire to the xfmt c.t. in place, the current at the c.t. would still be flowing toward it from one half of winding and away from it in the other, wouldn't it? Putting an earth on the c.t. and adding a neutral wire doesn't change that.

Yes, but when measuring the currents with respect to the mid point then they will be out of phase.

Put two 120V lamps in series and plug them into a 13A socket providing single phase power. Now measure the currents in each lamp with respect to the mid point between the lamps and you'll still see the current flowing towards that mid point from one lamp and away from it to the other. Has that somehow created a two phase system?

 

[ban-all-sheds]

In particular, if one used such a three wire supply by distributing the CT (as N) and one of the L's to various circuits in an installation, with unbalanced loads, I think it might perhaps be more logical (if one were inventing the terminoogy from scratch) to call that 2-phase.

Out of interest, if you were presented with a black box with 3 terminals, L1, N & L2, and were told it was a 2-phase 3-wire system with a 180° angle, how would you show that it was different to what used to be called split-phase?

Would you struggle to measure N -> L1 and N -> L2?

Would you struggle to know how to connect a load between L1 and L2 if it needed L1+L2V?

If you had a 2-channel scope would you deliberately connect one probe the opposite way round to the other with respect to N/Ln, and say "but look - the two Ls are in phase"?

However, what I do agree, and always have (although maybe I haven't said it) that to change terminology such as this after it's been established for decades is, at best, meddlesome.

Why?

If you think that the nature of the beast is such that, were it a new beast, it would best be called 2-phase 3-wire, then why should a term which by definition you now regard as less logical not be changed?

Is there some rule which says that if we've been calling something by an inappropriate name for more than x years we mustn't change it?

And if so what is the reason for the rule? People affected by the change becoming confused and not know what the nature of the beast is? Pandering to people who are being obdurate and just refusing to countenance changes?

 

[ban-all-sheds]

What if you connected those two speakers in parallel across a single transformer winding supplying a suitably low voltage at an appropriate frequency? No centre tap connections to worry about. You wouldn't try to argue that the source is anything but single phase, would you?

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I think maybe the problem is that that's all you seem to be looking at. With no neutral current, at the xfmr mid point the current flowing towards the centre tap from one half of the winding is matched by that flowing away from it to the other half. How could it do anything else?

Select any random point along the conductor in a circuit in which current's flowing and the current must be flowing toward that point from one direction and away from it in the other. It has to. where else could it go?

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Why? The absolute direction of the current through each lamp at any given moment hasn't changed just because you've added the neutral connection. And if you didn't have that third wire to the xfmt c.t. in place, the current at the c.t. would still be flowing toward it from one half of winding and away from it in the other, wouldn't it? Putting an earth on the c.t. and adding a neutral wire doesn't change that.

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Put two 120V lamps in series and plug them into a 13A socket providing single phase power. Now measure the currents in each lamp with respect to the mid point[/i] between the lamps and you'll still see the current flowing towards that mid point from one lamp and away from it to the other. Has that somehow created a two phase system?

It's starting to look as if, despite what you said earlier, you do not believe that a 2-phase 3-wire system with a 180° angle can actually exist.

 

[Tech99]

and whether the thousands today who will continue to refer to this arrangement as single phase are wrong.

By definition they will be, just as if they started calling it Norman.

By whose definition? Only that of the IET (maybe CENELEC etc.).

I very much doubt that the whole of North America, Japan, and other countries which use the arrangement are suddenly going to change the name. So are you saying that everyone in those places who continues to correctly refer to it as a single phase system is wrong?

It's a name change, and the point I have been trying to make is that the new name is not without merit.

What merit is there in causing potential confusion (no pun intended) by changing a long established term which everybody knows and which has been in use for over a hundred years? Especially when not everybody is going to be singing from the same hymn sheet?

No, but as we have already established, the characteristics of the load do not affect those of the supply

Hang on, haven't you just been trying to use the example of speakers (the load) to "prove" the supply characteristics by saying that they'll move in opposite directions at any given moment? (Even though that's not necessarily so anyway, since it depends which way round they're connected.)

You can make a 2-phase motor which works on a 4-wire 2-phase system with a 90° angle.

Of course. Many such motors were made in earlier days.

No. You are only seeing anti-phase voltages because you're using a mid point as your reference.

That's the whole point.

Yes, I think that is the whole point, because (correct me if I'm wrong) you seem to trying to insist that it must be a two phase system simply because the current will be moving in (two) opposite directions relative to the c.t. of the xfmr. So why isn't a simple 240V secondary with an earth on one end a 2 phase supply then? Go to the mid point of that winding and the current will be doing exactly the same thing. Putting an earth on the c.t. doesn't alter that.

The mid point is all that is different from a single-phase 2-wire system, so it makes perfect sense to recognise it's existence.

That's why it's always been known as a single-phase 3-wire system, to distinguish it from the simple 2-wire arrangement.

But then you are not referencing the current with respect to the mid-point.

Why do you think you need to? The current at any given instant is flowing in the same direction in the xfmr secondary regardless of whether the c.t. is earthed and regardless of whether there's a neutral connected to that c.t. which is being used. The magnitude of the currents might be different with a connected neutral, but the directions are exactly the same. So where has putting a c.t. earth and, optionally, adding a neutral conductor introduced any new current which wasn't there before and which is out of phase with the original current in the non-c.t. setup?

 

[JohnW2]

The argument does work, because you are using the loudspeaker to measure the current with respect to the mid-point, so if you deliberately connect one of your measuring instruments the wrong way round so that it inverts the reading then you can't turn round and say you've proved that the two readings are always the same.

You've lost me. I think/hope we agreed that if one has two speakers connected in series between the two ends of the transformer wiring, then the same current will flow through both, in phase (i.e. in the same direction at any pit in time), whether or not ther point of joining is connected gto the centre tap of the transformer.

Sure, by deciding which way around to connect the speakers, you can dictate whether in response to the same current flowing through the two speakers in series, their diaphrams move in phase, or 180 degrees out of phase, but so what? Reversing one of the speakers doesn't alter the direction in which the curent goes through the speaker relative to any external reference, it merely alters what direction the current flows through the speaker relative to the speaker's 'in' and 'out' terminals - but, again, so what?

If you had two 'speakers' connected to two sockets on the same final circuit of a standard domestic installation, but one with it's connections 'reversed' in comparison with the other, the diaphragms would move in antiphase - but you surely wouldn't then say that it was a '2-phase circuit' ... or would you?

Kind Regards, John

 

[Tech99]

The argument does work, because you are using the loudspeaker to measure the current with respect to the mid-point, so if you deliberately connect one of your measuring instruments the wrong way round so that it inverts the reading then you can't turn round and say you've proved that the two readings are always the same.

What do you mean by "the wrong way round"? I could just as easily point out that when connecting in series across the ends of the winding, ignoring the c.t. neutral, the "wrong way round" would probably be with the -ve of the speakers connected together, since that would result in anti-phase operation. So it should be

Supply ------ (+) Spkr 1 (-) ------- (+) Spkr 2 (-) ------ Supply

to get the speakers in phase (which would be the normal way you'd want them to work if actually planning to use them for sound reproduction).

So I could just as easily argue that in wanting the +ve of each speaker to the respective ends of the xfmr winding (with -ve together to the c.t.) to "prove" your 2-phase argument that you're deliberately connecting one of your measuring instruments the wrong way round.

But weren't you saying that how the load is connected is immaterial as to whether the supply is single phase or not anyway?

I'd be interested to know if you, or Tech99 actually believe that a 2-phase 3-wire system with a 180° angle can actually exist.

You can't generate a 2-phase 3-wire system with a 180° angle because it then becomes single phase. Put two secondaries in series and feed the primaries from the same 2-wire source, and you can only get single phase by definition.

 

[Tech99]

If you had a 2-channel scope would you deliberately connect one probe the opposite way round to the other with respect to N/Ln, and say "but look - the two Ls are in phase"?

The voltages at L1 & L2 are obviously out of phase with each other relative to the neutral. But that doesn't make it a two phase system. Select any point along the supply winding of the xfmr, including one which has no tap, and relative to that point the voltages at each end will be 180 out of phase. That's just the way transformers work, and a feature which has been made use of for a very long time in power systems, audio and r.f. amplification, telephone communications, etc.

The current in the xfmr winding is always flowing in the same direction, looking at the winding as a whole. And it's that single direction and phase of current which makes it a single phase system.

Take your dual-channel 'scope and (making sure it's isolated from earth), clip its common to L1 of your supply. Now put your two probes on N & L2 and what would you see? Move the common to L2, and clip the two probes onto L1 & N. What do you see?

Is there some rule which says that if we've been calling something by an inappropriate name for more than x years we mustn't change it?

The whole point is that it isn't inapproriate. Only one phase of current, thus single phase.

 

[ban-all-sheds]

OK - thanks for the confirmation that, as far as you are concerned, there simply is no such thing as a 2-phase 3-wire system with a phase angle of 180°.

There seems little point, therefore, in carrying on - it's pointless discussing whether <Name A> should be changed to <Name B> when you do not believe that something described by <Name B> can ever exist.

 

[Tech99]

Sure, by deciding which way around to connect the speakers, you can dictate whether in response to the same current flowing through the two speakers in series, their diaphrams move in phase, or 180 degrees out of phase, but so what? Reversing one of the speakers doesn't alter the direction in which the curent goes through the speaker relative to any external reference, it merely alters what direction the current flows through the speaker relative to the speaker's 'in' and 'out' terminals - but, again, so what?

If you had two 'speakers' connected to two sockets on the same final circuit of a standard domestic installation, but one with it's connections 'reversed' in comparison with the other, the diaphragms would move in antiphase - but you surely wouldn't then say that it was a '2-phase circuit' ... or would you?

Exactly. This is all down to merely which way the two wires from the devices are connected into the circuit. However they're connected up to the source, if you want your two speakers working in phase (as you would normally) and you find that they're actually operating anti-phase, you just swap the connections to one (and only one) of them.

The same happens with transformers in their many applications, e.g. 2- to 4-wire hybrids in telephone systems which employ multiple xfmrs (or did, before electronics largely replaced them). Get the connections to a winding the wrong way round, and the circuit won't work as intended. Swap the connections round so that the current at any given instant is flowing in the opposite direction, relative to some other current, and it will work.

No different than if you tried to use two separate 240V to 55V xfmrs to make your own 110V CTE site supply. If you end up with 55V from each end to the centre tap (formed at the link between the two secondaries) but next to nothing across the two ends, it just means that the phasing of one the xfmrs is wrong. Swap the two wires to any one winding (240 primary or 55V secondary) and you'll get the 110V you want.

 

[ban-all-sheds]

You've lost me. I think/hope we agreed that if one has two speakers connected in series between the two ends of the transformer wiring, then the same current will flow through both, in phase (i.e. in the same direction at any pit in time), whether or not ther point of joining is connected gto the centre tap of the transformer.

You can't say "whether or not", as that tap, and the use of it as a common reference point, is absolutely fundamental.

Sure, by deciding which way around to connect the speakers, you can dictate whether in response to the same current flowing through the two speakers in series, their diaphrams move in phase, or 180 degrees out of phase, but so what? Reversing one of the speakers doesn't alter the direction in which the curent goes through the speaker relative to any external reference, it merely alters what direction the current flows through the speaker relative to the speaker's 'in' and 'out' terminals - but, again, so what?

The "so what" is again absolutely fundamental.

Consider again the black box with 3 terminals, and instead of 2 speakers 2 DC DVMs which don't mind reverse polarity, and either superpower vision on your part or a supply frequency low enough for you to observe the readings changing.

If you wanted to establish if, how, and in what way, the potential difference between N and L1 differed from that between N and L2, would you connect the same terminal on each DVM to N, or would you connect the + on one and the - on the other?

I think essentially we need you also to answer the Q which Tech99 has - do you or do you not believe that a 2-phase 3-wire system with a phase angle of 180° can exist?

 

[Tech99]

As a matter of curiosity, what do these new regs say the colours should be on this "two phase" system? The extract (table 51) I have - which I assume is copied verbatim from when the harmonised colours were introduced - doesn't specify them. Has it been changed as well?

 

[JohnW2]

Out of interest, if you were presented with a black box with 3 terminals, L1, N & L2, and were told it was a 2-phase 3-wire system with a 180° angle, how would you show that it was different to what used to be called split-phase?

Eh? I thought the one thing on which we were all agreed (and, indeed, the whole basis of this discussion) is that there is one thing which has traditionally been called 'split phase' but which the IET now seem to want to call 2-phase 3-wire 180°. What on earth are these two things that you are postulating one might want/try to show were different?

Why? If you think that the nature of the beast is such that, were it a new beast, it would best be called 2-phase 3-wire, then why should a term which by definition you now regard as less logical not be changed?

I don't personally really give a jot what people want to call it, given that there is no ambiguity or safety issue. To the best of my knowledge, there is only one thing which 'split phase' can mean, and only one thing which 2-phase 3-wire 180° can mean - just as, in the world in general, there are often two or more different words or phrases that can be used to refer to the same object or concept.

Kind REgards, John.

 

[JohnW2]

I think essentially we need you also to answer the Q which Tech99 has - do you or do you not believe that a 2-phase 3-wire system with a phase angle of 180° can exist?

This is getting very very silly. Of course it can (and does) exist - since, as we all know, it's a collection of words which the IET is now using to describe something which has been around for decades. Whether the thing which has been around for decades should be refered to by that collection of words is a different matter - but the existence of the thing to which the IET are referring is not in dispute.

... and that's more than enough of this discussion for me.

Kind Regards, John.

 

[Tech99]

You can't say "whether or not", as that tap, and the use of it as a common reference point, is absolutely fundamental.

Why do you want to use that c.t. as your common reference point? What's so special about that point?