Now BS7671 says that 1-phase 3-wire is a 2-phase system

[JohnW2]

Does merely swapping the two connections to a simple 2-wire load constitute a change of phase?

IMO, Yes.

Consider two resistive loads each connected (2-wire) to their own separate a.c. generators (identical frequency), with one side of each generator connected together to provide a reference point. Imagine also that you had a control which enabled you to vary the phase angle between the two generators. If that phase angle was 45, 60, 90 or 120 degrees, I presume that you would have no problem in agreeing that the supplies to the loads were out-of-phase? Hopefully, you'd even say the same if the phase difference was 179 or 181 degrees. If so, what's so magic about 180 degrees which makes it an exception?

... 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.
....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.

I agree that, in both cases, there would be a bang regardless of whether or not there was a neutral. The point is that, if the supplies to adjacent properties were truly in-phase (a.c.) or of the same polarity (d.c.) then, in a perfect siutuation, there would be no bang when they were connected. The significance of the neutral is, of course, that it enables consumers to be provided with 240v supplies from a 480v winding or battery, some consumers having a supply which is out-of-phase (a.c.) or of opposite polarity (d.c.) to others.

Kind Regards, John.

 

[JohnW2]

So a site transformer creates a 2-phase supply?

Are you talking about a transformer which has a 230v input and provides two 110v (or whatever) outputs?

If so, then the answer depends entirely on how the windings were arranged/connected.

If the transformer had a single centre-tapped secondary providing 110-0-110 (and hence the neutrals of both outputs connected to the centre tap for 110v supplies), then, yes, I would call it a 2-phase supply. As I've said to Paul, if you have doubts, don your PPE and touch the two Ls together (touchiung together the L's of two supplies of identical voltage, frequency and phase does not require PPE!)

Kind Regards, John.

 

[Paul_C]

If the transformer had a single centre-tapped secondary providing 110-0-110 (and hence the neutrals of both outputs connected to the centre tap for 110v supplies), then, yes, I would call it a 2-phase supply.

I think BAS was referring to the CTE 110V transformers used on U.K. building sites - A 110V secondary winding with its center tap earthed. In this case you don't have a neutral, since all the loads are 2-wire 110V. Would you want to call that a 2-phase system?

As I've said to Paul, if you have doubts, don your PPE and touch the two Ls together

And if you short the two live 55V poles of that 110V CTE transformer, it will go bang as well, but purely because you're putting a short across the whole 110V secondary winding. The fact that the midpoint of the winding is earthed plays no part in that short resulting in a high current flowing.

 

[JohnW2]

I think BAS was referring to the CTE 110V transformers used on U.K. building sites - A 110V secondary winding with its center tap earthed. In this case you don't have a neutral, since all the loads are 2-wire 110V. Would you want to call that a 2-phase system?

No, of course not. Two-phase (or n-phase) supplies obviously require more than two wires, with phase differences between the Ls. If the transformer has only one output (i.e. a 2-wire system), then it obviously cannot be more than one phase, even if multiple loads are connected in parallel to that one output.

I remind you that what we are talking about is a 3-wire system. If you connected a common ('neutral') to that centre tap and then used that and the two ends of the winding to provide a 3-wire 55v supply (and then used that two provide two single-phase supplies to end-users), then I would call that 2-phase.

As I've been trying to explain, if and where earth is connected to the system is irrelevant.

And if you short the two live 55V poles of that 110V CTE transformer, it will go bang as well, but purely because you're putting a short across the whole 110V secondary winding.

Of course, but then you're just shorting the two sides of a 2-wire supply, not connecting the two Ls of a 3-wire system which (for single phase loads) has one 'common' and two Ls.

The fact that the midpoint of the winding is earthed plays no part in that short resulting in a high current flowing.

Exactly. As above, if and where earth is connected to the system is totally irrelevant.

Kind Regards, John.

 

[ban-all-sheds]

I remind you that what we are talking about is a 3-wire system. If you connected a common ('neutral') to that centre tap and then used that and the two ends of the winding to provide a 3-wire 55v supply (and then used that two provide two single-phase supplies to end-users), then I would call that 2-phase.

So the output of the site transformer changes from single phase to two phase if you use it to supply 55V loads?

 

[JohnW2]

So the output of the site transformer changes from single phase to two phase if you use it to supply 55V loads?

If you use the centre tap to provide a common for two single-phase 55V supplies which are out-of-phase, I'd call that a 2-phase set-up, wouldn't you?

I think that one problem/confusion with this discussion is that a 2-phase 180 degree supply is of no use as a polyphase supply, so the only context in which it's relevant is if it's used to provide single phase supplies to end-users - and if those single-phase supplies are out-of-phase, I find it totally logical to call the 3-wires supplying them a 2-phase supply. However, although BS7671 now apparently agrees with me, it seems that maybe not many other people do!

I have a 4-wire 3-phase supply to my house (separating into three separate single-phase installations immediately after the meter). If the house were a bit smaller, I guess there's no theoretical reason why the house could not have been supplied with just two of the phases (120 degrees apart) via 3 wires, with a neighbouring smaller house getting the third phase. Would you want to call that hypothetical set-up 'single phase'?

Kind Regards, John.

 

[ban-all-sheds]

If you use the centre tap to provide a common for two single-phase 55V supplies which are out-of-phase, I'd call that a 2-phase set-up, wouldn't you?

So the nature of the supply from a site transformer changes according to what loads I connect to it?

What if I have both 110V and 55V loads connected to it at the same time? What is it then?

 

[JohnW2]

So the nature of the supply from a site transformer changes according to what loads I connect to it?

Not really - it depends upon how the transformer is wired to its output terminals/sockets/whatever - regardless of what, if any, load(s) are connected.

What if I have both 110V and 55V loads connected to it at the same time? What is it then?

What if I had both 230v (single-phase) and 400v (3-phase) loads connected to the supply to my house. Would it then cease to be a 3-phase supply that my house was supplied with?

Kind Regards, John.

 

[Paul_C]

Does merely swapping the two connections to a simple 2-wire load constitute a change of phase?

IMO, Yes.

Let me rephrase that slightly: Does it constitute a change of phase with regards to overall operation of the system? The instantaneous current though that load on any given half cycle will be the opposite direction than it would have been if it were connected the other way round, but that instantaneous current is still flowing in the same direction as far the whole system is concerned. (And, of course, as we're talking about a.c. that 2-wire load won't know anything is different anyway since the current changes direction for each half cycle.)

If so, what's so magic about 180 degrees which makes it an exception?

Maybe examining the example of a house fed with 2 phases from a 3-phase 4-wire wye system will help with regard to the point about you seeing voltages being out of phase only because you're using the winding midpoint as your reference.

So 2 phases plus neutral into a house from a regular 3-phase 240/415V system. If you use N as your common reference point, then the voltage waveforms you see on L1 and L2 will not be in phase with each other. Now use L1 as your common reference point, and compare the voltage waveforms at L2 and N. Again, you will see a phase difference. The same happens if you use L2 as your common reference point to compare the voltage waveforms on L1 and N.

Now do the same with a 240/480V 3-wire supply. You will see those phase differences only when you use N as your common reference point, because it's connected to the center point of the winding which is feeding L1 and L2. If you use L1 as your reference, the voltage waveforms appearing at L2 and N will be in phase with each other. Similarly, if you select L2 as your reference point, the voltages at L1 and N will be in phase. There's the difference.

 

[Paul_C]

http://en.wikipedia.org/wiki/High-leg_delta

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!

That's not specific to the high-leg delta arrangement. Any delta supply can be arranged as an open delta with just two single-phase transformers if required, although it has certain drawbacks. It might be particularly relevant to certain applications of the high-leg delta supply though, given a situation where a small commercial installation needs relatively high amounts of power for single-phase 120 & 240V loads but only a much smaller amount for 3-phase motors.

These two statements in that article are also completely wrong:

Since one phase-to-neutral voltage (phase 'B') is higher than the others, no single phase loads can be connected to this phase. This eliminates the ability to use one third of the breakers in a panel for single-phase loads.

Connecting a single-phase 208V load between the high leg and neutral is quite possible, although not particularly common. But single-phase 240V loads can, and often are, connected between any two phases with a D.P. circuit breaker. If the supply is from a full delta bank (and not the cut-down open delta arrangement), then using the high leg for single-phase 240V circuits can be a way to achieve better load balance against the 120V loads which can be connected only to A & C phases.

 

[JohnW2]

Does merely swapping the two connections to a simple 2-wire load constitute a change of phase?

IMO, Yes.

Let me rephrase that slightly: Does it constitute a change of phase with regards to overall operation of the system? The instantaneous current though that load on any given half cycle will be the opposite direction than it would have been if it were connected the other way round, but that instantaneous current is still flowing in the same direction as far the whole system is concerned.

The real problem here is that one obviously cannot talk about phase in relation to a single 2-wire system. Phase is, by definition, relative to some reference waveform and one just doesn't have that with a single 2-wire set-up. That's why I've always been talking about the phase difference between two 'commoned' 2-wire systems (i.e. a 3-wire system).

Maybe one of the issues here that I have never been anywhere other than on the consumer side of an electrical supply. As such, I see 'the supply' as a number of conductors (usually 2 or 4) entering a property. Furthermore, since I've never experienced an environment (if one exists!) in which all of the loads are 3-phase, I also think of 'the supply' (enetring the building) as having a 'common' ('neutral') and one or more lives/lines - hence, when there is more than one L, my view of the type of supply is based on the phase relationships of the potentials (relative to N) of the two or more Ls. In contrast, you seem to be thinking more in terms of what is happy on the supply side of those cables entering a property.

However, its not just semantic - there is a major safety issue here, too. With the 240/480v 3-wire supply you're talking about, if one property were served by all 3 wires, then there would be potential differences of 480v RMS (around 680V peak) between some parts of the installation, because of the 180 degree phase difference (whether you want to call it that or not!).

Now do the same with a 240/480V 3-wire supply. You will see those phase differences only when you use N as your common reference point, because it's connected to the center point of the winding which is feeding L1 and L2. If you use L1 as your reference, the voltage waveforms appearing at L2 and N will be in phase with each other. Similarly, if you select L2 as your reference point, the voltages at L1 and N will be in phase. There's the difference.

No argument with any of that, but the question which occurs to me is 'so what?'. You obviously feel that this means that it is not a 2-phase supply. However, as I've said before, all one can really do with such a supply is use to to provide two single-phase 2-wire 240v supplies (to the same or different properties) - supplies which are refernced to the common N and which are obviously in antiphase.

We're not disagreeing about anything other than the terminology and, as above, I think the main difference is that you are thinking 'supply side' whereas I am thinking 'consumer side'. To the consumer who is provided with 3 wires, or even possibly to a consumer who shares 3 wires with an adjacent consumer, what really matters is whether 2-wire supplies derived from pairs of those 3 wires are in phase or not, and hence whether the pd between them is around zero or unusually high (in the case we're discussing, even higher than exists within a standard 3-phase installation).

Kind Regards, John

 

[Paul_C]

The real problem here is that one obviously cannot talk about phase in relation to a single 2-wire system. Phase is, by definition, relative to some reference waveform and one just doesn't have that with a single 2-wire set-up.

Agreed, hence my comments about swapping the connections around to a 2-wire load - It might mean that the instantaneous current is flowing through the load in the opposite direction on any given half cycle from the supply, but that current is still going the same direction in the overall system. And as it's a.c., that 2-wire load sees no difference with the reversal anyway. (There might be issues with which of its connections is earthed, if either, in relation to fuses, switches, etc., but that's a side issue.)

However, its not just semantic - there is a major safety issue here, too. With the 240/480v 3-wire supply you're talking about, if one property were served by all 3 wires, then there would be potential differences of 480v RMS (around 680V peak) between some parts of the installation, because of the 180 degree phase difference (whether you want to call it that or not!).

No argument on that point. This is one area where the Wiring Regs. have become rather less strict over time. Go back to the 14th edition and you'll find rules which were very much aligned with minimizing any risk from this (both for 3-wire a.c./d.c. systems and 3-phase systems):

A.20 All socket-outlets in any one room shall be connected to the same phase (or pole of a 3-wire system).

Exemption.- In non-domestic premises, if it is clearly impractical to comply with Regulation A.20, more than one phase (or pole) of the supply may be utilized provided that all socket-outlets on one phase (or pole) are grouped together and are not intermingled with socket-outlets connected to a different phase (or pole); and provided that in no circumstances may a socket-outlet be installed at a distance less than 6 feet from any socket-outlet connected to a different phase (or pole).

 

[JohnW2]

No argument on that point. This is one area where the Wiring Regs. have become rather less strict over time. Go back to the 14th edition and you'll find rules which were very much aligned with minimizing any risk from this (both for 3-wire a.c./d.c. systems and 3-phase systems):

A.20 All socket-outlets in any one room shall be connected to the same phase (or pole of a 3-wire system).
Exemption.- In non-domestic premises, if it is clearly impractical to comply with Regulation A.20, .....

Indeed. That was one of the first issues which I addressed when I moved into my current house over 20 years ago (that's pre-Part P, BAS, so I didn't tell the LABC anything ). The three phases were 'roughly' split between three floors - but with all sorts of 'exceptions' that had probably resulted from evolution of the installation over time - e.g.'just adding a socket' on a drop from a different phase which was easily accessed from under floorboards above. There was even a cooker supply on a different phase from most of the rest of the ground floor, and more than one case of nearly adjacent sockets on different phases.

I sorted all that at a very early stage, so that it has hence been almost strictly 'one phase per floor', with just a tiny number of necessary exceptions. Indeed, I've now done away with almost all of those 'necessary exceptions' (e.g. two-way multifloor light switching, cross-floor switching of immersion heaters etc.) - for example by using ELV and relays where 'cross-phase switching' is required.

Kind Regards, John.

 

[Paul_C]

Maybe someone can confirm, but I think I recall reading in an article somewhere that the 17th edition also relaxed the requirement for warning labels where voltages over 250V are involved somewhat as compared to earlier editions.

 

[JohnW2]

Maybe someone can confirm, but I think I recall reading in an article somewhere that the 17th edition also relaxed the requirement for warning labels where voltages over 250V are involved somewhat as compared to earlier editions.

I can't answer the historical question, but a couple of observations:

514.10.1 of both the BRB and BGB (BS7671:2008 and BS7671:2008(2011) respectively) say that a warning label should be used when an enclosure contains a nominal voltage greater than 230V "where such a voltage would not normally be expected". Those emboldened words clearly are open to interpretation, and maybe were not present in earlier editions. There are also two differences in 514.10.1 between BRB and BGB:

• 1...In the BGB, "230 volts" has been changed to "230 volts to earth"
  
  2...To my surprise, the second and third paragraphs of 514.10.1 have disappeared in the BGB. This means that there is no longer a need for warning signs when 'nearby' (can be simultaneously reached) separate enclosures contain voltages >230v relate to one another - and nor is there any longer a need for voltage labelling when different nominal voltages exist in 'switchgear and other fixed live parts' (whatever that means!).

So, whether or not there have been relaxations in the past - we seem to be experiencing some in 2011.

Kind Regards, John.