Quick question - Reg number prohibiting SP RCBOs on TT

[ricicle]

. I (and the BGB) believe that a supply derived from a centre-tapped transformer is 2-phase, whereas Tech99 (and plenty of others in the past) appear to regard it as a single-phase supply.

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It must be single phase as there is only one current.....

 

[ban-all-sheds]

//www.diynot.com/forums/viewtopic.php?t=283088&start=0

 

[Tech99]

The 110V site supplies in the UK can be derived from either a centre-tap earth 110V transformer, in which case it's a single-phase supply, or from a 3-phase secondary with star-point earth, in which case the supply is obviously three phase.

That's the point we've debated before, and opinions clearly vary. As I said, I (and the current regs) would call the former a two-phase supply.

Since when? All the editions of the IEE regs as far back as the year dot recognise it as a single phase supply (like the 3-wire 200/400 - 250/500V distribution systems found in rural areas). Is this some recent change? If so, then it's quite simply wrong.

Indeed, to those of you who do not agree with that, if you don't regard that arrangement as two-phase, what would you regard as two phase?

The old 2-phase 4-wire system - True two phase with each phase delivered over a separate pair of wires with a 90º phase angle between them.

I suppose that if you ran 2P+N from a three phase supply you could argue that the circuit is two phase as well, even though the supply itself is three phase.

What makes you feel that the centre tap of such a supply differs conceptually from the star point of a 3-phase one?

It's the centre-tap of a single winding. The 240V primary winding has only two connections and a single current running through it.

The star point of a three-phase wye secondary is different entirely - There are three separate secondary windings, each having current induced in it by a separate primary in which the currents are out of phase.

Yes, as I almost got as far as conceding in my previous message, I think I have to agree with that. Indeed, as you say, the same would probably be true if one took a 2-wire 400V circuit from a 3-phase supply. Whether that is necessary sensible is another matter - particularly in the latter case (3-phase supply), whilst it wouldn't normally matter, there are times when it would be desirable to know which of the two phases were present in the 2-wire circuit.

As I read the colour table, then yes, if you run a 2-wire (plus earth) 415V circuit from two phases of a 3-phase supply then both lines should be brown. No different in principle from running a 2-wire 110V circuit from a three phase site transformer which provides 110V between phases from a wye secondary.

 

[Tech99]

It must be single phase as there is only one current.....

Exactly. A 110V CTE site transformer is supplying only one current to the load. The fact that there's an earth on the centre tap of the winding doesn't change that. You could move the earth from the centre tap to one end of the winding and it would make no difference to the current, only in the relative voltage of each leg to earth.

 

[Risteard]

I would regard the non three-phase variant of the Reduced Low Voltage system to be a split phase supply (i.e. single phase).

 

[Tech99]

I would regard the non three-phase variant of the Reduced Low Voltage system to be a split phase supply (i.e. single phase).

Yes - It's the same arrangement as the split-phase distribution arrangement which can still be found in rural areas, just at about a quarter of the voltage and without the neutral being distributed.

But I see from that other topic that the IEE has now decided this should be called "two phase" even though it isn't. Daft.

 

[Risteard]

But I see from that other topic that the IEE has now decided this should be called "two phase" even though it isn't. Daft.

It's a somewhat illogical development alright.

 

[ban-all-sheds]

There will be 2 AC voltages present, and they will not be in phase.

 

[JohnW2]

The 110V site supplies in the UK can be derived from either a centre-tap earth 110V transformer, in which case it's a single-phase supply, or from a 3-phase secondary with star-point earth, in which case the supply is obviously three phase.

That's the point we've debated before, and opinions clearly vary. As I said, I (and the current regs) would call the former a two-phase supply.

Since when? All the editions of the IEE regs as far back as the year dot recognise it as a single phase supply (like the 3-wire 200/400 - 250/500V distribution systems found in rural areas). Is this some recent change? If so, then it's quite simply wrong.

Since the BGB was published. As I said before, look at the left-hand diagram in Fig. 3.3 ("Two-phase 3-wire") of 312.1.1 in the BGB. The only way one can get "Single phase 3-wire" is to have two separate secondaries wired in-phase (rather than centre-tapped) - see Fig. 3.2, immediately above Fig.3.3.

I suppose that if you ran 2P+N from a three phase supply you could argue that the circuit is two phase as well, even though the supply itself is three phase.

Maybe that's where the confusion is coming from. A 2-phase supply (or circuit) has to be 3-wire with 2P+N. If it's just 2-wire, whether a supply or a circuit, then it clearly is only 'one-phase'.

Kind Regards, John.

 

[JohnW2]

There will be 2 AC voltages present, and they will not be in phase.

Indeed, provided only that it is a 3-wire supply. If one only has 2 wires, all one can observe/measure is the instantaneous pd between the two - such that, without the third wire as a reference, 'phase' has no meaning.

Kind Regards, John.

 

[ricicle]

There will be 2 AC voltages present, and they will not be in phase.

You're incorrect. There is only one AC voltage. It's just you are referencing each end of a winding to a centre tapped earth ie measuring half of the winding voltage.

 

[bernardgreen]

Indeed, provided only that it is a 3-wire supply. If one only has 2 wires, all one can observe/measure is the instantaneous pd between the two - such that, without the third wire as a reference, 'phase' has no meaning

If there are only two wires that are taken from a 3 wire 3 phase supply then it has to be considered as a two phase supply when looking from the supply side, especially if the supply transformer has any reference to ground ( earth )

But at the same time looking at it from the user's side it does work as a single phase supply.

Two phases can go to a transformer's two terminal primary and the secondary produces a single phase output.

 

[ban-all-sheds]

There will be 2 AC voltages present, and they will not be in phase.

You're incorrect. There is only one AC voltage. It's just you are referencing each end of a winding to a centre tapped earth ie measuring half of the winding voltage.

There will be 2 AC voltages simultaneously accessible, one from the centre tap to one end of the winding, and one from the centre tap to the other end of the winding, and they will not be in phase.

 

[ericmark]

Come on guys what has this got to do with all pole isolations.

 

[ricicle]

[
There will be 2 AC voltages simultaneously accessible, one from the centre tap to one end of the winding, and one from the centre tap to the other end of the winding, and they will not be in phase.

In that case there are infinite accessible voltages available along the length of a winding depending on where your test probes are put. In the case of a centre tapped winding, yes there are three measurable voltages but only one is any use as the other two to the centre tap are a reference for safety operation only.
I think we're saying the same thing two different ways.