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

As mentioned recently on the IET forum, it seems that the revised version of BS7671 just published has decreed that a 1-phase 3-wire a.c. system is henceforth to be known as a 2-phase 3-wire system.

To my mind, this is absolutely ridiculous, and an application of completely incorrect terminology. The single-phase 3-wire distribution arrangement has been in use since the late 19th century. It's always been recognized and known as just that - A single-phase system.

And what is the purpose of this seemingly out-of-the-blue change? What reason is there to arbitrarily redefine a system in this way? And has the committee responsible for passing this considered the confusion it will cause, not just here but elsewhere in the world where everybody is still going to call a 1-phase 3-wire system a 1-phase 3-wire system?

Does that mean that a real 2-phase system would be known as a 2-phase 4-wire system?

Paul_C said:

As mentioned recently on the IET forum, it seems that the revised version of BS7671 just published has decreed that a 1-phase 3-wire a.c. system is henceforth to be known as a 2-phase 3-wire system.

To my mind, this is absolutely ridiculous, and an application of completely incorrect terminology. The single-phase 3-wire distribution arrangement has been in use since the late 19th century. It's always been recognized and known as just that - A single-phase system.

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No. You're wrong. It's the number of LIVE conductors that are counted.

The standard single phase installation has only ever been supplied by two live conductors and continues to be described as single-phase two wire system.

You've been ticking the wrong box on your documents

Stoday said:

Does that mean that a real 2-phase system would be known as a 2-phase 4-wire system?

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Presumably, although I haven't seen any specific references.

dingbat said:

No. You're wrong. It's the number of LIVE conductors that are counted.

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There's more to it than simply counting the number of conductors, live or earthed, as the 2-phase 4-wire example demonstrates.

You've been ticking the wrong box on your documents

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So what do you think the box marked 1-phase 3-wire is there for?

Paul_C said:

So what do you think the box marked 1-phase 3-wire is there for?

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For a split phase supply. As Dingbat has said, you're wrong.

Anyway why would you care? As you keep pointing out that BS 7671 is not law and you don't work to it.

Bill Stickers said:

For a split phase supply.

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But that's what we're talking about: Single-phase 3-wire, 240/480V as commonly found in rural Britain.

Without reading new book I would have considered if two phases from a three phase system are taken then with the neutral included this would be a two phase system as the two line wire are not in phase with each other so there are truly two phases.
If however a centre tap is taken as a neutral from a single winding of a transformer feed with two phases but no neutral with a 230 - 0 - 230 or line to line 460 volt then since the two outputs are in phase then it's split phase.
Both systems would have a three wire output as would a three phase system from a delta wound secondary (IT system) so I would have considered there always has been three system types for a three wire system.
2 Phase
Split Phase
3 Phase (IT system)
But I am undecided as yet as to if to spend out on new book. I no longer work so not really required.

ericmark said:

If however a centre tap is taken as a neutral from a single winding of a transformer feed with two phases but no neutral with a 230 - 0 - 230 or line to line 460 volt then since the two outputs are in phase then it's split phase.

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And that's a single-phase system, not a two-phase one. It's been a single-phase system for over a century. Every electrical institution has acknowledged it as such in all that time.

But I am undecided as yet as to if to spend out on new book. I no longer work so not really required.

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I haven't read the actual book either, and certainly don't intend to waste money on buying any new editions of BS7671, but for anyone who does have the new amendment, apparently it can be found in 3.2.1.1.

I haven't come across this type of supply in the field, I always thought it was just called "split phase", but if the IET want me to call it 2 phase 3 wire then it isn't much of a problem. No doubt there'll be a corigendum shortly which will change it back

Paul_C said:

ericmark said:

If however a centre tap is taken as a neutral from a single winding of a transformer feed with two phases but no neutral with a 230 - 0 - 230 or line to line 460 volt then since the two outputs are in phase then it's split phase.

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And that's a single-phase system, not a two-phase one. It's been a single-phase system for over a century. Every electrical institution has acknowledged it as such in all that time.

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Although it sounds as if I may be flying in the face of traditional terminology, I'm a bit confused here. Despite what Eric wrote, if neutral is connected to the centre tap of a single 230 - 0 -230 winding, then the two outpurs will surely be 180 degrees out-of-phase - in which I would have thought that the correctt (apparently 'new') description would be 2-phase?

Kind Regards, John.

I have a feeling that my side of the meter we'll continue calling it split single phase.
This differentiates from a 2 phase supply where the two phases are 120deg apart.

Then somewhere down the line a nice bit of confusion will arise

Paul_C said:

I haven't read the actual book either, and certainly don't intend to waste money on buying any new editions of BS7671, but for anyone who does have the new amendment, apparently it can be found in 3.2.1.1.

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It's actually 312.1.1.

It's more-or-less consistent with what I just wrote. If the transformer is arranged with N to the centre tap of a single winding, such that the two output lines are 180 degrees out of phase, they call it "Two-phase 3 wire". However, if there are two separate windings (not a single centre-tapped one), and they are wired such that the two output lines are in phase (i.e.'the bottom of each of the windings in connected to N), then they call it "Single-phase 3-wire". As I implied in my previous post, that makes sense to me.

Kind Regards,John.

The sense of what is described does not exactly match the reality of the situation.

Two sizes of transformer (25kVA and 50kVA) have two windings that can be configured in series with a centre tap with a 3 wire distribution system or in parallel with a two wire distribution system. It is not normal to have a single phase 3 wire system if for no other reason that it is more expensive than a two wire system and of course needs a double sized neutral!

The third size (100kVA) is a fixed centre tap winding so would be used with a split single phase 3 wire system

JohnW2 said:

if neutral is connected to the centre tap of a single 230 - 0 -230 winding, then the two outpurs will surely be 180 degrees out-of-phase

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Only because you're using the mid-point of the winding as your common reference point. The voltages appearing at the two ends of a winding will always be 180 degrees out of phase with each other as measured relative to any other point on the winding.

If you took a 480V secondary winding without using a center tap (or two 240V windings wired series-aiding to give you 480V), earthed one end, and then extended as a simple 2-wire 480V service, that's clearly single-phase. If you then ran a third wire from the center point, leaving the earth at one end, you'd have two live conductors, one at 240V and one at 480V relative to the earthed conductor, but the voltages - again with earth as the common reference point - would be in phase.

All that the common 240/480V 1-phase 3-wire system is doing is moving the earthing point. Merely moving the earth from one end of the winding to the middle doesn't change it from single-phase to a two-phase system. It's still effectively a center-tapped single secondary winding (and the primary is just a simple 2-wire connection to the H.V.).

westie101 said:

Then somewhere down the line a nice bit of confusion will arise

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That's what I think too. Arbitrarily decreeing such a change is bound to lead to confusion. And what purpose is it supposed to serve?

JohnW2 said:

However, if there are two separate windings (not a single centre-tapped one), and they are wired such that the two output lines are in phase (i.e.'the bottom of each of the windings in connected to N), then they call it "Single-phase 3-wire".

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There's little advantage to such an arrangement for distribution, since the currents from each live pole would be additive, and the common earthed conductor (which would not then be a neutral in the true sense of the term) would have to be sized to carry the sum of the maximum currents possible in the two lives. You might just as well do what is done in practice as mentioned already - Connect the windings in parallel and use simple 2-wire 240V distribution.

westie101 said:

Then somewhere down the line a nice bit of confusion will arise

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Like when I rang UU and they told me one of ours was a 3-phase supply? It was off a two-wire overhead.