Quick question - Reg number prohibiting SP RCBOs on TT

[ban-all-sheds]

If you use the centre tap as your reference point, then obviously the voltages at each end are 180º out of phase.

Well that's probably why they renamed it two phase.

That's a natural consequence of the way the transformer works,

Indeed it is.

But when you present 3 simultaneous voltages wrt a common point using 4 wires, and those voltages are out of phase because of the natural consequence of the way the transformers and generating equipment work, nobody seems to have a problem with it being called 3-phase.

 

[Tech99]

Well that's probably why they renamed it two phase.

Which is bizarre. Everybody's known it as a single phase for well over a hundred years of electrical engineering. Look in any decent book on the subject published in all that time and you'll find it referred to as a single phase arrangement.

But when you present 3 simultaneous voltages wrt a common point using 4 wires, and those voltages are out of phase because of the natural consequence of the way the transformers and generating equipment work, nobody seems to have a problem with it being called 3-phase.

Because you can connect three different things - lamps, motor windings, whatever - in wye or delta configuration between those points and get three simultaneous currents in those loads which are all out of phase with each other. You need to look at the currents involved, not the voltages.

 

[ban-all-sheds]

Everybody's known it as a single phase for well over a hundred years of electrical engineering. Look in any decent book on the subject published in all that time and you'll find it referred to as a single phase arrangement.

Things change.

Because you can connect three different things - lamps, motor windings, whatever - in wye or delta configuration between those points and get three simultaneous currents in those loads which are all out of phase with each other. You need to look at the currents involved, not the voltages.

You could connect two loads to a 3-wire 2-phase supply like that and the currents would be out of phase.

 

[Tech99]

Things change.

But single phase doesn't suddenly become two phase, any more than AC suddenly becomes DC.

You could connect two loads to a 3-wire 2-phase supply like that and the currents would be out of phase.

If it's something like 2P+N from a 3 phase supply then yes. But you can't do it with a single phase supply.

 

[securespark]

Although you've now said that twice, I'm not sure I understand the problem you see. As I understand it, the 0.2 second disconnection time relates to L-E 'faults' - which, as far as the regs are concerned means 'bolted shorts' - i.e. of zero impedance. Even with a poor TT electrode, that will result in at least an amp or two of L-E fault current - and a 30 mA RCD or RCBO will operate in under 0.2 sec at less than 60 mA L/N imbalance, let alone 'an amp or two'. So what's the problem?

Will they definitely operate within the required timeframe?

New standard RCD's and RCBO's have an operating limit of 300ms, and being electromechanical devices...

 

[JohnW2]

Although you've now said that twice, I'm not sure I understand the problem you see. As I understand it, the 0.2 second disconnection time relates to L-E 'faults' - which, as far as the regs are concerned means 'bolted shorts' - i.e. of zero impedance. Even with a poor TT electrode, that will result in at least an amp or two of L-E fault current - and a 30 mA RCD or RCBO will operate in under 0.2 sec at less than 60 mA L/N imbalance, let alone 'an amp or two'. So what's the problem?

Will they definitely operate within the required timeframe?

Yes, if they are compliant with the Standards that apply to them...

New standard RCD's and RCBO's have an operating limit of 300ms, and being electromechanical devices...

Yes, a max of 300ms at the rated trip current (i.e. 30 mA in most cases we are talking about) - but 150ms at twice that current and 40ms at 5 times that current - and the IET's 'bolted short' L-E fault (for which 200ms disconnection is required with TT) will be at least 30 or 40 times the rated trip current, even with a mediocre TT electrode.

Kind Regards, John.
Edit - typo corrected

 

[ban-all-sheds]

But single phase doesn't suddenly become two phase, any more than AC suddenly becomes DC.

But what if it should never have been called single phase?

If it's something like 2P+N from a 3 phase supply then yes. But you can't do it with a single phase supply.

So if I got 2 loads and connected one between the centre tap and an end point, and the other load between the centre tap and the other point, neither would work?

 

[Tech99]

But what if it should never have been called single phase?

Why shouldn't it have been? The basic electrical theory shows that it is single phase. If it had been called something wrong in the first instance, I think the ooportunity to correct the incorrect terminology would have been taken long before now. And just because, apparently, the IEE/IET/whatever it wants to call itself now has made some peculiar decision to adopt an incorrect term, I hardly think that the rest of the world is going to change.

So if I got 2 loads and connected one between the centre tap and an end point, and the other load between the centre tap and the other point, neither would work?

Of course they'd work. Not sure what your point is.

 

[ban-all-sheds]

Why shouldn't it have been? The basic electrical theory shows that it is single phase.

Are there restrictions on what the phase angle can be for it to be polyphase?

We've already established beyond doubt that there are two out-of-phase voltages present, and that 2 loads would have out-of-phase currents flowing in them.

If it had been called something wrong in the first instance, I think the ooportunity to correct the incorrect terminology would have been taken long before now.

Maybe it's an officially sanctioned change.

And just because, apparently, the IEE/IET/whatever it wants to call itself now has made some peculiar decision to adopt an incorrect term, I hardly think that the rest of the world is going to change.

Do you know for sure that the change didn't happen through CENELEC?

Do you know for sure that phase angles may not be integer multiples of 90°, or 180°?

So if I got 2 loads and connected one between the centre tap and an end point, and the other load between the centre tap and the other point, neither would work?

Of course they'd work. Not sure what your point is.

My point is that the currents in them would be out of phase, which you said couldn't happen with a single-phase supply:

You could connect two loads to a 3-wire 2-phase supply like that and the currents would be out of phase.

If it's something like 2P+N from a 3 phase supply then yes. But you can't do it with a single phase supply.

 

[Tech99]

Are there restrictions on what the phase angle can be for it to be polyphase?

We've already established beyond doubt that there are two out-of-phase voltages present, and that 2 loads would have out-of-phase currents flowing in them.

No - The currents will be in phase (assuming we're not complicating things with non-resistive loads).

Do you know for sure that the change didn't happen through CENELEC?

Could be. But CENELEC isn't the world.

Do you know for sure that phase angles may not be integer multiples of 90°, or 180°?

Of course a phase angle can be 90, 180 or 270°. That doesn't mean that just because you can see a 180° difference in voltage waveforms between two points as referenced to some other common point that it's a polyphase system. If you choose the right locations, you could see a similar difference with a simple 2 wire single phase circuit.

My point is that the currents in them would be out of phase, which you said couldn't happen with a single-phase supply:

The currents won't be out of phase though.

 

[bernardgreen]

The power station generates 3 phase.

The user at the far end of the network has equipment.

If the equipment has only two wires ( ignoring the safety earth ) then no matter where those two wires come from the equipment can only be single phase.

If those two wires are connected to two of the three wires in a three phase cable the equipment is still single phase even though it is connected across two of the supply phases.

Only when the equipment has more than two supply wires can it be more than single phase.

 

[Tech99]

If those two wires are connected to two of the three wires in a three phase cable the equipment is still single phase even though it is connected across two of the supply phases.

Exactly. With a two wire connection it cannot be anything but single phase. There are many single phase distribution transformers in rural areas which are connected across two phases of 11kV. The transformer and resulting supply on its secondary are still only single phase.

Only when the equipment has more than two supply wires can it be more than single phase.

However, that does not mean that if it has more than two supply conductors it must be more than single phase. To be polyphase it must have at least three connections, but three connections does not automatically mean polyphase.

 

[ban-all-sheds]

No - The currents will be in phase (assuming we're not complicating things with non-resistive loads).

No.

At any point in time their absolute magnitudes will be the same, but there's also a spatial dimension to consider.

Could be. But CENELEC isn't the world.

Indeed not, but it's our world wrt things like this, and if it was done through CENELEC then it wasn't the IET making a peculiar decision to adopt an incorrect term.

But even if it was the IET going it alone, do you really think that all those engineers with all that collective knowledge would not have had a good reason? Do you really think they would make a mistake?

Of course a phase angle can be 90, 180 or 270°. That doesn't mean that just because you can see a 180° difference in voltage waveforms between two points as referenced to some other common point that it's a polyphase system.

Sorry - I worded my question badly. From your perspective, do you think that the phase angle has to be other than an integer multiple of 90° for it to be a polyphase system?

In everyday life if you take one thing and split it into two then you have two things. Not with everything, granted, but if I have a piece of wood and I split it into two I then have two pieces of wood. And if I split it in the middle I have two pieces of wood of equal size, each one half the size of the original

I don't think it is obviously wrong to say that if you take one phase and split it that you then have two phases.

The currents won't be out of phase though.

If you are happy with the concept that at any point in time the potential difference between the centre point and one end will be a positive value and between the centre point and the other end it will be a negative value, then you must be happy that the currents have different directions.

If you could provide me with a supply with a voltage low enough, and a high speed video camera, I could connect two loudspeakers as the loads, and when filmed and played back at a slower speed you would see the diaphragms moving out of phase.

 

[ban-all-sheds]

If the equipment has only two wires ( ignoring the safety earth ) then no matter where those two wires come from the equipment can only be single phase.

Indeed, but that doesn't stop the supply from being 3-phase.

If those two wires are connected to two of the three wires in a three phase cable the equipment is still single phase even though it is connected across two of the supply phases.

Yes, and if you had more than one single phase load and connected them to different pairs of wires then the currents in them would be out of phase.

Just like if you connected two loads to two different pairs of a 2-phase 3-wire supply.

 

[bernardgreen]

Point A.................................Point B.............................. Point C
Negative----------<<---------Midpoint----------<<---------Positive
......................Current 1.............................Current 2

Igmore the dots they were to only way to get the text lined up

seen from Point A both currents are in phase towards Point A
seen from Point C both currents are in phase away from Point C

But at Point B those same two currents appear to be in antiphase as one is arriving and one is departing. But they are travelling in the same direction.