Quick question - Reg number prohibiting SP RCBOs on TT

[ricicle]

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

Come on Eric - you're the king of waffle

 

[ban-all-sheds]

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

Please sir, he started it sir...

Strictly speaking according the wiring regs, they should be brown, brown, and green/yellow, as it's a single-phase circuit.

 

[ban-all-sheds]

In that case there are infinite accessible voltages available along the length of a winding depending on where your test probes are put.

I was using the context of something where 3 points are presented - each end of the winding and a centre tap.

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.

So (assuming the voltage was right) a load connected between the centre tap and one of the other two points would not work?

In the US they have just such an arrangement to support 110V and 220V appliances.

I think we're saying the same thing two different ways.

Are we both saying that the 3-wire centre tapped arrangement we've been talking about is 2-phase?

 

[ricicle]

I thought the discussion was on RLVS's - in which case no it is not 2 phase.

 

[Tech99]

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

It all depends upon the point you choose as a reference. For a 110V CTE supply, if you use the centre-tap as your reference, then yes, the voltages on each live leg will be 180º out of phase which each other. Use one of the live ends as your common reference point then the voltages as seen at the other end and at the earthed centre point will be in phase (but at different amplitudes).

 

[Tech99]

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.

Sorry, what's the BGB? The current edition of the wiring regs? If so, then I don't have a copy.

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.

That's all you have with a 110V CTE supply. The centre-tap earth isn't a current-carrying conductor (except under fault conditions), merely a connection to reference the supply to earth. It's really no different with a 3-wire single phase supply, except that the centre-tap connection is distributed as a neutral. It doesn't change it into a two phase system just because you've introduced a third wire.

 

[Tech99]

So (assuming the voltage was right) a load connected between the centre tap and one of the other two points would not work?

In the US they have just such an arrangement to support 110V and 220V appliances.

And in Canada, parts of Latin America, and some parts of the Caribbean. The same system is also used in Japan, just at a slightly lower voltage (100/200). And in all cases it's known as a single phase supply system, just as the similar 3-wire systems in Britain (just at double the voltage) have always been known.

What on earth the IEE is thinking if it's suddenly decided out of the blue to call this "two phase" is anybody's guess.

 

[JohnW2]

Sorry, what's the BGB? The current edition of the wiring regs? If so, then I don't have a copy.

Yes, as you kindly emboldened in red when you responded to one of my messages:

(and the current regs)

a.k.a BS7671:2008 Amendment 1 (2011). If you don't have a copy of the current regs, it's obviously understandable that you don't know what they say. I had assumed that you were a practising electrician, and therefore would understand 'BGB', and have a copy thereof - my apologies.

Kind Regards, John.

 

[Tech99]

Thanks for clarifying that. No, not a practising electrician, I'm in data & telecoms work, so although I'm familiar with the IEE regs to some extent, I don't keep up with them and don't know all the latest changes in recent editions.

 

[JohnW2]

Thanks for clarifying that. No, not a practising electrician, I'm in data & telecoms work, so although I'm familiar with the IEE regs to some extent, I don't keep up with them and don't know all the latest changes in recent editions.

You're welcome - and before someone else decides to pull you up on it, FYI the IEE is no more, having metamorphosised into the IET some time ago ... and 'BGB' is 'The Big Green Book'!

Kind Regards, John.

 

[securespark]

Thought there was a 0.2 disconnection time on TT?

Which makes it a bit tricky to comply?

 

[JohnW2]

Thought there was a 0.2 disconnection time on TT? Which makes it a bit tricky to comply?

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?

Kind Regards, John/

 

[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?

Since no-one has yet commented on what I recently wrote (above), it is perhaps worth reminding ourselves that (whatever the earthing system) an RCD is the only way to achieve short disconection times in the presence of L-E faults of sufficient impedence to preclude very rapid operation of an MCB.

For example, given a B32 MCB, any current less than 160A will result in a disconnection time of at least 10 seconds. In the worst case, that could potentially raise the 'touch voltage' (MET/CPCs relative to truth earth) to 56V with TN-C-S (if Ze=0.35 &#937 or 128V with TN-S (if Ze=0.8 &#937 for at least 10 seconds. Whilst high touch voltages (relative to true earth) are theoretically not a hazard if all appropriate bonding is in place, it's hardly a nice situation. Furthemore, this could arise with a (quite possible) fault impedance only slightly above zero if the Zs of the circuit was already marginal (in terms of MCB disconnection times). With worst-case TN-S, and a marginal fault impedance, the 'touch voltage' could be above 50V for something like 200 seconds before a B32 operated.

Kind Regards, John.

 

[ban-all-sheds]

What on earth the IEE is thinking if it's suddenly decided out of the blue to call this "two phase" is anybody's guess.

Are the two voltages wrt the centre tap in phase or not?

 

[Tech99]

Are the two voltages wrt the centre tap in phase or not?

If you use the centre tap as your reference point, then obviously the voltages at each end are 180º out of phase. That's a natural consequence of the way the transformer works, and is made use of not only for 110V CTE supplies and 240/480 distribution in the UK (or 100/200 - 120/240 elsewhere in the world), but also for driving balanced audio & data lines, as a phase splitter for class B amplification, and similar applications. But you only see that phase difference in the voltages because you're using a mid point of the winding as your reference point.