No, you aren't wrong, although people don't [CYA](usually)[/CYA] worry about the external L-N loop impedance, figuring that if Ze is OK then that will be too. It'd be the same for TN-C-S supplies anyway, and for TN-S and TT it would be the same as it would be if the supply were converted to TN-C-S. PFC is established to make sure that the current won't be too much over the "at least" value, but that's it.
Which is why I always point out to people attempting DIY is that that level of ignorance is unacceptable.
Without an EIC you can trust you should measure the ring to find its resistance and then work out if your proposed addition will push it over the edge.
All very true, but perhaps I wasn't clear enough as to the point I was making. Eric's statement that one gets much the same answers for maximum circuit length whether one calculates on the basis of VD or disconnection considerations is essentially a 'coincidence' resulting from what the external L-N impedance is, or is taken to be. If I've done my sums right, the 0.96Ω external L-N impedance figure which seemingly results in the 'coincidence' is pretty high. With more realistic figures, one would end up with a calculated maximum circuit length (for disconnection) which was far beyond the maximum allowable in terms of VD.
True, but to get 5V drop (hence 100W 'wasted') in 2.5mm cable with 20A, the cable would have to be nearly 300 metres long - hardly likley in a domestic environment!
In terms of the big picture, I think it's much moire daft than that. The vast majority of electricity we use (other than that we use deliberately for heating) ends up as 'wasted' heat, regardless of what 'desired' things the load is actually doing - but one doesn't hear people getting very emotional about that!
With the caveat that I am not 'qualified' ... what should be apparent from this discussion is that, in practice, for domestic installations in most 'standard' houses (even those with extensions), it is very unlikely that you would get anywhere near the maximum length for a 2.5mm² ring final circuit. The "106 metres" we're talking about is almost exactly the length of a football pitch.
Not being a follower of such activities, I have to confess that, before I pressed 'submit', I had to ask Mr Google to confirm my gut feeling about how long a football pitch is
Particularly since it's 2012, would "a bit more than twice the length of an Olympic swimming pool" be any more acceptable to you?
I don't think there was anything wrong wiuth your VD calculations - and we know, using the IET's load assumptions, the maximum length of a 2.5mm² 32A ring final which will satisfy (5%) VD requirements is about 106 metres. The point is that, depending on the external L-N impedance, the maximum length that would satisfy disconnection time requirements could be very different from that. As you've said, to achieve satisfactory disconnection times with a B32 requires a total L-N loop impedance of 1.4375Ω (assuming 230V). If you take the example of TN-C-S, for which the extrenal L-N impedance will be equal to Ze, and assume that to be 0.35Ω, that leaves 1.0875Ω for the cable. By my calculation, that would allow a 2.5mm² ring final length of about 241 metres - very different from the maximum which would satisfy VD requirements.
Indeed, but that's because the 5% VD limit is arbitrary and, as the regs say, acceptable VD depends upon the equipment being supplied. Particularly in the case of machines involving motors, with very high start-up currents (hence very high VD during the start-up period),one might need much fatter cable than that which would result in <5% VD during 'normal operation'. Indeed, some machines might not even be able to tolerate a 5% drop during 'normal operation', so the circuit design would have to take this into account.
I once went shopping for a petrol mower, and was asked the size of my lawn in tennis court units.Not being a follower of such activities, I have to confess that, before I pressed 'submit', I had to ask Mr Google to confirm my gut feeling about how long a football pitch is
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