Main earth bond

[JohnW2]

Haven't you counted something twice?

Like what? ... L impedance back to transformer = 0.125Ω, MPB conductor = 0.05Ω, 'Ze' of extraneous-c-p = 0.25Ω.

Doesn't the Ze of 0.25 include the 0.125

Oh, I've produced some confusion here - my apologies! Yes if my " 'Ze' " of 0.25 really was a Ze value, then it would include the 0.125. However, when I suggested the 0.25Ω figure as being more realistic (than zero!), what I was actually thinking of was the 'Re' of the extraneous-c-p.

Anyway - (0.125+0.05+0.25) = 0.38, 230/0.38 = 605

I don't think so Per my figures above, (0.125 + 0.05 + 0.25) = 0.425, 230/0.425 = 541 - and least, using the rules of arithmetic I was taught

Oh. yes. I put the 0.05 in the wrong column (in my head). Sorry.

We all do it

What about with the correct 541A? (I can't be bothered to do it myself, and doing the calculation can be your penance for your previous arithmetical error ).

Lazy person 541A ... disconnection time 9s ? I shall use 10s ... EC 14.88mm² ... MEB 7.44mm² ... Still not too bad.

Indeed. Thanks.

Kind Regards, John

 

[JohnW2]

Ah, you misunderstood my tongue-in-cheek comment It was not a typo - don't forget that it was written before you reminded me of the 6mm² minimum, and I was suggesting that 0.5mm² (perhaps fractionally larger) would probably be enough with a PEFC of 3.3A. Such a conductor could carry that PEFC indefinitely without harm, so disconnection times and the adiabatic calculation would become irrelevant!

I beg your pardon. I thought it referred to half of ten.

Not at all - I meant what I wrote (when I had forgotten about the 'minimum'). If there were no minimim CSA imposed, then what I wrote would, I think, be true.

Kind Regards, john

 

[JohnW2]

541A ... disconnection time 9s ? I shall use 10s '... EC 14.88mm² ... MEB 7.44mm² ... Still not too bad

It's just occurred to me .... 10 (or 9) seconds is far too long for one to be able to assume that the process is adiabatic, so that the equation given in 543.1.3 can't really be used. Calculating the required CSA under non-adiabatic conditions is a much more complicated business (and requires additional information that may be hard to find) and, I suspect, is probably beyond the scope of most electricians. I therefore assume that, in such a situation. most would resort to using Table 54.7 - which, for virtually all domestic installations (of any supply type), would effectively call for a 16mm² EC and 10mm² MPB conductor.

Kind Regards, John

 

[EFLImpudence]

Would that apply to the rather specific unlikely fault being discussed?

 

[JohnW2]

Would that apply to the rather specific unlikely fault being discussed?

Sure, it applies to any situation in which current is passing down a conductor for a period of time (more generally, any heat-producing or -consuming process). The process is 'adiabatic' if the current flow is over so quickly that there is no time for any significant movement of heat to occur. In many contexts, a couple of seconds is regardede as 'the limit', but 543.1.3 says that one can use its adiabatic equation for current durations up to 5 seconds.

However, as far as we are concerned, moving into the realm of non-adiabatic conditions may make things 'better', since a little of the heat will escape and therefore the temperature rise of the conductor will be a bit less than if the same amount of energy were dissipated in a shorter period of time (adiabatically). Whether or not that overcomes the fact that current flowing for a longer time will result in more energy dissipation requires the 'clever' sums to be done!

Kind Regards, John