Nicely done, all respondents! Thank you.


Sparkymania.

"intensité du courant", hence the symbol I, is an origin I only realised in the last year or few, when I saw a quotation from Ampère (some 200 years ago) about the two essential features of electricity: tension et intensité.  I'd occasionally wondered, before that.



mapj1:

As your list of FTE conductor-sizes implies, the 4 mm^2 is a particularly extreme case. I chose the 32 A rating to try to push the reader into this size, without going up to such high ratings (e.g. 16 mm^2) that the 0.4 s requirement mightn't apply. I'm ashamed to say I didn't even notice the need for an erratum in your first post - but might have done if wandering back.


AJJewsbury:

Good point - I specified (indirectly) the external loop impedance in order to ensure that the final-circuit cable would be the major part of the total, so that the voltages from mapj1 would be roughly right.



I couldn't post the actual document of the retired IEC technical report I mentioned, but the following may be of interest, at least about my 'Q1' although  not the OP.  It is a plot I made for an instructive purpose, including the "L" and "Lp" curves defined in that IEC report. 


The solid green "L-curve" is defined in the report (perhaps elsewhere too?) based on assumptions of (1) a time-versus-current curve rather below the threshold of danger indicated in IEC 60479-1, and (2) a particular case of skin contact based on normal, non-wet conditions.  The latter, I feel, is the weaker assumption. It's admittedly quite pessimistic for the stated sort of condition, as it uses values of resistance that the great majority of tested people actually exceeded. But there are doubtless plenty of "normal" situations where people actually aren't so dry.  Note that the body (contact) resistance itself can change with voltage as shown also in IEC 60479-1, which is taken into account here.  (For bigger or wet contact areas the voltage-dependence is less, so it's not a huge influence here. But for small dry contacts, like a wire-end on a hand, the resistance can zonk down by more than an order or magnitude if the contact point gets much of the 230 V compared to, say, 100 V: I've tested this first-hand, having first been a little suspicious.)  The Lp-curve is for 'particular' conditions, i.e. not dry. The other clutter is the 0.4 s limit that is used as a simplification for systems above 120 V and up to 230 V to earth, the 5 s limit that's the subject of this thread, and then some funny line and point I've derived from a bonding requirement ... I can look up what I based it on if anyone really wants to know, but I don't have it in my mind just now.




Now the final part of the process:  the 0.4 s is based on the L-curve together with the assumption that a person is exposed to about 80% of 50% of the voltage to earth. This is 92 V, the corresponding time for which is rounded down to a neat 0.4 s. The 50% is based on equal ratios of line and protective conductor resistance in the final circuit. The 80% is based on the point made by AJJewsbury, where halving of voltage in the final circuit isn't the only factor in the voltage exposure. Increase of 230 V to the high end of the permitted range isn't done here (but 10% increase would be compensated by the rounding down to 0.4 s).


The main point of my Q1 is that the 0.4 s certainly isn't based on the ratio of conductor sizes found in UK FTE cable design.


[IEC TR 1200-413 1996] 413.1.3 says: "Table 41A of IEC 364-4-41 specifies a single time of 0.4 s. This time corresponds to a mean value of the factor c of 0.8 and a ratio m of 1, values which exist in general in final circuits. This time is confirmed by long experience in many countries as providing satisfactory protection against electric shock."

To clarify: the factor 'c' describes the relation of impedances up- and down-stream of the bonding closest to the final circuit; and the factor 'm' is the ratio Rpe/Rl in the final circuit.  From mapj1's numbers, m=2.7 would be more appropriate to FTE "4mm^2" UK cable.  (What's the situation in Ireland? Rpe=Rl ?)


[IEC TR 1200-413 1996] 413.1.3.5.  "The limitation to 5 s is conventional."  (And reasons justifying the longer time are given as the lower probabilities of faults in such circuits or of persons being in contact with or particularly gripping such equipment, and the hope that main bonding will suitably reduce touch voltage.  Just as already mentioned on this thread.)

Nicely done, all respondents! Thank you.


Sparkymania.

"intensité du courant", hence the symbol I, is an origin I only realised in the last year or few, when I saw a quotation from Ampère (some 200 years ago) about the two essential features of electricity: tension et intensité.  I'd occasionally wondered, before that.



mapj1:

As your list of FTE conductor-sizes implies, the 4 mm^2 is a particularly extreme case. I chose the 32 A rating to try to push the reader into this size, without going up to such high ratings (e.g. 16 mm^2) that the 0.4 s requirement mightn't apply. I'm ashamed to say I didn't even notice the need for an erratum in your first post - but might have done if wandering back.


AJJewsbury:

Good point - I specified (indirectly) the external loop impedance in order to ensure that the final-circuit cable would be the major part of the total, so that the voltages from mapj1 would be roughly right.



I couldn't post the actual document of the retired IEC technical report I mentioned, but the following may be of interest, at least about my 'Q1' although  not the OP.  It is a plot I made for an instructive purpose, including the "L" and "Lp" curves defined in that IEC report. 


The solid green "L-curve" is defined in the report (perhaps elsewhere too?) based on assumptions of (1) a time-versus-current curve rather below the threshold of danger indicated in IEC 60479-1, and (2) a particular case of skin contact based on normal, non-wet conditions.  The latter, I feel, is the weaker assumption. It's admittedly quite pessimistic for the stated sort of condition, as it uses values of resistance that the great majority of tested people actually exceeded. But there are doubtless plenty of "normal" situations where people actually aren't so dry.  Note that the body (contact) resistance itself can change with voltage as shown also in IEC 60479-1, which is taken into account here.  (For bigger or wet contact areas the voltage-dependence is less, so it's not a huge influence here. But for small dry contacts, like a wire-end on a hand, the resistance can zonk down by more than an order or magnitude if the contact point gets much of the 230 V compared to, say, 100 V: I've tested this first-hand, having first been a little suspicious.)  The Lp-curve is for 'particular' conditions, i.e. not dry. The other clutter is the 0.4 s limit that is used as a simplification for systems above 120 V and up to 230 V to earth, the 5 s limit that's the subject of this thread, and then some funny line and point I've derived from a bonding requirement ... I can look up what I based it on if anyone really wants to know, but I don't have it in my mind just now.




Now the final part of the process:  the 0.4 s is based on the L-curve together with the assumption that a person is exposed to about 80% of 50% of the voltage to earth. This is 92 V, the corresponding time for which is rounded down to a neat 0.4 s. The 50% is based on equal ratios of line and protective conductor resistance in the final circuit. The 80% is based on the point made by AJJewsbury, where halving of voltage in the final circuit isn't the only factor in the voltage exposure. Increase of 230 V to the high end of the permitted range isn't done here (but 10% increase would be compensated by the rounding down to 0.4 s).


The main point of my Q1 is that the 0.4 s certainly isn't based on the ratio of conductor sizes found in UK FTE cable design.


[IEC TR 1200-413 1996] 413.1.3 says: "Table 41A of IEC 364-4-41 specifies a single time of 0.4 s. This time corresponds to a mean value of the factor c of 0.8 and a ratio m of 1, values which exist in general in final circuits. This time is confirmed by long experience in many countries as providing satisfactory protection against electric shock."

To clarify: the factor 'c' describes the relation of impedances up- and down-stream of the bonding closest to the final circuit; and the factor 'm' is the ratio Rpe/Rl in the final circuit.  From mapj1's numbers, m=2.7 would be more appropriate to FTE "4mm^2" UK cable.  (What's the situation in Ireland? Rpe=Rl ?)


[IEC TR 1200-413 1996] 413.1.3.5.  "The limitation to 5 s is conventional."  (And reasons justifying the longer time are given as the lower probabilities of faults in such circuits or of persons being in contact with or particularly gripping such equipment, and the hope that main bonding will suitably reduce touch voltage.  Just as already mentioned on this thread.)