Touch voltage
- Thread starter studentspark
- Start date
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If you don't mind, I would like to know, Thank you.
I see my last post is awaiting moderation again ???
but not this one ???
MOD: Sorry didn't see it in the queue. Now approved, don't ask me how the algo works, I have no idea as not party to it's implementation but it's not personal. Computer just said "Maybe"
but not this one ???
MOD: Sorry didn't see it in the queue. Now approved, don't ask me how the algo works, I have no idea as not party to it's implementation but it's not personal. Computer just said "Maybe"
Last edited by a moderator:
There seems to be no rhyme or reason about it - the software definitely has 'a mind of its own'!
Kind Regards, John
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OK. I'll try to do this without a diagram ...
... think of having R1 and R2 in series with 230V applied across the two of them, through a fault (a dead short) between them at the end. The same ('fault') current flows through both of them, so the voltages across the two resistances will be proportional to those resistances (Ohm's Law - voltage across each resistance = resistance x current). Hence, if there is 50V across R2 and 230V across (R1+R2), then:
50/230 = R2 / (R1+R2)
after which it's just a little simple algebra ....
50 x (R1+R2) = 230 x R2
then (omitting the multiplication symbols for simplicity) ...
50R1 + 50R2 = 230R2
50R1 = 230R2 - 50R2
50R1 = 180R2
R2 = (50 / 180)R1
R2 = 0.28R1 (approx)
hence R2 is approx 28% of R1
... can you follow that?
Kind Regards, John
Bond what to what?
One can obviously minimise potential differences ('touch voltages') between two exposed-c-ps, or between an exposed-c-p and something else which was 'earthed' (like a pipe) by connecting them together with a bonding conductor, but to apply that approach generally would require a local bonding conductor ('SB') between every exposed-c-p and every other simultaneously touchable exposed-c-p or 'earthed' item - which would not really be very practical, would it?
Kind Regards, John
My turn now (and it ain't 'not visible to normal visitors' any more 
) !!!
) !!!
The voltage between e-c-ps - exposeed and extraneous - is what touch voltage refers to but we only have to consider it in bathrooms etc. when you might be earthed (effectively).
Obviously a live exposed-c-p is always at 230V.
Ah, I now see what 'C' in Studentspark's diagram is.
The 23,000Ω, including body resistance, to the MET is the value above which parts are not considered to be an extraneous-c-p - although, as the voltage is likely to be 240V, 24,000Ω is more realistic.
This limits the touch voltage to 10mA below which is not considered dangerous so the figure in the diagram is 0.01A - 10mA.
Having said that some like to limit it to 5mA so a value of 48,000Ω will be used - although it is a compromise between the two.
The 23,000Ω, including body resistance, to the MET is the value above which parts are not considered to be an extraneous-c-p - although, as the voltage is likely to be 240V, 24,000Ω is more realistic.
This limits the touch voltage to 10mA below which is not considered dangerous so the figure in the diagram is 0.01A - 10mA.
Having said that some like to limit it to 5mA so a value of 48,000Ω will be used - although it is a compromise between the two.
It that qualification relevant? - if you are talking about the p.d. between two conductive parts, then whether or not one 'might be earthed' is surely irrelevant (or, at least, 'different'), isn't it?
Not if it is an exposed-c-p which is connected to an 'effective' CPC. That's what I've been talking about (and doing calculations about) - the potential that would exist on a (CPC-connected) exposed-c-p if L came into contact with it.
Kind Regards, John
Yes you are right. I only added it afterwards for some reason trying to think of everything at once.
Probably not. I was trying to think of the example fault taking a long time to clear.
I should have said 'unless' you are earthed when it (touch voltage) makes no difference. I think.
When you wrote "Obviously a live exposed-c-p is always at 230V" you were presumably thinking of a situation in which not only was the exposed-c-p connected to L, but also in which it had lost its connection to an effective CPC. If one considers 2.5mm²/1.5mm² T+E then, even if there are no 'parallel paths', if the CPC is still connected the potential of the exposed-c-p will only be about 142V (about 62% of 230V), and if there are parallel 'earth' paths it will be less than that.
Kind Regards, John
Yes, I knew you'd get it in the end.
I'm going in the bath now. No exposed nor extraneous-c-ps in the room.
I'm going in the bath now. No exposed nor extraneous-c-ps in the room.
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