A completed 250VDC circuit will hurt, and could be fatal if you were unlucky with the contact areas and skin conditions.

As per the AC case, in the absence of a second contact you are as safe as the birds on the overhead  lines, until something touches you and completes the circuit.

One may estimate the capacitance to ground of a typical pigeon sized bird as being more or less the same as that of a 30cm sphere.

(4.pi.e0.r) Where r is 0.15m, e0 is 8.8pf/m and so C= 18pF or so. (compare the human at about 25-50 times this)

at 50Hz this is about 170Milion J  ohms.

So when the bird sits on the 11kV line (6.6kV to deck of course) the current into dicky's feet is about 40uA . This is driving the displacement current required to create and then remove the reverse the polarity of the distortion to the electric field that it creates every cycle.

There is no second contact. If there was we'd be looking at a flash and a bang.

Note however that while we see birds on LV and on 11 and sometimes 33kV overheads, we do not see them on lines at 132kV and higher - suggesting that their perception current like ours is also between the 100uA and 1mA marks.

Single wire earth return HV supply I think. Never done even in the roughest bits of Euroland. Common in Africa and south America.

On antennas and things with gaps and resonant RF traps in the lines the situation is more complex, and birds may well  fly off when quite modest amounts of RF are involved.

Mike.

Thank you Mike.

Does that mean that the belt that the careless electrician gets up a wooden ladder not a current to earth, but charging up the body like a capacitor? 

Broadly yes- and the same effect is how the 'single contact' neon screwdrivers that we are not supposed to trust also work...

In the wooden ladder case it may be a bit of R and C in parallel - if you megger between 2 screws in a piece of wood the result is a good proxy for how dry it is (actually a surveyors 'damp meter' is doing more or less this.) A piece of fire wood dry wood is a very good insulator (think hundreds if megohms - the full height of set of steps may well be off the end of the scale for the average meggar - but the same ladders after a roof-rack journey on a wet day, or being left out overnight, rather less reliably so.)

But even if the ladder were a perfect insulator or the electrician were suspended by some other means, such as a nylon rope he or she would have some residual capacitance to the rest of the planet - albeit probably under  hundred pF region.

If we approximate the electrician by a  sphere of 1m diameter (I'm not naming names at this point, this is a hypothetical case to ease the sums, and any association to any sparks who really is spherical and 1m in diameter is purely accidental..) we can use the same approximation and formula for the bird.

(4.pi.e0.r) Where r is 0.5m, e0 is 8.8pf/m and so C= 60pF or so.

But that is an isolated  body floating in free space. If you actually measure this, and as part of work for antennas on body worn radios I have, when you get  within a few sphere diameters of the ground, the 'isolated sphere' model becomes pretty pants, as we are not really looking at the asymptotic limit of the second plate being a sphere at the edge of the universe ,but something much nearer, acting as the 'other plate' - terra-firma, bits of building, cars etc brings the capacitor 'plates' together are often within an arm length or so - and so the capacitance as measured increases to more like a few hundred pF, rising to 1000pF or so if there are soles of size 8 shoes and a reasonably good ground beneath them,  and the figure is very person and environment dependent.

Which makes a covert body worn antenna a really awkward thing to design, but that is another story.

And yes, 50 times a second, that capacitance charges and discharges, and that 'displacement current' is what we feel as a bit of a tingle.

If it is more of a belt than a tingle, I would suggest there is some resistance giving a real if rather nebulous  2 point contact  as well, and assuming you are not in contact with a second wire, then perhaps your shoes or your ladders are not as good an insulator as they could be.

also note that in summer humans are better conductors - less clothing, more sweat, and the electric shock figures reflect this.

Mike

Mike, thank you. I think that I understand.

Of course if the lamp holder is metal and, accordingly, earthed and is held in one hand, it will be more R than C.

If you continue with that line Chris, you will discover if an Earthed lampholder is more dangerous than an unearthed one. The answer to that is decreed but could well be wrong!

Chris Pearson said:

Of course if the lamp holder is metal and, accordingly, earthed and is held in one hand, it will be more R than C.

There are Class II appliances with (unearthed) metal casings.

Chris Pearson said:

Of course if the lamp holder is metal and, accordingly, earthed and is held in one hand, it will be more R than C.

There are Class II appliances with (unearthed) metal casings.