Electric shock path of current

We all know not to touch a live (line) conductor, but how does the current flow to give an electric shock or perceived electric shock?

Conventional wisdom says its via resistance of the hands , skin and feet but we all wear thick rubber soles and could be on a carpet on a timber floor.

I have done test at 250v DC touching one probe while the other is connected to the MET the meter was unable to detect anything and i felt nothing but i would not try this with AC.

Therefore to me that leaves inductive and / or capacitive coupling.

Does anyone here have any explanation of this effect?

  • Despite conventional wisdom, there is no problem touching a live mains part, so long as you are not in contact with anything else. Standing on an up turned plastic bucket ought to do ;-)

    The hard part really is avoiding the anything else.... If you watch live working crews they are very, very careful to cover N and E wires before sawing into the L.

    In terms of displacement currents, your body capacitance to earth varies a bit with body mass and where you are standing in relation to other earthed items but is typically 500-1000pF for a body well away from earth and not leaning on anything massive,  so the current into you is set by the voltage and 1/(2.pi.F.C) in the normal way.(at 50Hz this is a few j megohms, so the currents are fractional mAs)

    At 50Hz or so the limit for perceived  sensation is about half a mA and limit of no let go about 10-20mA, but it depends quite a bit into which  body part as to how strongly which muscles are paralysed.

    If there is no second contact you need quite a lot more than 230V RMS to reach a significant effect by capacitance alone.

    Mike

  • There is no problem touching a live mains part, so long as you are not in contact with anything else.

    I can vouch for that from my mis-spent youth (I seemed to have a very conscientious guardian angel). Not so long ago, and not very far from here, having a non-conductive location (e.g. bedrooms, living rooms) was considered sufficient to provide protection against electric shock and earthing of sockets etc. wasn't bothered with - only places like kitchens and bathrooms had earthing contacts on their sockets.

    It is relatively easy to find something "earthy" to complete the circuit for a shock though - indoors metallic plumbing and attached accessories (e.g. radiators) - class 1 appliances plugged into an earthed socket, even damp floors or walls (e.g. older properties that date before the days of damp proof membranes) and of course outdoors there's likely to be all sort of metalwork stuck into the ground (from gates/posts, fencing, pretty rose arches, lamp posts etc etc).

       - Andy.

  • The first time that I got a belt was at the age of about eight. I reached up to turn on a bed-head lamp (the things that we had in the sixties!) but it had fallen off in the night, so I grasped bare wires. The shock must simply have gone through my hand, but it didn't half give me a turn!

    So now let's change a lamp fitting using aluminium step ladders, but forget to isolate the circuit. The fault circuit (as it were) is terminal, screwdriver, finger, body, shoes, ladder floor. OK, for this exercise we can have leather shoes rather than rubber ones, but it is a suspended timber floor. Or we can use wooden step ladders so now we have even more insulation. Surely, that must be comparable with Mike's upturned bucket!

    So perhaps the reality is that you get 2 mA, but I still don't really know how the circuit is completed.

    That said, a shock off a static electricity machine can give you quite a tingle.

  • We all know not to touch a live (line) conductor, but how does the current flow to give an electric shock or perceived electric shock?

    Conventional wisdom says its via resistance of the hands , skin and feet but we all wear thick rubber soles and could be on a carpet on a timber floor.

    I have done test at 250v DC touching one probe while the other is connected to the MET the meter was unable to detect anything and i felt nothing but i would not try this with AC.

    Therefore to me that leaves inductive and / or capacitive coupling.

    Does anyone here have any explanation of this effect?

    All of this information is available in the IEC 60479 series of standards.

    Please don't take your personal DC experiments further at higher voltages ... 630 V DC has up to now been considered roughly equivalent to 250 V AC, although research is ongoing. There's also the issue of, at what point does DC become AC (e.g. voltage drop exceeding 10 % in DC systems with non-DC currents, becomes an AC touch voltage according to IEC 60479 series and effectively BS 7671).

    The series of standards contains recommended thresholds for different circumstances (shock path, type of footwear ... or none ... etc.)

    Worth remembering, though, that this series of standards is for guidance of standards-making committees to take into account the relevant circumstances for the standards themselves, so there's not "direct read-across" to the parameters in a particular product or installation standard, nor any implied "requirement" or "acceptability for compliance to essential requirements of safety legislation" using the IEC 60479 series

  • Some time ago I remember accidently touching the bare end of a live fuse in a 4 way extension lead I was upstairs in my bedroom standing with bare feet on a nylon carpet I never felt a thing which was lucky for me. The exact opposite happened years before I borrowed a metal cased HF receiver and put it in our outhouse rigged up a temporary areal to test it make sure it worked  as I was tuning my bare ankle touched a gas pipe I got a right belt and used some un ladylike language. The plug was wire correctly  so don't know why I got a shock  house was PME supply if that made any odds.

  • 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!