Down the rabbit hole again…
I got a cheque through the post the other day (I know; how exciting!).
For the grand total of £79 (oh well – I guess that holiday in the Bahamas will have to wait another year; as if any of us are going anywhere anyway).
To explain; I’m involved in the running of a local charity in west Devon that is rewilding a small parcel of common land. The cheque was from Western Power Distribution (WPD), one of the various regional power companies that takes power from the National Grid and links it to end-users like you and I. It was for payment of ‘wayleave’ or rent for poles that carry high voltage power cables across the land.
Interestingly there was a leaflet and a sticker included in the envelope with information about the dangers of overhead power lines and general precautions. Copies of the leaflet can be found here (https://www.westernpower.co.uk/downloads/13982). Here is a photo of the sticker:
The sticker is for putting in the cab of vehicle (such as a tractor, lorry or tele-handler) to give instructions if any part of the vehicle comes into contact with live overheads.
Note the instruction to stay in the cab. And to jump well clear if you can’t stay in the cab (because, for example, the vehicle is on fire). Note in particular the instruction not to climb down.
And my personal favourite; to move away from the vehicle using bunny hops (i.e. feet together).
What on earth is going on?
Well exactly; what on earth, or rather what in earth, is going on.
When any part of a vehicle comes into contact with a live overhead, say an 11,000 volt (11 kV) cable the body of the vehicle will likely be raised to the same voltage potential as the cable. So all metalwork etc. is now at 11 kV. It may be that the large rubber tyres of the vehicle are effectively insulating the vehicle from the ground. In which case, if you do step carefully down from the cab holding on to the vehicle with your hands and putting a foot down onto the ground you will act as an electrical conductor. The end result; a massive shock current flowing through you that would kill you and likely cause horrendous burn injuries.
WPD have made a short public information video aimed at the agriculture industry to warn of the dangers of accidental contact with overheads:
If you remember your O-level physics (yes I am old enough) Ohm’s Law is: voltage = current x resistance.
If we rearrange that equation: current = voltage / resistance.
[These aren’t the correct technical terms for some of the characteristics but it’s good enough for jazz]
If we say that the hand to earth resistance is 100,000 ohms (which it might well be when wearing shoes) then:
The shock current would be 11,000 volts/ 100,000 ohms = 0.11 amps.
0.11 amps doesn’t sound like much does it? Especially when you compare it to the current passing though say a 2 Kw electric kettle (8-9 amps).
But 0.11 amps from the mains (alternating current; AC) is more than enough to kill you by stopping your heart.
But why the bunny hops?
So it may be fairly obvious that you should not touch any part of your live ‘at voltage’ vehicle and the ground at the same time. That’s why the instruction is to jump out of the cab and not climb down.
But why should you then bunny hop away?
Well the answer is that if electricity is leaking into the ground at any point from your vehicle then it would be creating an increase in the voltage potential at that point (a phenomenon called ‘earth potential rise’). In other words the ground near the vehicle will be at raised voltage potential relative to the ground further way from the vehicle.
The potential drops in a gradient as you move away from the point of contact. So as you stride away from the vehicle you can have one foot on the ground with one potential and another foot on the ground at a different potential. The result would be a potential difference between your feet. And since your legs probably conduct electricity better than the soil does the end result is current flow through you from one point to another.
The potential difference between the feet of a person striding away is called the ‘step voltage’.
See this great video from Northeast Utilities (US): https://www.youtube.com/watch?v=uCAgsKaA_YY
To avoid creating the potential difference you want both feet kept very close together. They will then be at the same potential and so no shock current will flow.
Hence the bunny hops.
People and animals
People are susceptible to harm caused in this way. But large animals even more so. Animals such as sheep, cattle and horses have a big distance between their front and rear legs and shock current travelling between the two passes through the torso where the heart is. And they stand bare footed (O.K. wearing metal shoes in the case of some horses).
Two race horses died in the paddock at Newbury Race Course in February 2011 as the result of this phenomenon caused by a faulty underground cable. See here for the news report: http://news.bbc.co.uk/sport1/hi/other_sports/horse_racing/9400599.stm.
There is video of the event on YouTube.
Sheep are also susceptible; see http://news.bbc.co.uk/1/hi/england/devon/5150638.stm. Yes you read that right; it was 92 animals dead in a single incident.
And a group of 11 cattle were killed in my backyard on Dartmoor just last year by earth potential rise caused by a lightning strike.
And finally Ronnie…
I should point out that as you bunny hop away bunny ears and bunny teeth are entirely optional.
Stay safe in these strange times.
Additional references just for fun:
Dr Jim Phelpstead BSc, PhD, CMIOSH
RRC Consultant Tutor