Electric shock via ground that becomes live.

[rollingstein]

Was reading a crane-operations manual and it had a Section on what to do if your boom accidentally contacted overhead electrical wires. It advised not trying to step out of the cab (of course); but in case of imminent danger advised jumping out (makes sense).

The interesting part came after this: It said to take tiny steps or hop on one leg to get away from the live equipment. Apparently the reasoning is the ground may have become live and a bigger step would be across a potential high enough to injure.

Does this strategy make sense? What's the typical potential contour and could a potential drop across a stride be so high to shock via two legs?

 

[jedishrfu]

I think they're afraid that if your two legs both touch the ground they will become a conductor so by having only one in contact that lessens the chance.

 

[Studiot]

Many years ago I was the victim of live ground.

This was due to lightning and I was on a bicycle at the time, cycling past the aptly named Bay of Storms in Italy.

I can confirm you get a definite feeling in the legs.

 

[dlgoff]

Here's a video about step potential safety due to earth potential rise (EPR)/ground potential rise (GPR).

http://vimeo.com/29931214

 

[Studiot]

dlgoff, A most interesting link, many thanks.

 

[jim hardy]

it's part of substation design to provide a ground mat sufficiently 'stout' that, should a fault occur, the voltage drop across the ground over the distance of a man's stride is small enough to not electrocute him.

There's no probably such ground mat under a crane out in the boondocks, so OP's surmisal seems correct..

Re studiot's bicycle - i was once swimming in a lake when lightning struck a couple miles away. Definitely felt the shock. Scared the daylights out of me.
I believe American Flamingoes stand on one leg for that reason. Florida Everglades is one of world's most active lightning zones.

oops - i see two posts appeared while i was typing...
nice find, dig...

old jim

 

[jedishrfu]

Here's a video about step potential safety due to earth potential rise (EPR)/ground potential rise (GPR).

http://vimeo.com/29931214

Cool video.

 

[rollingstein]

I believe American Flamingoes stand on one leg for that reason. Florida Everglades is one of world's most active lightning zones.

Ha!That's a new one. I thought it was to do with their circulation / thermoregulation.

 

[jedishrfu]

Ha!That's a new one. I thought it was to do with their circulation / thermoregulation.

Same goes for the Original Karate Kid in his famous finale standing on one leg ready for the shock of winning.

 

[mtb856]

I agree with the others. I work for an electric utility and we have warnings about walking in wet ground around power lines. We have to wear dielectric boots if working in those conditions.

If there is a phase to ground fault on a power line (like a crane contacting a line) and you are standing in the ground return path to the substation, there is a risk of a ground potential rise between your legs. Whatever fault current is flowing through the crane travels through the ground back to the neutral of the substation transformer, so there can be considerable ground current flowing (hundreds or thousands of amps depending on the line).