What happens if "hot" wire touches Earth ground?

[William White]

He was safe in the tractor because it has great big 6 feet rubber tyres.

The tractor was at the same voltage as the overhead line.
The ground was at zero Earth potential.

As he stepped out of the tractor and put one foot down on the ground there was a massive voltage across him and he fried (HV, because of the power involved, tends to cook you alive rather than shock you into death)The auto-reclose is normally a relay on the supply CB.
Overhead lines are prone to nuisance tripping. Wildlife can short out the conductors, so can flying debris in the wind (branches etc). High winds can clatter the lines together.

All these sorts of faults clear themselves (the dead animal falls to the ground, the branch falls down etc). So after a few seconds (generally 3-5 seconds) the relay sends a signal to the CB to close. If the CB trips again (the fault is still standing) the relay is either programmed to lock the breaker open or try again.A load spike at 11 kV or LV would generally not even be a blip at 33kV. Thats one of the reasons for discrimination.

A dead short across a 33kV line can be several hundred MVA. There is no load spike that could require such power - the maximum load of the 33 transformer is typically 15/30 MVA. (15 continuous/30 short time, or with coolant pumps and air fans running(

(an oil filled CB contails several hundred litres of mineral oil - a large fault will cook the oil black)

Earth faults generate lower fault levels, but last longer. So the trick is to set different protection schemes to operate at different rates.

Designing protection curves is not a trivial task. The protection closest to the fault should trip first and it should diffentiate between load and faults, and different types of faults (ie do you want to take out the LV breaker and just disconnect load, or the HV breaker and take out the transformer)In the UK 33 is a now distribution voltage, but many still call it subtransmission (distibution was below 22kv; subtransmission was historically up to 132 kv; transmission 275 kv and the "supergrid" 400 kv. The distinction was generally due to the different departments that looked after the work)

 

[rollingstein]

He was safe in the tractor because it has great big 6 feet rubber tyres.

The tractor was at the same voltage as the overhead line.
The ground was at zero Earth potential.

As he stepped out of the tractor and put one foot down on the ground there was a massive voltage across him and he fried (HV, because of the power involved, tends to cook you alive rather than shock you into death)

Understood. Stepping out is a sure way to get yourself killed.

I was imagining a guy who didn't step out but jumped and then while standing on the ground the recloser closed and the step potential between his feet got him killed. That would sure be bad luck.

As I understand, it's very tricky to estimate the effect of a step potential while standing on the ground close to a downed conductor. e.g. the gait / stride varies, ground resistivity varies etc.

Have you heard of fatalities due to the step potential in real life? i.e. People who didn't actually touch a conductor nor a piece of live equipment nor were in the water but got killed merely by the current flowing through the ground from one leg and out through the other?

I've seen calculations of this but never heard of any anecdotal electrocutions of this nature.

 

[William White]

Understood. Stepping out is a sure way to get yourself killed.

yes, if he jumped clear, he would (could) have been okay

As I understand, it's very tricky to estimate the effect of a step potential while standing on the ground close to a downed conductor. e.g. the gait / stride varies, ground resistivity varies etc.

They teach people who operate HV switches to keep their feet planted firmly together when operating the switch in case the earth-mat fails (or is missing - farmers love to plough up Earth mats near switches in fields)

Have you heard of fatalities due to the step potential in real life? i.e. People who didn't actually touch a conductor nor a piece of live equipment nor were in the water but got killed merely by the current flowing through the ground from one leg and out through the other?

I've heard of plenty of people getting to close to the safety clearance and getting fried - idiot metal theives.

And this
http://news.bbc.co.uk/sport1/hi/other_sports/horse_racing/9400599.stm

horses being much more likely to die than people

 

[rollingstein]

I've heard of plenty of people getting to close to the safety clearance and getting fried - idiot metal theives.

Indeed! But that's more like an arc from the conductor to the idiot's body.

 

[rollingstein]

horses being much more likely to die than people

Reminds me of another anecdotal story I've heard that stray potentials can cause cows to reduce milk production. Sometimes a tingle below the perception threshold of a human will be enough to severely impact the milk production of a dairy barn.

 

[jim hardy]

horses being much more likely to die than people

"Step" potential is called that because it's the voltage that would appear between a man's feet when he takes a step
as opposed to a step change in time
it'd be volts per meter or yard or foot ; volts/lengthclearly a horse has more distance between his front and rear feet than the span of a man's step
and the path from horse's front to rear legs goes through horse's chest, unlike a man on two legs,

Lightning strikes produce a lot of Earth current and it goes a surprisingly long way from the strike.
Once while i was swimming in a freshwater lake lightning struck about a mile away and the shock was surprising but not very painful. "Wow , " i thought, "glad that wasn't any closer !"Once upon another time when returning from a fishing trip in Everglades, with the afternoon thunderstorms menacing us from behind, we passed a flock of flamingos standing on one leg.

That's when it dawned on me, that one legged stance protects them from a shock should lightning strike anywhere nearby.
i wonder...

 

[rollingstein]

That's when it dawned on me, that one legged stance protects them from a shock should lightning strike anywhere nearby.

If a one legged stance would protect a flamingo from feeling a shock then how come a "zero legged" swimmer you still felt a shock?

 

[William White]

make up some numbers (just for example - it doesn't matter what the numbers are for illustration)

There is a lightning strike in the water which results in a voltage gradient. Say it is 1000 volts per metre in the region of interest.

The body of the swimmer is 1 m long. There is a voltage across the swimmer of 1000 V AND the flow of current is across his organs!

The flamingo has one leg in the water. Its tiny little foot is 2cm across. There is a voltage gradient of just 20V across its foot BUT the current will not flow into its organs because there is no circuit.

Its exactly why birds can sit on a live wire. The voltage gradient along the wire is (for all intents) zero. They sit on the wire and their entire body is at the same voltage as the line and there is no voltage gradient.

If it was a big bird (like a swan) and it opened its wings and touched the adjacent line then there is the full voltage across the bird body. Its fried.

 

[rollingstein]

Its exactly why birds can sit on a live wire. The voltage gradient along the wire is (for all intents) zero. They sit in the wire and their entire body is at the same voltage as the line and that voltage is fixed. No gradient.

But humans too can sit on live wires right? In spite of having much larger bodies. e.g. Those guys doing hot work on high voltage transmission lines? Or is their protective suit a Faraday cage?

 

[William White]

But humans too can sit on live wires right? In spite of having much larger bodies. e.g. Those guys doing hot work on high voltage transmission lines? Or is their protective suit a Faraday cage?

well they wear a protective suit but the principle is that same with the bird

at any two points on the same line the voltage difference is (for all practical purposes) zero (significant volt drops occur over very long distances - hundreds of metres).

What the linemen cannot do is touch two lines that are at different voltages.What is interesting, is that you will not see birds sitting on EHV lines (above 132kV). The air is conductive around the line and there is a voltage gradient in the air. When the birds get close enough it is very uncomforable for them and they fly away.slightly off topic - but if you have a flourescent tube and live near a supergrid line (400kV); walk under it with the tube and it will light up. You can plant the tubes in the ground and they will light up. An artist made a electric forest of flourescent tubes

http://www.richardbox.com/

 

[jim hardy]

If a one legged stance would protect a flamingo from feeling a shock then how come a "zero legged" swimmer you still felt a shock?

The voltage gradient was down to some number of volts per meter that's not lethal but still impressive .

I'm about 1.72 meters long , a bit more with arms extended
so whatever voltage was developed across the roughly two meters of water surrounding me was impressed across me.try a search on 'electrofishing'

 

[jim hardy]

But humans too can sit on live wires right? In spite of having much larger bodies. e.g. Those guys doing hot work on high voltage transmission lines? Or is their protective suit a Faraday cage?

try a youtube search on high voltage suit

great video here
www.youtube.com/watch?v=9tzga6qAaBA

 

[rollingstein]

try a youtube search on high voltage suit

great video here
www.youtube.com/watch?v=9tzga6qAaBA

@jim hardy

Oh that one's one of my favorite vids. But the thing that confused me is whether it's the faraday cage that protects him or the fact that he's always on an equipotential surface.

i.e. If someone tried to go on the live line using the same careful technique but sans the special suit, would he get electrocuted? I've always wondered about this.

PS. Are live lines hot to touch? How much would the surface temperature of a line like this one likely to be?

 

[William White]

The suit is used because of the potential gradient AROUND the conductor.

The air around the conductor has voltage gradient.

When the voltage is large enough, this gradient can become substantial

Birds can sit on a 11kv or 33kv line; they will not sit on a 132kV line

Likewise, the worker needs to be at an equipotential; so at lower HV, the suit is not needed (linemen use hot glove technque). at EHV, the voltage gradient is large enough to set up a current that can flow through the worker. I have spoken to live line workers and they say it feels like insects crawling all over.The temperature of the line depends on the current flowing through it.
You can think of the line as a single-bar electric heater.

A single bar electric heater is about a foot long and is rated at 1kWA 33kV is tens of miles long (supergrid line is hundreds of miles long)

Power is i^2 R (where R is the resistance of the line and i is the current flowing). Current can be several hundred amps. Its important that the line has as low a resistance as possible (the electricity company is losing money if the line is acting like a long bar heater!)

In short, no they do not get appreciably hot, there is just not enough power loss per short unit length. Icing can be a big problem in the winter: the weight of ice can bring the line down.

You can work out how hot they get compared to a bar heater. Just work out the reistance of the line. Say its 150 sq mm aluminium (for arguments sake). Say its 10 miles long. There you have your resistance. Then say it has 200 Amps flowing. You can then divide into a short length (say a foot, like a bar heater) and compare the power output to a bar heater.

Hot spots can occur at loose connections. Helicpoters regularly patrol EHV lines with an infrared camera to keep an eye on the growth of hot spots.

 

[rollingstein]

In short, no they do not get appreciably hot, and icing can be a big problem in the winter: the weight of ice can bring the line down.

Thanks @William White

The other problem that ice build up reminds me of it conductor gallop. I remember reading that when impending gallop risk is high due to icy conditions in an area the power dispatchers can, to some extent, route extra power through a branch bringing up I-squared-R heating to the point where it could melt off some of the ice. Not sure how realistic this is and whether dispatchers actually have the kind of routing flexibility needed to achieve this.

 

[jim hardy]

Oh that one's one of my favorite vids. But the thing that confused me is whether it's the faraday cage that protects him or the fact that he's always on an equipotential surface.

i.e. If someone tried to go on the live line using the same careful technique but sans the special suit, would he get electrocuted? I've always wondered about this.

good question.

I think of the phenomenon as one of capacitance , where the line's surface area is one plate of a capacitor and the rest of the universe forms the other plate

Think for an instant in DC - freeze frame your thinking... AC is after all at any instant going in only one direction
You saw the sparks to the helicopter as he attached the clamp to the HV wire.
That current brought the helicopter to same potential as the line. The helicopter became a 'wide spot' on the capacitor plate to which it connected.. the spark charged the capacitance of the helicopter to line voltage.

Now back to AC thinking: Since that helicopter's capacitance must be constantly charged between positive and negative high voltage as the power line cycles, AC current must flow. That's why the sparks persist, were that a HVDC line there'd be only one spark.

The lineman sits inside his HV suit which has capacitance to the rest of the universe. So charging current flows into and back out of his suit at line frequency. But since he's inside the suit which is an equipotential surface he experiences no voltage gradient.

Were he to grab the line without a protective suit, the current necessary to charge his body's capacitance would flow through his hand.
Would it be enough to feel ? I suspect so. Let's put a number on it.
I know from tinkering with analog meters that i can't feel 20 microamps but a couple milliamps will make me jump.

The Human Body Model is the oldest and most commonly used model for classifying device sensitivity to ESD. The HBM testing model represents the discharge from the fingertip of a standing individual delivered to the device. It is modeled by a 100 pF capacitor discharged
through a switching component and a 1.5k Ωseries resistor into the component.

http://www.esda.org/assets/Uploads/documents/FundamentalsPart5.pdf

at 60 hz, 100 pf = 26.5 megohms, compared to which the 1.5K resistance is insignificant
so 500 KV / 26 megohms = 19 milliamps, a surely painful shock. 20 ma through the chest can stop your heart.
Were the helicopter's capacitance added to the lineman's i expect it'd be lethal.The field way of thinking also explains it - the suit being conductive makes an equipotential surface , reducing field strength in its immediate vicinity to zero.

So the two ways of thinking - electric field or capacitance - lead to the same result. Either mental model works for me.

Birds begin avoiding power lines around 40KV i think it tickles their feet.

 

[rollingstein]

Now back to AC thinking: Since that helicopter's capacitance must be constantly charged between positive and negative high voltage as the power line cycles, AC current must flow. That's why the sparks persist, were that a HVDC line there'd be only one spark.

@jim hardy

Ah, that's a neat observation. I never realized that the duration of the spark (i.e. initial one time versus continuous) was related to AC vs DC.