What happens if "hot" wire touches Earth ground?

[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.