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Power line workers and voltage

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Bhiggs
#1
May9-12, 08:32 PM
P: 2
I was watching a fascinating video about the people who work on live power lines- they are lifted to them by helicopter and wear metal-fabric jumpsuits in order to create a Faraday cage around their bodies so they can work on the live wires without getting shocked. But then I started thinking about voltage in general- do they really need to wear the suit?

All of the simplified examples that are used to explain current talk about the grounding effect- a current is created between a voltage source and the ground since the ground has a voltage of 0V. But if the worker accesses it by helicopter, he doesn't have that voltage difference, so a current shouldn't run through him anyways (bird on a wire don't need a metal jumpsuit!)

But then, what is the voltage of a person who approaches a live wire (or a van der graaff generator, or a socket, or anything with charge?) The demos always show that a person who is insulated from the ground can touch the charged object (at least for the examples I've seen when it doesn't have a huge amount of charge- like a Van der Graaff generator) - but the person must have SOME voltage difference, or else we would be walking around holding 20,000V! But it's not 0V, because as long as the person is insulated they don't get shocked, the charges just collect around them (the typical hair standing on end.)

So my questions: Is the faraday suit really necessary? And what is the voltage of a person insulated from the ground?
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256bits
#2
May9-12, 08:52 PM
P: 1,414
well, they wesr the suit so there must be some reason ither than just looking 'cool'.

In the video, you might see a probe being sent from helicopter to the live wire so as to equalize potentail before the workers embark. The metal suit, rather than being a Farady cage, ( was that what they said in the video?or your term ?) is so the worker ( or suit ) is at the same potential as the helicopter and wire. It is more easily done to equalize potential when everthing is metal.

Now considering that I am not a hightension engineer or worker, there may be other reason to having to wear the suit.

I would suspect that the helicopter when landing back down to the ground would also have to equalize potentail before the workers disembark so as to avoid a shock from residual charge collected onto the helicopter.
Bhiggs
#3
May9-12, 09:32 PM
P: 2
The video called it a Faraday cage. And yes, there was a probe that contracted some visible charges. Does that mean that the copter was 0V? Even though it was off the ground? Or maybe just lower voltage than the wires...

willem2
#4
May10-12, 01:20 AM
P: 1,395
Power line workers and voltage

A helicopter will have a self-capacitance of 100 pf or so. A current

[tex] I = C \frac {dV}{dt} [/tex]

must flow to keep the helicopter at the same potential as the wire.

For a 380 kV 60 Hz line

The maximum I = 10^-10 * 3.8 * 10^-5 * 2 pi * 60 = 14 mA.

If the current has to go through a person, this would also dissipate approx.
14 mA * 380 kV = 5.5 kW, most of at probably at the point of contact.
Bobbywhy
#5
May10-12, 02:35 AM
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Particles present in the air and coming in contact with helicopter rotor blades while the helicopter is in flight cause an increase in the static-electricity charging current and result in an increase in the accumulated voltage on the helicopter. Static wicks containing resistors installed on fixed points of the aircraft are installed to bleed off charge in forward flight, but rarely discharge the static buildup totally.

Connecting the helicopter to the power line has to do with the electrostatic charge on the helicopter. The aircraft can carry hundreds of kilovolts in some conditions and this must be discharged safely before the worker touches the power line. We would not want that worker to discharge this great static charge. See the video below.

http://www.youtube.com/watch?v=K3djcxUCcvQ
davenn
#6
May10-12, 03:18 AM
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Quote Quote by Bhiggs View Post
..........But if the worker accesses it by helicopter, he doesn't have that voltage difference, so a current shouldn't run through him anyways (bird on a wire don't need a metal jumpsuit!)
Ahhhh but you wont see birds pirched on EHT power lines. ( well I have never have anyway ) 120, 240 even 400V yes
The electric field around a multi kV power line is so hi that they easily sense it and stay well away.



cheers
Dave


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