Still confused about how a helicopter bonds to a transmission line

Amped_86

Joined Jun 28, 2018
26
I posted a thread a few days back wondering about how a helicopter bonds to a transmission line and I'm still not quite clear on the matter. I'm approaching this as simply as I can.

In the education section on this website there is some great information regarding how birds can land on transmission lines without getting shocked. They don't get shocked because the voltage is the same at every part on the line. Even the points of the line on which the bird's feet land are electrically common so even though a path exists between the bird's feet for current to travel, there is no voltage between the bird's feet and therefore no current.

Moving to the helicopter I thought I could just thing of the helicopter as a large bird that experiences the same thing when 'landing on' or touching the transmission line.

But when I watch the Youtube videos, a fairly substantial arc occurs whenever the helicopter contacts or removes contact with the line (I'm referring to the stick the lineman uses). I'm assuming that there is current travelling between the line and the helicopter and I'm assuming it's quite a bit of current as the arc seems quite substantial. Is this wrong?

In the initial post several people said that the helicopter can be thought of as one plate of a plate capacitor (the ground being the other plate). I'm not really able to fully comprehend a capacitor of this size (compared to capacitors I'm familiar with the gap between the ground and helicopter seems quite large and how is the area of the ground plate determined?) so I'm not sure if the capacitance would be large or small.

Even so, I'm not sure what capacitance has to do with the scenario anyway. If the helicopter is one plate of a capacitor and the arc represents the charging and discharging to match the voltage of the line, is current still not flowing through the helicopter during the charging and discharging processes? And again, if so, it seems like quite a bit of current judging by the arc. Wouldn't this be bad for the workers inside the helicopter?

AlbertHall

Joined Jun 4, 2014
12,269
Look up 'faraday shield' and why one of the safest places to be in a thunderstorm is in car.

Ylli

Joined Nov 13, 2015
1,069
When the helicopter is bonded to the line, there *is* a current flowing through the helicopter, a capacitive current. The helicopter is one plate of a capacitor, and the the earth is the other. Yes, it is a very small capacitance, but there is also a rather high voltage.

Let's try 10 pF as a guestimate for the capacitance.

Xc= 1/(2piFC) = 1/(2*3.14*60*10e-9) = 265 K ohms.
If the line voltage is 750 KV, then the capacitive current will be"
I = E/Xc = 750,000/265,000 = 2.8 amps.

The lineman does not want to be standing on the helicopter and touching the wire if the bonding connection opens.

OBW0549

Joined Mar 2, 2015
3,566
Let's try 10 pF as a guestimate for the capacitance.

Xc= 1/(2piFC) = 1/(2*3.14*60*10e-9) = 265 K ohms.
If the line voltage is 750 KV, then the capacitive current will be"
I = E/Xc = 750,000/265,000 = 2.8 amps.
10 pF is 10E-11 farads, not 10E-9.

Still, your point is valid, even if off by a couple of decimal places.

@Amped_86: Simply put, bonding the helicopter to the transmission line with a cable & clamp equalizes the potential between the two, meaning the lineman never has to bridge a large potential difference when moving around as he does his work.

It really is that simple.

Ylli

Joined Nov 13, 2015
1,069

OBW0549

Joined Mar 2, 2015
3,566
Good point; I should have written that as 1.0E-11.

Amped_86

Joined Jun 28, 2018
26
Look up 'faraday shield' and why one of the safest places to be in a thunderstorm is in car.
Thanks for the advice. I didn't know about Faraday Shields or Cages but after studying them a bit things are making more sense. A Faraday Cage uses electrostatic induction to cancel out the applied field the cage is subject to inside of the cage. So in the case of a car or airplane that is struck by lightning, the metallic body of the car or airplane acts as a Faraday Cage causing the charge to flow on the outside of the metallic body but not on the inside, keeping the contents inside of the cage (people) safe.

In the helicopter transmission line videos that I've watched, they've mentioned in a couple of them that the current/charge is flowing 'around' them rather than through them. They also mention the use of Faraday suits which is essentially a fine mesh Faraday Cage that a person can actually wear. The suits must allow for current to travel around the wearer of the suit rather than through them.

So going back to my initial question about the arc, can I say that the helicopter is acting like one plate of a capacitor, drawing current to match the voltage of the line, but it's also acting like a Faraday Cage and the workers are wearing Faraday suits which causes the current to travel harmlessly around them rather than through them? Also, in the case of a car or plane struck by lightning, where does the excess charge go? Does it just dissipate into the air?

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BobaMosfet

Joined Jul 1, 2009
2,082
I posted a thread a few days back wondering about how a helicopter bonds to a transmission line and I'm still not quite clear on the matter. I'm approaching this as simply as I can.

In the education section on this website there is some great information regarding how birds can land on transmission lines without getting shocked. They don't get shocked because the voltage is the same at every part on the line. Even the points of the line on which the bird's feet land are electrically common so even though a path exists between the bird's feet for current to travel, there is no voltage between the bird's feet and therefore no current.

Moving to the helicopter I thought I could just thing of the helicopter as a large bird that experiences the same thing when 'landing on' or touching the transmission line.

But when I watch the Youtube videos, a fairly substantial arc occurs whenever the helicopter contacts or removes contact with the line (I'm referring to the stick the lineman uses). I'm assuming that there is current travelling between the line and the helicopter and I'm assuming it's quite a bit of current as the arc seems quite substantial. Is this wrong?

In the initial post several people said that the helicopter can be thought of as one plate of a plate capacitor (the ground being the other plate). I'm not really able to fully comprehend a capacitor of this size (compared to capacitors I'm familiar with the gap between the ground and helicopter seems quite large and how is the area of the ground plate determined?) so I'm not sure if the capacitance would be large or small.

Even so, I'm not sure what capacitance has to do with the scenario anyway. If the helicopter is one plate of a capacitor and the arc represents the charging and discharging to match the voltage of the line, is current still not flowing through the helicopter during the charging and discharging processes? And again, if so, it seems like quite a bit of current judging by the arc. Wouldn't this be bad for the workers inside the helicopter?
I'm going to explain this simply, because it is a simple concept. That doesn't mean it isn't dangerous. This brave men sometimes have things go wrong, and electricity at these levels is absolutely unforgiving.

Every single physical thing has an electric or electromagnetic charge. This simply because it has electrons. The _only_ reason electrons move anywhere is if they can connect the item they are in through a medium (a dialectric, like air or any physical medium) to another (one or more) item(s) that have fewer electrons. Once the connection is made, the electrons move- a lightning bolt is an example (just an oversized spark).

A helicopter has it's own electric charge, aka 'potential'. This is different than the potential in a transmission line. The transmission line is isolated from ground. The helicopter is isolated from ground. If the two get close enough, a spark will jump between them, usually lethal to the lineman and chopper pilot. So what a lineman, wearing basically a chainmail isolation suit (to make the current flow around his body, not through it) uses a dialectric probe to reach out and place it within harms way, rather than himself, so that the line voltage will leap to the probe, current will flow, and then both the chopper and the line are at the _SAME_ potential. Where the potential is the same, the current will not flow and so now, the helicopter is part of the transmission line circuit. Upon completing a repair, or inspection, the lineman then removes the probe, breaking the circuit, and the chopper will move away and dissipate charge.

How the above may apply in circuits:

Now, use what you've learned to apply to a circuit you build. The reason ground is tied together in circuits is because everything can have it's own voltage level. If we didn't tie them together through a common point, you could have unknown different potentials and those circuits would not work, or worse- they could be dangerous. Ground enforces a _common_ reference point, just like the probe, so that everything operates from the same potential.

People talk about voltage, but don't really understand it. Voltage is not real (like a chair or an atom or part of an atom). It is a man-made concept to describe the difference between n-points of differential electron charge. It represents a ratiometric value- that's what Ohm's Law expresses. A voltage level is a value that lets us express a value of charge differential, aka 'potential' for electrons to move if a path is made. In some cases potential can be increased such that the electrons will move via almost non-existent paths- like through the air (which takes about 30K-50K volts if potential to make move).

In daily lives- Remember the 70's-90's computers. Every time you touched a keyboard or mouse a spark happened and the computer would reset? Basically there was a voltage difference high enough between you and the computer that when you got close enough, a spark formed, electrons flow, and charge between you was equalized. This is why technicians touch metal objects near devices they work on constantly, or carry grounding FOBs, or grounding straps. To do exactly what the lineman is doing.

wyzarddoc

Joined Jul 30, 2018
3
Why has no mentioned the emf generated by the high voltage power line and the blades of the helicopter?? Or the fact the helicopter is generating it's own electricity by induction in the field.

WBahn

Joined Mar 31, 2012
29,167
I posted a thread a few days back wondering about how a helicopter bonds to a transmission line and I'm still not quite clear on the matter. I'm approaching this as simply as I can.

In the education section on this website there is some great information regarding how birds can land on transmission lines without getting shocked. They don't get shocked because the voltage is the same at every part on the line. Even the points of the line on which the bird's feet land are electrically common so even though a path exists between the bird's feet for current to travel, there is no voltage between the bird's feet and therefore no current.

Moving to the helicopter I thought I could just thing of the helicopter as a large bird that experiences the same thing when 'landing on' or touching the transmission line.

But when I watch the Youtube videos, a fairly substantial arc occurs whenever the helicopter contacts or removes contact with the line (I'm referring to the stick the lineman uses). I'm assuming that there is current travelling between the line and the helicopter and I'm assuming it's quite a bit of current as the arc seems quite substantial. Is this wrong?

In the initial post several people said that the helicopter can be thought of as one plate of a plate capacitor (the ground being the other plate). I'm not really able to fully comprehend a capacitor of this size (compared to capacitors I'm familiar with the gap between the ground and helicopter seems quite large and how is the area of the ground plate determined?) so I'm not sure if the capacitance would be large or small.

Even so, I'm not sure what capacitance has to do with the scenario anyway. If the helicopter is one plate of a capacitor and the arc represents the charging and discharging to match the voltage of the line, is current still not flowing through the helicopter during the charging and discharging processes? And again, if so, it seems like quite a bit of current judging by the arc. Wouldn't this be bad for the workers inside the helicopter?
If the helicopter is neutrally charged, then there would be little to no arc when the helicopter is bonded to the transmission line. But helicopters are great static electricity generators due to the friction of the rotors and the air -- particularly if the air is very dry. So the helicopter almost always has a significant static charge on it when the bonding strap contacts the transmission. Also, the transmission line itself can carry a significant static charge. The arc is simply the result of the static potential difference caused by the different static charge levels equalizing.

Joined Jan 15, 2015
7,220
Why has no mentioned the emf generated by the high voltage power line and the blades of the helicopter?? Or the fact the helicopter is generating it's own electricity by induction in the field.
I believe that was mentioned in the previous thread on the subject the thread starter mentions in his first post. That would be this thread.

Ron

Amped_86

Joined Jun 28, 2018
26
@
I believe that was mentioned in the previous thread on the subject the thread starter mentions in his first post. That would be this thread.

Ron
I apologize for starting two threads that are the same, I was getting a little overwhelmed with all of the info offered in the first one. I think I more or less have my answer. The part that was tripping me up was if there's an arc, then there must be some current flowing. I was wondering how the workers survive this current flow and I think the answer is in the form of the helicopter acting as a faraday cage and the use of faraday suits by the workers.

Joined Jan 15, 2015
7,220
No need to apologize my post was to inform the poster before me as to why the rotating blades of the helicopter weren't mentioned. No problem at all.

Yeah, I believe you pretty much have it. Like as mentioned once the helicopter and line are at the same potential no problem like a bird on a wire. Pretty cool stuff really. A Google of Faraday Cage should yield some pretty cool images if I recall correctly. The Wiki has a few.

Ron

Amped_86

Joined Jun 28, 2018
26
Why has no mentioned the emf generated by the high voltage power line and the blades of the helicopter?? Or the fact the helicopter is generating it's own electricity by induction in the field.
As you mentioned there seems to be more going on below the surface (induced voltage of the helicopter) than just the voltage difference between the helicopter and line. I imagine AC and DC lines would affect the bonding differently. I'm pretty simple so I was just looking for a simple answer as to why the current that flows when the arc is created doesn't electrocute the workers and I think it's because of the faraday cages and suits.

wyzarddoc

Joined Jul 30, 2018
3
Maybe this will help!! The power lines that require helicopter linemen are 13kv or more at 60 Hz. This produces lines of flux according to Gauss's law. This means any conductor crossing the lines of flux will generate a electrical charge. The helicopter cannot hold completely still so it is generating an electrical charge. The blades of the helicopter rotor also generate a electrical charge in addition to a static electrical charge that is normal for rotor blades. ( this is why helicopters always drop a rope to ground before air-lifting a payload to disperse the charge) The electrical discharge observed is probably very high voltage going from the wire line to the helicopter and very low current. In any event the insulated pole that is used to connect the helicopter to the wire line is designed to not transfer the charge to the lineman but rather to the helicopter aluminum structure. The aluminum helicopter then distributes the charge since the lineman is in contact with the helicopter he is brought up to the same potential as the helicopter so can safely get out and work on the line. The lineman is just another item in the helicopter just like the pencils charts and tools. As long as the lineman doesn't get between the helicopter and the wire before they reach equilibrium he/she will be safe on the other hand if the lineman completes the circuit between the helicopter and the line before equilibrium they are toast.

KeepItSimpleStupid

Joined Mar 4, 2014
5,088
On the other end of the extreme where normal people don't have to deal with it. A wire blown around by a fan generates current. it's tiny, on the order of 1e-12 Amps, but it's there. It's especially there when your trying to measure around that current (Say 10 pA). Physics says a conductor in the earth's magnetic field would generate a current.

When I got annoyed enough, I started to do measurments of charge (picocoulombs to NANOcoulombs) and divided by the time to get current. It worked a lot better.

Triax cables are used and they have graphite between the shields and the wire. That reduces the tribo-electric effect. The outer shield is at ground and the inner shield is at the potential of the inner conductor, so very little stray current in the cables

This was in a small environmental chamber and the wires had to be taped down. You had a fan and vibration to deal with.

KL7AJ

Joined Nov 4, 2008
2,229
I posted a thread a few days back wondering about how a helicopter bonds to a transmission line and I'm still not quite clear on the matter. I'm approaching this as simply as I can.

In the education section on this website there is some great information regarding how birds can land on transmission lines without getting shocked. They don't get shocked because the voltage is the same at every part on the line. Even the points of the line on which the bird's feet land are electrically common so even though a path exists between the bird's feet for current to travel, there is no voltage between the bird's feet and therefore no current.

Moving to the helicopter I thought I could just thing of the helicopter as a large bird that experiences the same thing when 'landing on' or touching the transmission line.

But when I watch the Youtube videos, a fairly substantial arc occurs whenever the helicopter contacts or removes contact with the line (I'm referring to the stick the lineman uses). I'm assuming that there is current travelling between the line and the helicopter and I'm assuming it's quite a bit of current as the arc seems quite substantial. Is this wrong?

In the initial post several people said that the helicopter can be thought of as one plate of a plate capacitor (the ground being the other plate). I'm not really able to fully comprehend a capacitor of this size (compared to capacitors I'm familiar with the gap between the ground and helicopter seems quite large and how is the area of the ground plate determined?) so I'm not sure if the capacitance would be large or small.

Even so, I'm not sure what capacitance has to do with the scenario anyway. If the helicopter is one plate of a capacitor and the arc represents the charging and discharging to match the voltage of the line, is current still not flowing through the helicopter during the charging and discharging processes? And again, if so, it seems like quite a bit of current judging by the arc. Wouldn't this be bad for the workers inside the helicopter?
The Helicopter has a certain amount of capacitance simply because of its surface area. The arc you see on the shorting stick has a whole lot of voltage, but very little current. (The Helicopter probably has only a few hundred picofarads of actual capacitance).

The "trick" is just to bring the helicopter to the same potential (average, because it's AC of course!) as the power line....just long enough for the guy to step off onto the wire.