Why the arc when bonding-on during high voltage repairs?

Question

When working on high voltage power-lines via helicopter the technician will "bond-on" with a steel wand called a hot-stick. During this process an arc is conducted from the line to the wand. Now, since the helicopter is floating both literally and electrically, why is there any potential between the line and the wand?

My guess is that because of the alternating electric current in the power-line there is therefore an alternating expanding and collapsing magnetic field present in close proximity to the helicopter. This sets up a charge polarization in the frame of the helicopter 180 degrees out of phase with the current and therefore an electrical attraction occurs which produces an electric potential.

If this is true, even though the electrons are not free to move in a bird (as they are in the metal helicopter frame) they are able to rotate and still set up a charge polarization (much like a charged balloon sticks to a wall) and therefore when a bird "bonds-on" to a wire don't you think they feel a little tingle at first?

One other thing. Is it true that by virtue of the fact the the helicopter circuitry is an isolated one, they can bond-on to either line and that line will be the ground reference for the ground of the helicopter circuitry?

Answer 1

This question is somewhat related to yours.

The phenomenon has nothing to do with magnetic fields. There is an intense electric field around any high-voltage wire (AC or DC), and the intensity is inversely proportional to the distance from the wire. At very close distances, the intensity exceeds the breakdown threshold for the air itself.

The presence of the conductive helicopter (and the repairman with his metal mesh suit) distorts this field, concentrating it even further in the space between the wire and the helicopter. When the bonding strap gets close enough, an arc forms. The current in this arc is limited by the overall capacitance of the helicopter.

Once the bonding connection has been made, the helicopter and the mesh suit are all at the same potential as the line itself. The electric field is still distorted, but now the concentrated areas are moved away from the repairman and the connection between the wire and the helicopter.

And regarding the birds, when have you ever seen a bird sitting on one of these high-tension transmission lines?

Answer 2

Even floating objects have capacitance. For example clouds which have huge capacitance and the charge equalization current ( cloud to cloud lightning) can be >> 10KA depending on the ionization path resistance. But heli's being a relatively "large bird" will conduct a fair amount of current to reach the same potential as the UHV line whether it is AC or DC. Once "bonded" the extremely low frequency 50/60Hz and mesh suits protect the operators. As an aside high electric fields of DC or ELF (extra low) have no detrimental effects on humans unlike RF from sustained arcing.

*added I read from ABB it costs $10k~20K to train an operator for live wire maintenance with 2 yrs prior experience at lower levels. So information is not freely available. They once relied on live hand connections since 1989 with a Faraday suit but now use the fiberglass rods with special conductors for connect and disconnect instead.. They don't specify details or differences between live servicing say 400 kVac lines vs UHVDC at 800 kVdc. I suspect servicing HVDC has the advantage of only having initial discharge/charge charge arc currents and DC leakage determined by dust and humidity, so I suspect servicing is limited when risk of rain is 0%. However AC lines may have more leakage due to capacitance to free-space of the Helicopter. Since the wavelengths are extremely long there is not much difference in electric fields, but as Dave said high gradients before contact. as there is in leakage current due to series capacitance of the rod to the body capacitance of the Heli.

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More Aside notes... EMI affects humans roughly proportional to f until you get to infrared wavelengths then it picks up again starting with UV and continues to get even more sensitive to gamma ray wavelengths. I don't have the latest peer reviewed journals on non-ionizing risks, but there may be plenty of scare-ware reports. Microwave Engineers are extremely cautious about cell tower design to minimize risks to humans from dispersion to near field ground radiation and apartment spacing. Continuous exposure to Tesla generators with high current arcs are not good. Whereas low current plasma balls are harmless. Power Line operators are extremely careful about the cleanliness of their grounding safety rods that may reduce the dielectric strength.

I recall a Popular Electronics story decades ago, that warned people about AC power panels near bedrooms causing cancer in the home. I remember hearing circa Y2K that it was "fraud" and the perp. "scientist" was charged for defrauding the Gov't of grant money. Similar studies have been done on Cows. But the huge EM wavelengths create little potential risk to humans ( pun intended) HV lines are restricted from property development in most areas due to extreme weather on safety of fallen lines from Ice, tornadoes or hurricanes. Its not the HV ELF fields you have to worry about, but rather continuous arcing fields which contain RF, which if sustained can cause red eyes or worse fried brains in the case of Tank operators during WWII with high power transmitters behind their brain causing them to pass out to safety below deck.

So how much current does it take to charge up on connection? I think that would depend on the altitude & relative humidity and the coupling capacitance of air dielectric to the surface area of the floating "bird" capacitance is the other factor. So it is not truly "floating" unless you are a really tiny bird with no rotating "wings" that create also tribo-electric DC charges.

The altitude effect is relative to ground. As the chopper aproaches ground < 100ft it's capacitance increases rapidly from "free space" values thus as V=Q/C the charge Q results in a rapid decrease in voltage as C increases. In any case the landing feet touch first to discharge the bird and I have never heard of arcing on touch-down but this another question, but this effect reduces the risk.

Answer 3

It's about accumulated charge. While the helicopter itself can accumulate charge (rotors moving past air) and the high voltage leads themselves possibly will have a dc voltage (transformers don't pass DC). There is an additional factor though that you may not be aware of, the very high voltage transmission lines can be DC.

Aerial proximity fuzes in anti-aircraft rounds work on these principles. As the shell approaches the aircraft the charge differential as expressed in the electric field is detectable. When a certain threshold is reached the bomb goes off.

Answer 4

With regard to helicopters discharging as they touch down:

The passage of the rotary wings ("blades") through air is enough for them to build up a substantial static charge through friction with air molecules and for this reason alone if a hovering chopper drops a line, it's advisable to allow it to hit the ground before touching it.

Careless rescue personnel have been known to be knocked off their feet by the discharge, but it's not so much of a problem in wet weather (for obvious reasons)

The same thing happens on fixed-wing aircraft, which is why many have corona discharge wires on the trailing edge of their wings, for when the charges get to extreme levels.

Free-space coronal discharge is also known as "St Elmo's Fire" and can be quite damaging to an airframe.

See http://www.google.co.uk/patents/US3260893, which goes into a lot more detail about the phenomenon and mitigating it.

The visible arcing when bonding out is simple DC potential equalisation between the helicopter and the line (usually it's the helicopter at higher potential, bearing in mind that for AC lines the average potential of the lines is "ground level"). It's not particularly dangerous (only a few mA involved), but when you're that high off the ground you don't want anything to happen which might cause line staff to let go of whatever they're holding onto, should they get a jolt.

(If you think helicopter work is wild, some UK maintenance teams specialise in climbing towers and then climbing along the insulators onto live wires!)