See video. As helicopter approaches high-voltage transmission line, the worker is holding out a probe; at 0:15, when it gets within the final couple of feet of the wire, there’s some arcing, and after the probe finally makes contact, the worker leaves the helicopter and gets onto the line.
What is the probe for, and what’s with all the arcing? The helicopter is ungrounded - it’s hovering several yards above the ground. It’s not about static electricity, or helicopters would do this every time they landed. The line is presumably AC, so the voltage is fluctuating back and forth, so they surely aren’t trying to bring the heli to the same potential as the line. Except that sure does seem to be the case.
So what gives? Why is there arcing between the probe and the line?
The helicopter is 0 volts… The line is alternating high voltage. The probe brings the helicopter and workers up to the same voltage as the line (potential). Thus the initial arc.
Yes, it’s AC. So, once the helicopter is in firm contact with the wire, there will be alternating current flowing in/out through the connection, back and forth. But just before firm contact, this current tries to flow through an air gap - and eventually succeeds, when the gap is narrow enough. You want to make sure this doesn’t happen near the operator.
So based on this and on herman and bill’s post, if the helicopter hovered perfectly still so that the probe tip was a few inches from the wire, there would be constant 60Hz arcing going on?
Given that there’s current flowing into the body of the heli, does this mess with any of the onboard electrical systems in any meaningful way, or is the current all confined to the exterior surface?
It’s static, isnt it? The lines are insulated from the earth, so they can hold a considerable static charge. The helicopter is at a different charge state.
AIUI, the lines are not de-energized for this type of work; they are being driven by the AC voltage from the alternator at the power plant. So no, not static electricity at all.
It’s not the primary effect in this case. Static is still a real issue though. Properly grounding a hovering helicopter is a big safety concern during military sling load operations.
I know that if a helicopter drops a cable down to pick something up, you want to let it touch the ground before you touch it. Otherwise, you get quite a shock.
Wouldn’t the line, the helicopter, and the air form a capacitor? If so wouldn’t the probe act as a short to equalize the charge density on the helicopter and line?
I bet that arc is not good to look at for prolonged periods.
Birds probably do not carry a static charge like a helicopter. On the other hand, have you ever seen a bird sit on a high voltage line? My WAG is the line’s electrical charge might interfere with a bird’s internal (and sensitive) navigation so birds just don’t even alight on high voltage lines.
Just to be clear, after they use the probe for the initial arcing-connection you’ll notice they attach an actual ground cable for the duration of the work. This is the first clip I’ve seen where the helicopter leaves the workmen there, usually it just remains hovering with the ground cable is attached to the chopper itself while the guys work quickly.
In this seen fromThe Hunt for Red October they fail to initially ground the helicopter with a probe first and you see a large static charge jump to the First Officer when he accidentally makes contact. Right before this scene Scott Glenn’s character says something to the effect of, “The helo’s blades will be generating a hell of a lot of static electricity…” but I think most people didn’t really understand what happens (I didn’t the first time I saw it).
Yeah, no offense to Alaskan crab fishermen but this has got to be one of the most hazardous jobs in the world.
Well its not a ground cable, but I guess the importance is clear.
Without the conductor, the arc would otherwise go through the person, and that is bad…
The reason for the continuous arc is a form of capacitance. Due to the skin effect, the large surface area of the helicopter is holding onto more charge , like a capacitor would, than you expect …
Rescue helicopter people also have to ensure the helicopter is discharged from static electricity before they bridge the gap to ground, because a single static charge packs a whallop, the build up is worse near water… (Same reason rain clouds cause lightning… more static in them )
When I was in Colorado Ground Search and Rescue (an arm of CAP), we did some practice drops from a Chinook on the jungle penetrator.
You get winched down through the center body of the helo (it’s very windy ). We were told to NOT touch anyone coming down on the line until they hit the ground.
Same with sailing. I remained on my brothers boat for the entire time from water to trailer. Once on land, I climbed down the back ladder to the ground. When I hit the ground, I hit the ground. Actually did knock me on my ass.
). We were told to NOT touch anyone coming down on the line until they hit the ground.