How high is that wire... REALLY?

[Jim Cate]

Originally Posted by Dsanduril

Jim,

no personal experience at that location, but I've been in similar 'energizing' situations with our own 20m airdraft.

Have you seen this from RMS?

Upper-Clarence-River-insets-Grafton-Brushgrove-Ulmarra.pdf

It shows 17m.

My electronically updated chart NSW2C6 also shows 17m.

Lucas' 6th Ed. Update issued late 2017 says:



Sorry, but it looks like all the 'official' sources now say 17m.

Interesting... the cruising guide published by or under the guidance of the Clarence valley council, Clarence tourism and Australian Government Department of Resources, Energy and Tourism has the same chart, same inset, same appearance... and it says 21 meters!

Shows ya, never trust the gov or someone promoting tourism!

Thanks for the link.

Jim

[Snore]

Jim


Power lines have a “kill zone”. It has been a while since I retired. As I recall a bear wire 13Kv line will arc 10 or 15 feet onto a portable (truck mounted) crane.

If there is a sign with a stated max air draft, it is foolish to exceed it. Yes the mast may fit under, but the electricity could arc to the mast. It yields almost the same result as direct contact!

[valhalla360]

There is likely some fluff in the measurement as they have to account for expected changes in water level and with a long span temperature can even change the sag as the lines get longer as they get hotter.

But as mentioned, if it's high voltage, you don't have to touch it to get an arc.

[Dave_S]

Jim you probably remember my post on here about the lines at Russel Is that you helped me with they were said to be 20ish. When I finally got there they were more likely 30m. I could easily see there was a big gap from the deck looking up. Not sure about how far the power can jump but I assume less than from the cable past the insulator to the tower.

One of you could watch from the tender to get a better idea?

P. Ś-. I think you reminded me how big a deal it would be if I got it wrong :-)...........back at us :-)

[Snore]

Originally Posted by Dave_S

Not sure about how far the power can jump but I assume less than from the cable past the insulator to the tower.

One of you could watch from the tender to get a better idea?

P. Ś-. I think you reminded me how big a deal it would be if I got it wrong :-)...........back at us :-)

Your assumption is not correct. It can arc further than the insulator -

[Dymaxion]

Silly idea... tie a childs helium balloon to some fishing line at least a couple of meters longer than your air draft and take it under the wire via dinghy? Calm winds needed.

[Bill Seal]

If you use a balloon, just don't use a metallized Mylar one.

[transmitterdan]

The balloon idea is a bad one. Even a rubber balloon can cause an arc and the string will carry the current right down.

Specified draft under HT is not the same as under a bridge. The minimum gap below HT wires is based on some estimate of the distance needed to avoid risk of arcing to sharp objects (e.g. antennas) at the operating line voltage. It sounds like the new 17M distance is due to a revised formula rather than a change in the actual structure.

I am a crazy engineer and if I had to get under the wire i would remove the VHF whip and wind gauge from the masthead and steer as far from the center of the span as possible based on depth.

[Briney Bug]

One aspect of getting under high-tension wires that is often overlooked is allowing some distance to negate arcing. How much? The minimum would be the same as the distance that the wires are separated among themselves, though this is difficult to establish from the boat.
Check the chart, if a bridge is adjacent, then the minimum distance would be that of the bridge's clearance and the wires' clearance; usually 15 feet. So 15 feet is the generic amount that we use unless we have figures that suggest different.
Rudy Sechez

[Kita]

I ran across a wire once that the chart said was 45ft,from a distance it looked like more,my clearance is 47, so I called a marina nearby & they had the same info but I should call coast guard. ..lol ,they had no clue but did tell me the name of local electric company. ..so I called them & they have accurate data and told me it was 50 ft. Even though the chart said 45. Then the electrical people said well there's another 15 ft clearance as to what is shown on the chart as an extra space as an air gap so there will be no electrical discharge to a mast,they assured me I was good.... it helps to have accurate data

[oldjags]

The wires are spaced a good bit farther apart than the maximum possible arc length. If not, every time a bad thunderstorm came through, the lines could short out.

Starting an arc requires a much closer approach than maintaining one does. You have to actually get pretty close to the wire before it will arc, but once the arc has started, the ionized gasses will maintain the current flow even if you pull away.

What is that device called in the physics lab, that has 2 vertical wires arranged in a V-shape? At the base of the V, there is a small gap that is just wide enough for the current to arc across from wire to wire. Once the arc starts, the hot plasma arc rises up the V until the gap is too great. The arc collapses and a new one immediately starts at the bottom. You see them all the time in mad scientist horror films!

[Dave_S]

Originally Posted by Snore

Your assumption is not correct. It can arc further than the insulator -

Not being a smart ar$e but why would the electricity choose to arc to a boat with an incomplete path to earth over a nicely earthed steel tower.

[stormalong]

Originally Posted by oldjags

What is that device called in the physics lab, that has 2 vertical wires arranged in a V-shape? At the base of the V, there is a small gap that is just wide enough for the current to arc across from wire to wire. Once the arc starts, the hot plasma arc rises up the V until the gap is too great. The arc collapses and a new one immediately starts at the bottom. You see them all the time in mad scientist horror films!

It is called a Jacobs ladder.

[transmitterdan]

Originally Posted by Dave_S

Not being a smart ar$e but why would the electricity choose to arc to a boat with an incomplete path to earth over a nicely earthed steel tower.

The physics are a bit complex. It all comes down to the electric field strength and how much electric field air can take before the molecules are ripped apart which leads to the arc. If you look carefully at high tension insulators you will often see smooth metal rings at each end of the insulator. Also, many insulators have special shapes called "skirts". These shapes are specially designed to reduce the electric field and thus arcing. The skirts also help shed water during rain.

A mast on a boat has pointy things sticking up like antenna and wind instruments. Pointy things concentrate the electric field thus increasing the chance of an arc. Electricity will jump a longer distance to reach a pointy thing than it will jump between nice smooth things. It also depends on the temperature and humidity of the air. So it isn't a simple thing to calculate exactly where or when an arc will occur. Thus the calculation of minimum clearance has to have a safety factor added.

A boat on the water is at almost the exact same potential as the tower. The difference is so small as to be negligible.

[hellosailor]

As Snore said, high voltage can leap quite a ways to your mast and then to earth. That will depend on the humidity in the air, the materials, and of course the warmer it is (air temp or cable temp due to heavier loads) the lower they will hang.

Definitely worth standing clear.