ECN Electrical Forum
Discussion Forums for Electricians, Inspectors and Related Professionals

Some years ago Hawaiian Electric Company started suffering repeated trips on their main high voltage backbone.

They called in expertise from the Mainland.

He determined that salt -- ocean salt -- had been condensing down onto the pylons -- especially at night -- and accumulating until it compromised the dielectric strength of the design.

At great expense a helicopter crew was brought in from the Mainland. They were able to spray pure water -- even on energized lines -- and knock the salt levels down.

The expert informed HECO that such wash-downs are a standard routine elsewhere in the country.

The truck -- itself -- could have initiated the arc by tossing conducting/ionized particulates skyward. They would be strongly influenced by the A/C field.

Without knowing what the load was -- I can't reach any conclusion.

Salt fog is especially unfriendly to external insulation.

The insulating properties of air is about 1 inch of gap is around 5000 volts of insulation Now that is without an arc. So for your dump truck and spark gap he was likely less than 3 inches from the wire at the moment the arc struck. Next the nail in the tire would not reduce the trucks dielectric very much at all. since the frame is isolated by the tire at the axle. I don't find the truck drivers claim to be credible at all. unless their was a chain or something in contact with the ground and a space less than 3 inches from the box to the OH wire both occurred at the same time it would not likely have jumped across at all. A load break switch at that voltage uses simple arc chutes and air to break the flow and the blades only open a foot or so.
Once conducting the insulation value of air is a lot lower and might have been able to pull an arc for a couple of feet max. Limits of approach for that voltage is 10 feet. At that distance there should almost be no chance of jumping across that gap. In Cancun the plasterers I watched working on the exterior of a retail/condo was easily within 5 feet and no crispy plasterers in the area.

Unless there was a spider thread ,heavily contaminated by carbon particles, brought into the gap by crane.However 10 feet flashover is impossible at that voltage ,even in the worst case scenario.But 1 or 2 feet who knows..Such things are under research.

Did you guys ever see that video of a POCO station opening up a large switch the the arc jumped a long way? If not, I think I still have the video clip somewhere and I might be able to get Bill to post it for me. Or you can e-mail me and I can send it to you.

Harold:
I was at a PSE&G switching yard last week and they happened to 'open' a 235KV switch. It was a bright sunny day last week, but the arc was still awesome! I'd guess somewhere between 6-10'.

harold;HotLine,

Yes of course,I've seen HV switches actions in RL too.However,opening HV switches up and streching up long arcs this way is a different thing.
As regards maximum sparkover (flashover) distance ,so called thermodynamic "inertia" of arc is not the cause but consquence.