An overhead line carrying 13,200 Volts arced to a construction dump truck today when the operator raised the dump bed near the line. There were no injuries. The operator claims he was more than 15' feet away. The current exited at the left rear wheel where a nail was in the tire. The recommended "safe" distance from powerlines is typically 10' for this voltage. How much does relative humidity affect this recommended distance? The RH at the time of the incident was approx. 55%.

mlittle,
Welcome to ECN, mate!.
I work with HV stuff like this all the time and it never ceases to amaze me how people seem to treat it with such dis-respect.
However, to answer your question:
Are you sure the distance is 10 feet?.
Humidity, (or moisture in the surrounding air) would work to lessen the "Flashover" distance, which has happened in this case.
Pure water is a poor conductor of electricity, however, try and find somewhere on this planet where the rain or mist water is pure.
Add impurities and the water vapour will conduct electricity with relative ease.
It goes back to basic Atomic theory, you have to have "Free" electrons to enable current passage through the vapour.
Personally I'd like to know what the MAD (Minimum Approach Distance) was with this.
Other side of the coin, mate, should have this guy even been raised the bed without a Safety Observer watching the distance?.

Well of course he shouldn't have raised his dump box!!! We have strict internal policies about working around overhead power lines. Unfortunately you can establish all the policies you want, train people over and over again and they still make poor decisions. And before anybody asks, yes we do have a disciplinary policy and yes everyone knows about it and yes we've disciplined employees regarding this topic before. But that wasn't really my question so lets see if we can get back to that . . . Since my original post I've learned that the 3 overhead lines carried a total of 13,200 volts so the line that arced to the truck was carrying 4,400V. Do any experts out there have a good idea what the arc-over distance would be for this situation when the relative humidity was 55%? I'm trying to investigate this incident and be as fair to all employees involved as I possibly can.

That is not how it works. The minimum voltage to ground would be 7600 volts. There is no way that the guy was 15' away. I would expect that he was less than 15" away.
The following is from: http://www.highvoltageconnection.com/articles/highvoltagespacing.pdf
Don

What was the truck dumping at the site? I do not recall ever seeing a report on how dust would change relative conductivity of the air, but that could be something to think about.

Don (resqcapt),
I have to agree with you on the distance from the lines and the chance of flash-over.
15" sounds more like it, there is no way that an arc could bridge a gap like that, at a low voltage like that.
To be honest, the RH can have an effect on the Flash-over distance, but, you should not be using plant that close to the lines that Flash-over is an issue.
Over here in NZ, you are required to maintain a distance of 4 metres (12ft) from any O/H lines, when using plant that could contact the conductors.
As a side-line, the PoCo I work for will sleeve any lines for free, as in, we don't want our Network damaged either.
Mind you, most accidents involving HV lines from trades staff, the PoCo are never even asked what the risk is, until something goes wrong.

Could warm, moist, high carbon, diesel exhaust come into play here??? Sure is good you found that nail in the tire though!
Joe

Thanks resqcapt19! You answered the question directly and provided some great literature. I appreciate you help and interest.

Well,I guess he doesn't lie.Quite likely HE was more than 15' away.However, his truck must have been less than 1' away from that power line when the arcing happend.
Variations of humidity content (1 g/m³... 30 g/m³) and temperature (-40°C...+40°C) of air affect the nonuniform air gap breakdown strengt only up to ± 40 % if compared with standard atmospheric condition . This ,according to IEC, holds for gap lenghts in the range 0.1 m < d < 10 m stressed by HV AC 50/60 Hz.


Only excessive temperature could have large effect which wasn't the case.
But moist + dust,carbon and other polutions do have significant effect to surface conductivity of support and suspension insulators,and lower breakdown strenght of gaps where insulators are present. Jimmy doesn't now that

Ever build a "Jacobs' Ladder?" Well, I have.

Putting out 10kv, the transformer produced an arc that ordinarily required the wires to be within 2" of each other to start. Then, with the help of the ozone the arc created, the arc would climb until, at a distance of about 4", it would break.

Applying this to your example, I doubt the arc could jump even as much as 6".

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.