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#150791 - 11/17/05 04:01 PM 13,200V arcing distance?  
mlittle  Offline
Junior Member
Joined: Nov 2005
Posts: 3
Fort Collins, CO, USA
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%.


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#150792 - 11/19/05 02:48 AM Re: 13,200V arcing distance?  
Trumpy  Offline


Member
Joined: Jul 2002
Posts: 8,223
SI,New Zealand
mlittle,
Welcome to ECN, mate!. [Linked Image]
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:
Quote
The recommended "safe" distance from powerlines is typically 10' for this voltage. How much does relative humidity affect this recommended distance?

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?. [Linked Image]


#150793 - 11/19/05 03:42 PM Re: 13,200V arcing distance?  
mlittle  Offline
Junior Member
Joined: Nov 2005
Posts: 3
Fort Collins, CO, USA
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.


#150794 - 11/19/05 04:28 PM Re: 13,200V arcing distance?  
resqcapt19  Offline
Member
Joined: Nov 2000
Posts: 2,148
IL
Quote
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.

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
Quote
How much spacing is needed in high voltage circuits and setups? The general guideline in common use is to allow 7,500 to 10,000 volts, dc per inch in air. When dealing with ac, the general guideline is to multiply the rms voltage by three to determine the spacing that’s required.

Don


Don(resqcapt19)

#150795 - 11/19/05 07:43 PM Re: 13,200V arcing distance?  
ftl-eric  Offline
Member
Joined: Jan 2002
Posts: 15
Schererville, IN
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.


#150796 - 11/19/05 07:57 PM Re: 13,200V arcing distance?  
Trumpy  Offline


Member
Joined: Jul 2002
Posts: 8,223
SI,New Zealand
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. [Linked Image]


#150797 - 11/19/05 08:53 PM Re: 13,200V arcing distance?  
JoeTestingEngr  Offline
Member
Joined: Nov 2005
Posts: 791
Chicago, Il.
Could warm, moist, high carbon, diesel exhaust come into play here??? Sure is good you found that nail in the tire though!
Joe


#150798 - 11/21/05 10:07 AM Re: 13,200V arcing distance?  
mlittle  Offline
Junior Member
Joined: Nov 2005
Posts: 3
Fort Collins, CO, USA
Thanks resqcapt19! You answered the question directly and provided some great literature. I appreciate you help and interest.


#199778 - 03/08/11 04:11 PM Re: 13,200V arcing distance? [Re: mlittle]  
Vlado  Offline
Member
Joined: Feb 2011
Posts: 28
Croatia
Originally Posted by mlittle
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.
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. grin
Originally Posted by Trumpy

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.

Variations of humidity content (1 g/m... 30 g/m) and temperature (-40C...+40C) 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.

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

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 crazy


#199784 - 03/08/11 07:55 PM Re: 13,200V arcing distance? [Re: mlittle]  
renosteinke  Offline
Cat Servant
Member
Joined: Jan 2005
Posts: 5,316
Blue Collar Country
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".


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