25 Ohms

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bennie

Esteemed Member
I am getting close to discovering the creation of the 25 Ohm rule, for electrode to earth connection.

I have a friend, from my teenage years, who is now retired as a mathematics professor, and electronic engineer.

I asked him about the 25 Ohm resistance. He acted surprised when I asked; like saying " you mean you don't know?"

The answer is complex due to the math involved, which is beyond my capability.

A simple answer is; A static charge, on metallic objects, produced by a common high voltage surge,
Will reach equilibrium with the earth within 10 seconds when connected by not more than 25 Ohms resistance in the path to the earth.

The initial surge will have a time domain of 2 to 10 ms. The resultant electrostatic charge will not equalize in a short time, if there is a high resistance path between the negative and positive charges.

The math is from Charles Coulomb and Johann Gauss.
 
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roger willis

Member
Location
Texas
Re: 25 Ohms

Hi Benny,
I'm with all the rest. We have missed you.
I do more reading than posting, maybe after 43 years in this business I'll learn something.

Roger
 
dereckbc

dereckbc

Moderator
Staff member
Location
Plano, TX
Re: 25 Ohms

Bennie, is he reffering to time constants in capacitors? It takes five time constants to discharge a capacitor. 1 Time Constant = RC
 
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bennie

Esteemed Member
Re: 25 Ohms

Dereck: The same principal applies. A conductor, when energized, even an open circuit, will create an energy field. When the voltage is removed, the conductor is still charged. The charge will be slow to relax, on an ungrounded conductor. The difference in charge is between the conductor and the earth.

This is also the reason for a ground electrode at an outdoor light pole. A transformer high to low fault, will impress medium voltage on the branch circuit. When the primary cut out activates, the line is still charged. The 10 seconds to reach equilibrium with the earth, is a calculated time for safety purposes and also equipment operation.

I just completed a refresher course in the science of electrostatics. My first course was at Stanford University, 45 years ago. This was when I had plans to be an aeronautical engineer.

The theory, and application of electrostatics has not changed. Electrostatics was the biggest concern with high voltage transmission systems, when it was determined to ground one side of a source.

I have a different understanding of the reasons for earth grounding than I did before.
 
charlie

charlie

Senior Member
Location
Indianapolis
Re: 25 Ohms

Bennie, I just got back from vacation and I get to read your posts. What a great surprise and welcome back. Between just you and me, I missed you. Charlie :) :D :D :D
 
brian john

brian john

Senior Member
Location
Leesburg, VA
Re: 25 Ohms

Is it safe to assume based upon the above

That a lower earth ground resistance will dissipate a discharge faster and there by minimize the level and time sensitive electronic equipment will be exposed to a high voltage impulses such as and in particular lightning?
 
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bennie

Esteemed Member
Re: 25 Ohms

Brian: That's the way I read the phenomenon. A resistance more than 25 Ohms will cause any residual charge to take longer to go down to zero.

This is why it appears to not be a big deal with two rods. The resistance is the best that can be had. Just don't touch any grounded metal surface during a storm.
 
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karl riley

Senior Member
Re: 25 Ohms

Hi Bennie, I was hoping you would return to the hunt!
You have gotten some other info on the 25 ohms. About power line reclosers, etc. I am not sure the Prof. has the entire story. Sometimes mathematical physicists simplify things in order to use their formulas, but simplify them out of reality. Just a jaded comment based on some experience.

I would like to get Dereck's full comments on this.Hard to imagine tens of thousands of volts hanging around for 10 seconds.

Karl
 
dereckbc

dereckbc

Moderator
Staff member
Location
Plano, TX
Re: 25 Ohms

Karl, its bed time for me now. I will answer after I have had some rest. 10 seconds to discharge, not in my book, that spells dead!

Dereck
 
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bennie

Esteemed Member
Re: 25 Ohms

I am surprised that no one has high pot tested conductor insulation. I can understand that most inside wiremen are not experienced with high voltage line maintenance, and the reason for safety grounding the lines after deenergizing.
I can understand that not many have worked on electrostatic precipitators.

Believe me, a charged surface or conductor can retain a charge for a considerable time and can renew the charge after shorting is removed. This is created by the e field collapse.

[ August 25, 2003, 11:22 PM: Message edited by: bennie ]
 
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russellroberts

Senior Member
Location
Georgia
Re: 25 Ohms

Bennie,so good to see your voice again. Like Roger Willis,I do more reading than posting,but between you,Dereck and the others,I've managed to actually understand some things that went over my head before. :)

Russell
 
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bennie

Esteemed Member
Re: 25 Ohms

I have seen electrostatic dust filters at the old Kaiser Steel Mill in So. California, draw a two inch long arc to ground the day after shutting them down. The charging voltage was 75 KV DC.
The chain grounds had to be on at all times when they were inactivated.
 
brian john

brian john

Senior Member
Location
Leesburg, VA
Re: 25 Ohms

Bennie:

Have you ever meggered bus duct at 1000 VDC? Run a one-minute test and let some unspecting person touch the bus after completion of test but prior to discharging the bus. EOWWWWWWWWWW It was a long time ago but I remember the lesson well.
 
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bennie

Esteemed Member
Re: 25 Ohms

Brian: I haven't meggered bus duct, but can understand it would take a big charge. The electrostatic charge remains long after the source is removed. The time the charge remains is a good sign of the insulation quality.

I have been bit by conductors that were tested hours before and not grounded to discharge the electrostatic e field.

In Korea, I was knocked down, when I reached for the tag line on a stretcher being lowered from a helicopter. The static charge was huge.
 
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