Grounding electrode conductor sizing

[hurk27]

Re: Grounding electrode conductor sizing

The capability of a ground rod to dissapate the fault current into the earth is limited to what a #6 wire can deliver.

Derwith I don't think you have the understanding of it yet!
#1 Ground rod are not for fault current unless it is at a voltage much higher than what is delivered to our houses!
And what determins the amount of current a ground rod will return to source will be the ground (Earth) not the GEC by no means.
It is the resistance of the rod to Earth connection that comes into play. If you had a rod that was at the minium of 25 ohm's you could attacth a hot 120 volt wire to it and it will only draw 4.8 amp's of current. this would not trip a 5 amp breaker. now raise the voltage to 7200 volts and you now have 288 amps flowing through this wire but because POCO's would never have there primarys fused as high that would heat up a #6 the wire would still be protected. and as far a lightning goes.... Well it goes where it wan't to

 

[derwith]

Re: Grounding electrode conductor sizing

Wayne,

I agree with you. By fault, I mean any fault causing current to flow through the #6 to the ground rod. No matter where you put a ground rod, the impedence to earth will not go low enough to have current higher than what a #6 can handle.

 

[electricman2]

Re: Grounding electrode conductor sizing

Grounding (earthing) of electrical equipment doesn't serve the purpose of "providing a low-impedance fault-current path to clear ground faults." In fact, the Code prohibits the use of the earth as the sole return path-because it's a poor conductor of current at voltage levels below 600V [250.4(A)(5) and 250.45(B)(4)].
Grounding (earthing) the metal parts of electrical equipment doesn't protect electrical or electronic equipment from lightning-induced voltage transients (high-frequency voltage impulses) on the circuit conductors inside the building or structure. Nor does it protect equipment within a structure from transients generated from other equipment in that structure.

Excerpt from Mike Holt's newsletter 10-14 "Grounding versus Bonding Part 2 of 12"
Read the entire newsletter for more on this subject.

 

[oceanaradio]

Re: Grounding electrode conductor sizing

Gentlemen, I was confronted with the question at hand when when installing a communication station lightning protection system. All weather operation is required, and the area experiences a high rate of summer thunderstorms (50+ days/year).

This included bonding the radio station master ground to the building service entrance ground. It was a long distance (about 60'). My choice of GEC (#4 solid copper) would certainly be lower impedance than the home's AC ground, but no size of wire or even wide strap could prevent high potential due to inductance over that long a run. So, multiple earth references were added to provide high current capability, and prevent additive impedances across that distance.

The purpose of this bond is not to dissipate lightning energy as the Ground Electrodes at the end of downconductors do. This bond is to prevent Ground Potential Rise (GPR) from a nearby strike from running up through the station ground, backwards through connected equipment, and out via the high impedance path of station (and home) AC wiring. Theoretically, bonding conductors would be in a high voltage and never high current condition in a nearby strike. But as we know, the inductive effect predominates with lightning energy across any length of conductor, and 60' is a hopelessly "too long" bonding distance. Hence the multiple ground rod references to compensate for what could be extremely high current in a saturated-ground condition of a nearby strike.

A schematic (URL) below shows the bonding and grounding across the unfortunately long distance between service entrance and station grounds.

http://members.cox.net/pc-usa/station/schematic.htm

Comments are certainly welcome!

Jack Painter
USCG Auxiliary Oceana Radio
Virginia Beach VA

 

[hurk27]

Re: Grounding electrode conductor sizing

Jack I agree with you totally as since lightning is a HF event long GEC runs limits the amount of protection drastically and is of very little use in lightning protection. This is one reason I think the Water pipe should be brought into a house close to the service point if it is to be used as a grounding electrode. And any other GE that will be used should be within 20' of the service connection point.

 

[physis]

Re: Grounding electrode conductor sizing

So some of you guys are saying that 20 or 60 feet of GEC might have more impeadance to lightning than a couple feet of air?

 

[oceanaradio]

Re: Grounding electrode conductor sizing

Not saying that at all. But once lightning injects a voltage into all conductive materials in a building (and it does this both capacitively and by induction, the latter having a slightly faster rise time) a lot of interactive forces come into play. We no longer care what impedance was overcome for either the attachment, or energy from a nearby strike to travel to us, we are concerned about how it travels after it is here.

The studies I have read can be summarized by this: lightning will find the weak links in a system, and exploit them. All paths are possible, high or low impedance, as varying frequencies and magnetic forces pulse this energy several times in less than 100 milliseconds. Surge protectors have to act in microseconds, as the magnetic inductive forces are faster. In the end, the lowest impedance path is always sought, but many high impedance paths may be indiscriminately taken to get there. That serves your question "what about all the impedance overcome through air first?" Things that seem as insulators to us, aren't to lightning. But once it becomes in our "near field", then our equipment, structures, GEC's and BONDING PATHS begin to take control.

Bonding is the protection from those unexpected and undesirable paths which we cannot protect from lightning's influence. But in most cases, we can protect from it's damage!

There are probably limits to what we can call an effective bonding conductor. I do not know them, and have not yet seen a study that defines a conductor that is too long, depending on a few other factors. Code is what we use as a guide here, it isn't clear on the subject (reference what started this discussion), and it may or may not be reflective of the science behind several interactive forces involved at that point in a lighting attachment or near field effects of same.

My choice of oversized GEC and many additional GE's to compensate for the abnormally long bonding path from station ground to service ground was the result of studies that included an educated guess by a professional engineer, and my evaluation and decision to make the best of a less than ideal design condition.

I have experienced two tree strikes within 50' of my antennas and structure, and perhaps two dozen nearby strikes within 500' since the system was completed late last Spring. I am at least sure of the performance of the surge protection system. It is likely, but I cannot be sure, that several thousand volts have "floated" on my operating equipment during strikes that were practically on top of me. No evidence of arcing or flashover marking is apparent on GEC's from these events, and extremely sensitive equipment has not shown any latent effects so far. Does this make me comfortable? I'll invite anyone to sit at my operating station with a microphone in your hand while the skies light up outside, and you can tell me. ;-) But knowing I did the best I could, and continually striving to learn how to be safer, is why I appreciate your comments and suggestions here.

Jack

 

[physis]

Re: Grounding electrode conductor sizing

"what about all the impedance overcome through air first?" Things that seem as insulators to us, aren't to lightning.

Lightning ionizes insulators. That's it's mission from the instant it decides it wants to be lightning. That's what allows it to exist as lightning. But it will still avail itself of the least "impeded" path. Given ac dynamics that path can be different every time a bolt strikes, even at the same location.

lightning will find the weak links in a system, and exploit them. All paths are possible, high or low impedance, as varying frequencies and magnetic forces pulse this energy several times in less than 100 milliseconds. Surge protectors have to act in microseconds, as the magnetic inductive forces are faster. In the end, the lowest impedance path is always sought, but many high impedance paths may be indiscriminately taken to get there.

This is a pretty eloquent description of lightning! You almost seem to assign it a quasi intelligence.

 

[oceanaradio]

Re: Grounding electrode conductor sizing

Thanks Sam. I wish it weren't true that we have so much more to learn about lightning, in order to better protect ourselves and our property from it.

The IEEE is still enthralled in a bitter discussion of what to do about ESE lightning dissipator claims, thoroughly discredited as these claims seem to be. This distracts them from advancing other good science that blunt-tip air terminals are superior choices to pointed-tips for arc attachment that was going to happen in a given 20' area anyway. Hard to keep up with this stuff, and I'm glad to find another source of experience and education here in these forums.

I'll try to remember we're the intelligent beings, not lightning ;-)

Jack

 

[dana1028]

Re: Grounding electrode conductor sizing

The original question:

Originally posted by wwebb:
Articles 250.66 (A), (B) are causing some confusion in our jurisdiction. These articles state: "that PORTION of the conductor that is the SOLE CONNECTION to the grounding electrode shall not be required to be larger than 6 AWG copper wire...".

If a GEC leaves the svc panel, goes to a ground rod and terminates there, the GEC does not have to be larger than a #6 copper.

If the GEC leaves the panel, goes to a ground rod, then moves on and connects to the water pipe - then the GEC must be sized per T250.66.

And (I think), if a conductor leaves the panel and goes to the water pipe it must be sized per T250.66; if you then install a 2nd clamp and take a conductor on to the ground rod, that 2nd conductor can be a #6 copper (as long as it terminates on the ground rod and goes no further).