More Grounding Issues...

[bphgravity]

...resistance to ground of 25-ohms or less...

I still have an issue with this. It seems to be a contradiction to other requirements of the code.

We all know electrical system grounding serves four primary purposes, if nothing else. Lightning, line surges, high-voltage cross-over, and voltage stabilizing. Is it safe to assume the the lightning aspect present the greatest concern and demand the most out of the grounding system?

If so, the NFPA 780 appears to contradict the grounding impedance need by the NEC. Per section B.3.5,

Low resistance is desirable, but not essential...

that pretty much says it all, but I will continue,

...if the soil is of normal resistivity..., the resistance of a ground connection made by extending the conductor 3 m (10 ft) into the ground will be from about 15 ohms to 200 ohms, and two such ground connections on a small rectangular building have been found by experience to be sufficent.

Doesn't this basically indicate that ground impedance even as high as 200 ohms is of not much concern?

Now on another note. It is also well known that grounding of electrical systems presents an objectionable current path to the source. Ohm's Law shows the lower the ground resistance, the greater the current that can flow.

Doesn't this indicate that the lower the ground resistance, the greater the shock potential?

The final point. Lets assume you achieve a gorund resistance of 25 ohms as required by 250.56. A 40,000 ampere lightning strike imposed on the service equipment could create a potential of around 1 million volts. Lets now assume the lowest ground resistance that can be acheived is only 200 ohms. The same 40,000 ampere lightning strike could raise a potential of 8 million volts. In this case, it seems to indicate the lower the ground resistance, the safer the installtion. However, I again beleive this is really not true. Who cares what the potential rise over the system is during a lightning strike. If proper bonding is performed, all objects will will also be at the same potential and therefor there would be little danger for someone in contact with a properly bonded object.

So in conclusion, lightning creates the greatest energy event that can be imposed on an electrical system. Overall ground impedance is generally of little concern. Also, objectionable current shouldn't be acceptable for possible event. And finally, rise of potential and flash over are of no real concern if proper bonding is established.

Again, it appears that secondary provisions for lightning protection are more effective. Bonding, SPD's and other methods not even assoicated with grounding are the true protective devices and grounding really appears to be doing nothing.

 

[rbalex]

Re: More Grounding Issues...

It wasn't nearly as ambitious as Don's efforts, but one of my earliest Proposals (93 cycle) was to simply change the name of Art 250 to Bonding and Grounding to emphasize your points.

It was rejected.

It has been changed since, but ?Grounding? still gets top billing.

 

[dereckbc]

Re: More Grounding Issues...

Bryan, I am at work and cannot take time to go into this right now. You have have points I would like to expand on late but think about this statement for now.

"Who cares what the potential rise over the system is during a lightning strike. If proper bonding is performed, all objects will will also be at the same potential and therefor there would be little danger for someone in contact with a properly bonded object".

That would be true if no current is flowing through any of the equipment and circuit conductors. Truth is unless exteme measures are taken fault current will be flowing, and at 8000-amps rise in 6 microeseconds, about any conductor will drop around 100 volts per linear inch. Now imagine your 40K example. Later.

 

[bphgravity]

Re: More Grounding Issues...

Okay Dereck, that might be a little too strong of a statement. It's out of frustration. I have been trying to read a few IEC and IEEE publications and I don't get alot of it. The math is confusing and the terminaology is not familiar.

I wish there was a good course I could take on fundementals of electrophysics. I guess thats what engineering school is for.

 

[tom baker]

Re: More Grounding Issues...

What we really need is a separate article on Grounding and on Bonding. The Grounding article would be much shorter. Most of the time when the NEC referrs to grounding it is really meaning bonding.
Its interesting to note that in Article 314 where the new requirements for handhole enclosures are covered, they use the term bonding, where before it would of been bonding.

 

[dereckbc]

Re: More Grounding Issues...

Bryan, I hope you did not take my comments as condescending, they were not offered in that spirit. My goal was to get you thinking for a moment. I certainly understand your frustration, but you do not need to be mathematician or physicist to understand the principles or mechanics. If you understand basic ohms law, and ac frequency interaction with reactive components like inductors and capacitors, you can understand what is going on. To try to keep it short, lets build on a very good point you made about 25 ohms versus 200.

When you quote a 25 or 200 ohm GES impedance, it is in reference to very low frequencies. If you use a common meggar, you are talking about frequencies in the range of 130 to 190 Hz. These frequencies are meaningless with respect to lightning. Lightning has a very high frequency component and the inductance is the major factor of circuit impedance.

So does it matter if the impedance is 25 or 200, you decide. Let?s assume lightning has a fundamental frequency component of 100 Mhz. Our GEC conductor is 10-feet in length, size does not matter as the inductance does not change significantly from say a 4 AWG to a 4/0. What do you think the impedance would be from the GEC termination point in the service cabinet, to earth with both a 25 and 200 ohm GES?

Let?s assume the GES does not exhibit any frequency interactions, it is either 25 or 200 no matter the frequency to keep things simple. Using tables provided in IEEE Emerald book Table 4.1 the impedance of a 10-foot long conductor of say 4 AWG @ 100 Mhz is 2.6 K ohms. 2.6 K ohms in series with 25 ohms is 2625 ohms. 2.6 K ohms in series with 200 ohms is 2800 or 2.8 K ohms. Is there a significant difference between 2.8 and 2.625?

Keep in mind the numbers I used ignored the real GES impedance at lightning frequencies, and in reality the impedance would be many times higher. Once we cross into lightning protection systems, GES impedance is not that important, surface area is and ground saturation become very important. But that is another subject.

 

[bphgravity]

Re: More Grounding Issues...

Originally posted by dereckbc:
Bryan, I hope you did not take my comments as condescending, they were not offered in that spirit.

Absolutely not. I completely appreciate any opinion or comment you make.

 

[jwelectric]

Re: More Grounding Issues...

Now I am confused and need help with this subject. Would one of you please take a few minutes from your busy schedule and explain something to me. The flow of current during a lighting strike is a direct current flow. From a negative to a positive charge similar to that of a battery. Having said this, where does the theory that Lightning has a very high frequency come from?

 

[charlie]

Re: More Grounding Issues...

It is both high frequency and DC. it is like pulsating DC but with only one wave at a very high frequency. Dereck will be able to explain it much better than I.

Bennie has given us the reason for the 25 ohm threshold here.

 

[dereckbc]

Re: More Grounding Issues...

jwelectric, as Charlie said it is complex waveform of both AC/DC. The AC portion is from rapidly rising DC currents. Any arc contains energy from DC to light.

Take my comments to Bryan with a grain of salt. I am not really trying to say the impedance of lightning protection system is not important, because it plays a part. But note I am talking about lightning protection systems. These systems go well beyond the scope of the NEC and are performance based rather than safety.

Any lightning protection system I am familiar with involves more than a rod or two sunk in the dirt. They involve multiple rods, rings, and radials to divert energy away from the protected area, and have large surface areas in contact with earth. This large surface contact area shorts out the various inductance and capacitance associated with the conductors. This in turn shortens the discharge time constant and limits what is called ground saturation (the ability of earth to quickly absorb the charge in a short period of time).

So when I say the overall impedance is not that important with respect to lightning protection systems, I am saying the design and surface contact area are the important parameters. The impedance is a result, not the target. However the impedance of a said lightning protection is going to be much lower than that of a NEC minimum. Two different animals.

[ March 19, 2005, 10:39 AM: Message edited by: dereckbc ]

 

[physis]

Re: More Grounding Issues...

I read this and the thread Charlie put up a link for. Oddly, I don't have any comments at the moment. But it's a great topic.

 

[jwelectric]

Re: More Grounding Issues...

Okay Dereck help me with this question. First let me say that I understand that after the second rod that not much more can been done to help with the dissipation of lighting.
The question, would not a uffer ground made of a 20 foot 5/8 rebar encased in the footer be a better ground for lighting due to the exposed area than the two ground rods?
If you would be so kind would you please give your thoughts to the pro and cons to the uffer ground? Thanks

edited to add, everyone else feel free to pipe in as I need your knowledge on this matter, Thanks

[ March 22, 2005, 10:21 PM: Message edited by: jwelectric ]

 

[dereckbc]

Re: More Grounding Issues...

Originally posted by jwelectric:
[QB] First let me say that I understand that after the second rod that not much more can been done to help with the dissipation of lighting./QB]

I do not agree with this statement respectively. I work with a lot of tower designs. The main component is to use radials going outward from the towers with rods driven along the length of the radials at periodic intervals. The idea is to prevent ground saturation. Ground saturation works like a resistor and capacitor in a series RC circuit. The more rods and radials you add, shorts out the resistance and capacitance, which in turn lessons the time constant, or discharges the lightning quicker. Much like pumping a given volume of water through a 1/2 inch hose. Dramatically increase the size of the hose, and the faster you can pump the given volume of water through.

I don't know if I am the right one to ask about ufer grounds as I am not a big proponent of them. I have been fortunate to work for a large telephone company and exposed to thousands of sites we call regens ( a small 12 x 24 building along fiber optic routes). These regens all use ufer grounds. I have witnessed a few in NC and FL where the concrete exploded at the point where the conductor entered the concrete from water turning to steam.

After doing some investigation, we concluded the problem was a lack of specifications. We didn't spell out exactly what we wanted done, so what we got was just a NEC minimum 20 feet of wire in the bottom of a footer.

So to answer your question about would a ufer be good for lightning protection, IMO I would have to say yes and no. Yes if you bond the rebar together via ties, compression, and weld. No if you only perform NEC minimum requirement. That may get me in hot water with some people, but I base my opinion on personal observation and experience.

From that experience, I then specified all the rebar to either be double tied, or in mission critical sites (like FAA & DOD) to use Burndy Hy-Ground compression taps, stub up the rebar, then welded the GEC to the rebar. Never had a problem after that. Would you do that for residential, commercial, or most industrial jobs? No chance, it is only for special applications.

 

[hurk27]

Re: More Grounding Issues...

JW if you like to read here is a great site that has allot on it about lightning. and it also list books that you can get on the subject:

National Lightning Safety Institute (NLSI) Home Page

Browse around it has much to take in.

 

[charlie]

Re: More Grounding Issues...

Dereck, your experience is based on lightning protection of miles of exposed line or high towers that attract lightning. During a single storm, your facilities will get multiple hits and the NEC minimum requirements are not enough. The typical NEC installation will take the effects of a strike once every several years and will not have a problem with the Ufer ground or the NEC minimum.

The minimum we have to work with is four grounds per mile. Also, if we random lay with a communications company in a trench, we are required eight grounds per mile. Neither of these are permitted to count our customer's grounding systems.

Both of us have to install more grounds because of exposure to lightning. What we do doesn't translate to what is required for the average homeowner or business. On the other hand, if someone were to build on the top of a mountain . . .

 

[dereckbc]

Re: More Grounding Issues...

Charlie, I agree, that is why I tried to point out that lightning protection is a completely different issue.

 

[charlie]

Re: More Grounding Issues...

One of these days, I will read more thoroughly before I leap.

 

[hurk27]

Re: More Grounding Issues...

By Charlie: The typical NEC installation will take the effects of a strike once every several years and will not have a problem with the Ufer ground or the NEC minimum.

I kind of have a problem with this statement. I know that high towers and utility lines will be struck more often than the average house but even if the house only gets a strike once in ten years the lightning will do damage under the NEC minimums! In Florida we had many houses get hit and everything from burning it down to severe damage is usually the result. Here in Indiana we have at least one a year gets hit and the damage can be quit extensive. Last year I had one where the lightning blew a large hole in the roof and dropped all the drywall onto a 12 year old child who was sleeping next to the wall where all the home runs went to the basement panel. It could just of easily have killed him. They immediately checked into a lightning protection system that went way beyond what the NEC requires. Should the NEC be brought up to stronger requirements? I would say no. But it (NEC) should point out that the Earth grounding requirements in it does very little to protect against a lightning strike and those who live in a high risk area (like on a hill) should seek the advice of an expert to provide extra protection instead of misleading many to think it will protect the house, which is what many do think now.

Oh no another proposal AGH!!!