Yes, we must have 5 ohms impedance!

[dsteves]

A line surge is a voltage spike (really an energy spike)

http://en.wikipedia.org/wiki/Voltage_spike

According to the Standard Handbook of Electrical Engineers, thirteenth edition, by Fink and Beaty, table 27-2, Power System Overvoltages are tabulated into three categories.

Category 1 is "Power frequency overvoltages", described as "temporary overvoltages dominated by the power frequency component", caused by electric faults, sudden load changes, or ferroresonance.

Category 2 is "Switching Overvoltages", described as "temporary overvoltages resulting from a switching operation", caused by energization of lines, de-energization of capacitor banks, fault interruption/TRV, high-speed reclosing, energization/de-energization of transformers, etc.

Category 3 is "Lightning overvoltages", described as "temporary overvoltages resulting from a lightning stroke terminating at a phase conductor, shield conductor, or any other part of a power system", caused by lightning-cloud to ground flashes.

They are listed in increasing value of per-unit overvoltage magnitude.

The author of the section, Prof. A. P. Sakis Meliopoulos of Georgia Institute of Technology, goes on to say, "Grounding plays an important role in dissipation of lightning strokes and therefore controlling overvoltages resulting from lightning. Yet, grounding has been widely misunderstood and proper analysis models are scarce." (You said it, Buster!)


If your system can deal with lightning overvoltages effectively, it will deal with lesser magnitude and higher frequency of occurrence sources also.

Dan

 

[dsteves]

Dan
Are you sure about the <.050 Ohms???

I find that as a difficult value to believe could be achieved... but I could be wrong.

I went back and checked the email. The calcs are giving 0.084 ohms just for the loop of 4/0 copper. The CEE hasn't been factored in yet, but I'm reasonably sure the parallel combination should get us to < .050 ohms.

Dan

 

[dsteves]

Absolutley....not.

Customer: "It's not much of a cheese shop, is it?"
Grocer: "Finest in the district, Sir!"
Customer: "Explain the logic underlying that statement."


Dan

 

[roger]

Dan, in most situations you should not go back and edit your posts after they have been replied too.

Now as far as the 25 ohms to ground, we have our own test set (three point fall of potential) and have to perform tests for performance grounding from time to time, as Bryan says, it can (does) change daily or hourly depending on soil type, salts, moistures, etc....


Roger

 

[dsteves]

Thanks, Roger, I'll avoid the temptation to correct statements in posts

In your experience monitoring installations, what kind of range does the GES vary through? I haven't seen such a study, but I'd be suprised if the min/max values differed by much more than one order of magnitude. If it was 25 ohms one day, was it 2500 ohms another, or 250 ohms?

If the measured value on the building GES is 0.085 ohms at installation, would you expect the value to be more than 25 ohms at any time later in its life?

Dan

 

[iwire]

Dan many of us have been here a long time and we have often discussed grounding issues, code, reality, job requirements etc.

Take Bryan (BPHGravity) he has a seemingly endless apatite for all things lightning and grounding. Here is guy that has not just read the NEC, or the Handbooks, or the trade mags. He has spent vacation time researching both the NEC requirements and the current lighting research.

When Byran tell me that a low impedance is not that important for lighting protection I believe him.

When he tells me current research shows that shallow rods are as effective as deep rods I believe him.

I have the up most respect for Byran's knowledge of this subject. And that is just Byran, many of us here have a good grip on grounding and do not just accept on face value the importance of a low impedance connection to dirt.

Customer: "It's not much of a cheese shop, is it?"
Grocer: "Finest in the district, Sir!"
Customer: "Explain the logic underlying that statement."


Dan

I wish you spoke English Dan.

I think your asking why I said absolutely not to Peters opening question.

Does having 5 ohms of impedance connected to dirt really matter?

I happen to work with Peter, it is one of our jobs he is talking about.

I take that question to mean

Does having 5 ohms of impedance connected to dirt really matter compared to just going with what we get following the NEC?

So lets say the job did not have a spec and we just followed the NEC and had a total of 30 ohms when we do are 3 point fall of potential test.

Now keep in mind this building is not a communication center or IT building.

What is the difference in safety or reliability that you see gained by requiring 5 ohms instead of the 30 ohms?

Why should the customer pay us to meet some arbitrary number picked by an engineer not realizing what kind of money will be spent trying to achieve this 5 ohm level?

I ask you to explain what type of line surge a rod will control at the voltages in a premise wiring system?

About the only items I can see a connection to earth helps with are lighting (and low impedance is not necessary) or a high voltage line dropping on a MGN outside the premise. (again anything under 50 ohms will likely operate the high voltage cut out)

Say 7800 volts into 50 ohms = 144 amps well above a primary cut out.

One last thing I, like Pierre am having a hard time believing you will reach < .050 ohms

There is a point of diminishing return.

 

[iwire]

I'll avoid the temptation to correct statements in posts

Dan once people have answerd a post it really is bad Forum etiquite to go back and make changes.

If you find you must make a change than what many of us will do is add a line at the bottom noting what we changed.

For me it usally looks like this

Edit; Spelling

These are just suggestions, your ability to edit your own posts actually 'times out' I just don't know how long that is.

 

[dsteves]

Dan once people have answerd a post it really is bad Forum etiquite to go back and make changes.

If you find you must make a change than what many of us will do is add a line at the bottom noting what we changed.

For me it usally looks like this

Edit; Spelling

These are just suggestions, your ability to edit your own posts actually 'times out' I just don't know how long that is.

I get the point - I think

As far as your post prior goes, I don't know where to start. How about at the top?

1. If impedance doesn't matter, make it infinite. If it does matter, how low is "low enough"?
2. Who said anything about depth?
3. I've only taken single days of vacation to study grounding.
4. That *is* English, specifically Monty Python's "The Cheese Shop" skit.
5. You quoted my post; I figured you are commenting on it.
6. It's a school. Ask the engineer why 5 ohms. I'm with him.
7. The rod thing, and surge control, it's sort of like riding a Harley.
8. What's the I^2*t let-through on the primary cutout?
9. 0.050 ohms - oh well, I guess we'll just have to wait and see. We are installing test wells. I guess me saying it's so doesn't make it so. I'll post a link to the test report when it comes out. Never challenge another's dogma with facts.:wink:

You sure give a guy a lot of stuff to cogitate in a single post If it weren't for some "real life" work I had to deal with, I might have responded a little sooner...

Dan

PS - "If at first you don't succeed, destroy all evidence that you tried." *
* Unless you're not iwire, nudge-nudge wink-wink!

 

[peter d]

1. If impedance doesn't matter, make it infinite. If it does matter, how low is "low enough"?

No one is suggesting that the impedance doesn't matter. The question is which is more important, the low impedance path back to the source via the main bonding jumper, or back to source via the earth?

5 ohms versus 25 ohms on a 480 volt premises wiring system is irrelevant.

 

[coulter]

...One last thing I, like Pierre am having a hard time believing you will reach < .050 ohms ...

He maybe could. One facility I worked in Sitka had a several acre ground grid, concrete building foundations with the re-steel bonded to the grid, built on fill, on the waterfront, with the water table down 10 feet, rained 100"/year, sulphite paper mill so the ground was saturated with dilute sulphur dioxide (turns to acid). I couldn't accurately measure the ground resistance. Using the 1985 grade instruments that were available, the best I could say was "<<1 ohm".

carl

 

[dsteves]

No one is suggesting that the impedance doesn't matter. The question is which is more important, the low impedance path back to the source via the main bonding jumper, or back to source via the earth?

5 ohms versus 25 ohms on a 480 volt premises wiring system is irrelevant.

We're talking about surges, not faults. The path for a lightning surge *is* through the ground (earth), not back to neutral, per se. The neutral gets involved by virtue of the fact it's bonded to ground. The effectiveness of this bonding is the issue. The DC resistance and impedance of the GES has little to do with clearing faults, and it is not the intent of the Code to provide a current path through the earth to open a fuse or non-GFI circuit breaker in a ground fault situation.. See 250.4(A)(5), which finishes up with, "The earth shall not be considered as an effective ground-fault current path."

The matter is hotly debated, understood at varying levels, and we will not likely make any breakthrough realizations here, but we sure can talk about it!

Dan

 

[bphgravity]

The one and only thing that I can be certain about when it comes to issues of grounding and lightning is that the very moment you think you have a good grasp of the concepts, a new report or paper is published that puts everything you thought you knew into the trash can or places a new spin on the issue, or at least makes your head spin.

I think it is best when having these discussions that every sentence should begin with the words, "The evidence shows", or "It is my belief", or some other qualifier that indicates the idea is not absolute and may not be all together accurate.

So to practice what I preach, there does not appear to be any evidence to date that indicates earth resistance or ground surge impedance is of great significance. There is no direct evidence that shows grounding of an electrical system will protect against any damage to the system from direct or indirect lightning occurrences. It is my belief that other more conclusive and practical methods such as surge protection devices should be considered in lieu of attempting to achieve low ground resistance at any one point in time for any one particular installation.

And bonding is a very significant issue when dealing with lightning. And not because you are trying to open overcurrent devices, but because all non-current carrying objects and objects in contact with earth should be as close to the same potential to each other as possible to prevent flash over or excessive ground potential rise.

There isn't a person on this Forum that can explain the 25-ohm rule or why it is in the code. But there are dozens whom misinterpreted the section and apply it incorrectly.

I can assure you that the future of our current grounding practices and principles is in serious question and will not remain in place. I believe that in my lifetime, most what incorporates Article 250 and the NFPA 780 will not be recognizable to what is required today.

(Thanks for your vote of confidence BOB. It's good to know that at least a few of my posts have not been lost to World Wide Web.)

 

[karl riley]

Achieving 0.05 oyhms would have interesting consequences. Since the impedance through earth itself is negligible (which makes SWER systems feasible) you would be setting up a parallel path for neutral return to the transformer, and if the primary neutral is tied to secondary, an easy path for primary neutral to the earth.

All the problems arising from "stray voltage/stray current" would be established.

Actually, we need some impedance to minimize this parallel path.

Karl

 

[karl riley]

Oyms! Oops.

Karl

 

[tom baker]

Then the TVSS only deals with L-N or L-L, not N-G or L-G or L-L-G or L-N-G.

You'd better look at your TVSS. Every pic I just looked at of Eaton's TVSS's have a lug or whip for connection to GES. The home ones mount in the service panel where N and G are bonded. In that case you could forego the separate ground, but it's essentially a L-L-G system in that case.

[edit] There are some inexpensive home TVSS's which do only L-N, and look like a GFI recep, but I wouldn't waste my money on one of them.

Here's a link to IAEI's March/April 2000 article about TVSS's.

http://www.iaei.org/subscriber/magazine/00_b/manche.htm

Dan

Hopefully Dereck will answer on this, as he developed a TVSS for GE. The line an neutral are for back door surges where the ground or netral voltage is elevated.
As proof that TVSS do not require a ground connection, Art 285 had a change in the 2005 NEC to allow TVSSs to be used on delta systems (ungrounded). The tvss takes the surge from L1 and sends it to L2, not earth.

 

[dsteves]

Hi, Karl. Our system is ungrounded 480V 3 phase. No neutral will be used from the service, only phase conductors. There are 2 12.47kV D / 480V Y pad-mounted transformers. X0 is unconnected.

I don't know that the OP's system looks like.

Hi Tom, article 285 changed to permit delta ungrounded, impedance-grounded and corner-grounded TVSS if listed for the purpose. You still have a grounding conductor from the TVSS if it's a good one
Here's such a TVSS :
http://www.passandseymour.com/pdf/F06-F07.pdf

I'm afraid we're really going far afield from the OP questioning the need for 5 ohm DC resistance of the GES at a school...

Dan

 

[George Stolz]

(Thanks for your vote of confidence BOB...)

I second his vote of confidence.

Further, after Bob's post, I will refer to him as Bbo for the rest of the week, Bryan.

 

[iwire]

Give me a break....my glasses are in the truck.

 

[George Stolz]

There isn't a person on this Forum that can explain the 25-ohm rule or why it is in the code.

Bryan, I don't believe there's a panel member than can explain it either.

Although the panel doesn't come right out and say it, the expectation is that you'll meet the 25 ohm requirement with components selected from 250.52(A)(2) through 250.52(A)(4). These elements typically come into play in larger structures. These elements are not typically associated with, say, residential construction and wiring.

Dan, check out these ROP's from the 2008 NEC:

2008 5-171 Log #385 NEC-P05 Final Action: Reject (250.56)
____________________________________________________________
Submitter: Bryan P. Holland, Holland Electric
Recommendation: Delete the entire section. Delete reference to this section in 250.53(D)(2).

Substantiation: There is no substantiation for having a resistance to ground 25 ohms or less. There is no evidence having a resistance to ground less than 25 ohms provides any more safety or reliability than ground resistances over 25 ohms. The section is based on design and not a minimum standard. No other electrodes are required to meet minimum resistances to ground, nor should rod, plate, or pipe electrodes. There are thousands of single-family homes in my community alone with single rod electrodes. There have been no reported cases of problems or hazards due to this. Performance grounding of the electrical service should be left to the electrical designer and engineer, and not the electrical installer.

Panel Meeting Action: Reject
Panel Statement: Insufficient substantiation has been provided to remove this section from the NEC. The panel concludes that deleting 250.56 reduces current minimum requirements. The panel affirms that the provisions in 250.56 are needed for installations using rod, pipe, or plate electrodes. See panel action and statement on Proposal 5-144 (Log#1858 and Proposals 5-128 (Log #1856) and 5-133 (Log #1857) in the 2004 ROP.

2008 5-175 Log #3325 NEC-P05 Final Action: Reject
(250.56)
____________________________________________________________
Submitter: David Pattison, Sky Electric
Recommendation: Revise text to read:
...25 ohms or less shall be augmented by one additional electrodes of any of the types specified by 250.52(A)(2) through (A)(7) until 25 ohms or less is achieved.

Substantiation: It is unclear if the intent of the code would be to continue driving ground rods until 25 ohms is achieved. If not what is the significance of 25 ohms or less?

Panel Meeting Action: Reject
Panel Statement: The submitter has not provided substantiation to include the restrictions proposed. Single electrodes indicated in this section must meet the 25 ohm provisions of 250.56. One additional electrode is required where this resistance value exceeds 25 ohms. The panel affirms that it is not required to install multiple rod, pipe, or plate electrodes until 25 ohms or less resistance is achieved. It is not the intent of this section to achieve a resistance of 25 ohms or less due to varying soil conditions. See also panel action and statement on Proposal 5-143 (Log # 293) in the 2004 NEC cycle. George's fun note: Proposal 5-143 (Log #293) was written by none other than Bryan three years before. It was essentially the same proposal as this one, apparently trying to make sense of the requirement.

Number Eligible to Vote: 15
Ballot Results: Affirmative: 15

It's my opinion they don't know why it's there either. I think if they knew exactly why it was there, they'd defend it, instead of lobbing "insufficient substantiations" at the proposals. But, at any rate, they pinky-swear that getting anywhere near 25 ohms is not on their minds. Often.

P.S. - Nice Python quote, I didn't get it but have to show respect to the Pythons.

Since the impedance through earth itself is negligible (which makes SWER systems feasible) you would be setting up a parallel path for neutral return to the transformer, and if the primary neutral is tied to secondary, an easy path for primary neutral to the earth.

That is an excellent point, thanks for posting that!

Edit to add in Dan's quote, it added more relevance to the ROPs. Then I edited some more to keep yacking, and I also realized I'd inadvertently deleted things I meant to say. I'm done now, I promise.

 

[peter d]

The best thing to do with that job spec would be to cover it with the lid of a trash can. Unfortunately, I don't have the authority to do that.