Yes, we must have 5 ohms impedance!

[peter d]

I thought I would share a job spec that I saw on a plan the other day. We are updating the distribution system for a small school and this is what they require for the grounding/bonding of the new electrical room we are going to build. I am paraphrasing but you get the basic idea:

"Contractor must verify to engineer that grounding electrodes must provide 5 ohms or less impedance. Contractor must install additional electrodes to attain 5 ohms."

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

 

[dsteves]

For mitigation of lightning and line surges, yes - absolutely.

Depending on rho of the soil in the area and the square foot size of the GES, 5 ohms is fairly easy to attain.

Our new manufacturing facility will have < .050 ohms upon completion.

Dan

 

[bphgravity]

You should see some of the grounding specifications for typical commercial buildings I see come in with permit applications on a daily basis. One recently called for 3 - 30' copperweld ground rods installed 30 feet apart in "test-wells", connected to a ground ring circling the building. This was for a community clubhouse with NO special communications or computer equipment. The plans continued to specify a maximum earth resistance of 10-ohms. In the event 10-ohms were not achieved, additional 10' ground rods were to be installed until 10-ohms was reached. No other electrodes would be available.

The plans were approved...

 

[dsteves]

Hmmm. Sort of looks like a specification the government might come up with. The Oreo cookie spec in rations once included a color comparison with the cookie observed "in direct sunlight and held in such a way as to eliminate specular reflection..."

Dan

 

[coulter]

Dan -

With all due respect, I'm not so sure

For mitigation of lightning and line surges, yes - absolutely. ...

I don't recall that NFPA 780 requires a low Z ground. I agree there needs to ba a ground for mitigation of indirect lightning strike effects. I don't have any data that supports a low Z ground making any difference.

I'm going to need help on how a low Z ground mitigates a line surge.

carl

 

[Rockyd]

Is there something brewing in the industry? It's almosts sounds like the next"boogey man in the closet" issue. I don't see where the special noise is being set up over grounding. I'm referring to beyond the usual UFER, 2 ground rods, or standard grounding practices. Bonding, yes I see all kind sof reasons to be serious about bonding, but not grounding.

Perhaps someone that is a lightning expert, or at least familiar with the subject could clue me in. I don't have a whole lot of experience in dealing with lightning directly (originally from Alaska). Know what the NEC says, but have never got into NFPA780, or studies of validity, so let me know if I'm missing the boat on this one.

 

[bphgravity]

Grounding serves very little purpose for grounding of premise wiring systems of 600V and less.

Grounding is a significant element to lightning protection systems; however the earth resistance or surge impedance of the grounding system is not of great importance.

 

[dsteves]

Dan -

With all due respect, I'm not so sure

I don't recall that NFPA 780 requires a low Z ground. I agree there needs to ba a ground for mitigation of indirect lightning strike effects. I don't have any data that supports a low Z ground making any difference.

I'm going to need help on how a low Z ground mitigates a line surge.

carl

250.56 specifies 25 ohms max resistance to ground. 250.4(A)(1) states the requirement for limiting the voltage during lightning strikes, etc. A full treatise of the explanation of how grounding systems can affect voltage spike magnitudes is a profound understanding that comes with knowledge of physics, and is beyond the scope of this thread It may, however, be of some help if you consider the grounding and grounded systems to be series-parallel R-L-C circuits, where you're reducing R and increasing C with grounding electrodes.

Ic = C dV/dt. During a lightning strike, dV/dt can be really high. If the current through the capacitor Ic can be made sufficiently high by making C as high as practical, dV/dt will be minimized. The parallel runs of ungrounded and grounding conductors in feeders and branch circuits increase capacitive and inductive coupling between the line and ground. The low resistance and high capacitance of the GES maximizes the overall system capacitance, which in turn minimizes dV/dt for any given stroke.

Dan

 

[bphgravity]

250.56 specifies 25 ohms max resistance to ground.

Not true. This only applies to SINGLE rod, pipe, or plate electrodes.

250.4(A)(1) states the requirement for limiting the voltage during lightning strikes, etc.

Yes, but this means very little if all equipment is bonded properly.

 

[Bob NH]

Grounding serves very little purpose for grounding of premise wiring systems of 600V and less.

Grounding is a significant element to lightning protection systems; however the earth resistance or surge impedance of the grounding system is not of great importance.

On the other hand, there are inspectors who require two supplemental ground rods with a service upgrade (in addition to the underground water pipe) on the basis that "a ground rod is not a reliable ground to meet the 25 Ohm requirement".

A commercial device to measure the resistance is pretty expensive, but I have done it by putting 120 Volts to the rod through a 100 watt light bulb and measuring the voltage at the rod and the current through the grounding conductor.

The voltage on the ground rod can be measured relative to the grounded conductor if it is available, or to almost any nearby ground a reasonable distance from the grounding electrode that is being tested.

A little alum (aluminum sulfate) dissolved in water and put into a little hole around the ground rod will go a long way toward meeting requirements on a marginal resistance electrode.

SAFETY NOTE: You should use a fairly long wire to the rod and avoid the rod area while making the measurement so you don't step on a "hot" point that could exist if you have a high resistance rod.

 

[bphgravity]

The earth resistance of electrodes can literally change on a daily basis. An installation could measure below 25-ohms during the rainy season and yet be well above the 25-ohm during the dry season.

Too many variables and conditional circumstances goes into the earth resistance that mandating a certain value at any one time or moment is silly, especially at installation, which is not the most likely atmospheric or ground condition the electrodes will exist in.

 

[tom baker]

For mitigation of lightning and line surges, yes - absolutely.

Depending on rho of the soil in the area and the square foot size of the GES, 5 ohms is fairly easy to attain.

Our new manufacturing facility will have < .050 ohms upon completion.

Dan

Dan, A TVSS does not require a connection to earth ground to operate. I am not sure if you mean this when you state "line surges" and perhaps you meant contact with a higher voltage line.

 

[tom baker]

I thought I would share a job spec that I saw on a plan the other day. We are updating the distribution system for a small school and this is what they require for the grounding/bonding of the new electrical room we are going to build. I am paraphrasing but you get the basic idea:

"Contractor must verify to engineer that grounding electrodes must provide 5 ohms or less impedance. Contractor must install additional electrodes to attain 5 ohms."

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

We used to have this spec and I had it removed. It is meaningless and a big burden on the EC.
If you want 5 ohms, then the correct approach is to test the soil resistance and design the system first.

 

[dsteves]

Not true. This only applies to SINGLE rod, pipe, or plate electrodes.



Yes, but this means very little if all equipment is bonded properly.

I type slowly, so please read slowly to protract the misery like I have... It took me so long to type, I had to log back in again!

I'm not trying to argue for argument's sake here, but I must answer the post thusly.

It's a little tough to appreciate the Code by taking a single passage out of context. The Code requires that single components of 250.52(A)(5) or 250.52(A)(6) must either be <= 25 ohms, or be augmented by using one or more of the elements in 250.52(A)(2) through 250.52(A)(7) in order to reach the 25 ohm maximum value.

Most construction uses elements in the GES from the following:
250.52(A)(1) Metal Underground Water Pipe, and
250.52(A)(5) Rod and Pipe Electrodes.

The argument you are giving about 250.56 only applying to single electrodes is flawed because you aren't citing 250.53(D)(2). Basically, the requirement for the 25 ohms is pointed at cheaper installations trying to minimize cost and time by doing the least possible work. The Code permits the single rod or pipe to augment the metal underground water pipe alone if the rod or pipe makes 25 ohms. If it doesn't, you can't bond to the water pipe again; you have to buy another rod or pipe and pound it in the ground no less than 6 feet from the first one to try to get to the 25 ohm limit. [edited to show that only *one* additional electrode must be added. Thanks to Jim Dungar for the POI.]

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.

Equipment bonding, in your context (if I'm not mistaken), is for fault current path provisioning, and does not come into play for grounding. If everything's bonded properly, all of it will enjoy the same voltage stresses during the unmitigated lightning strike.

A five acre, 2 story metal roof manufacturing facility on a hilltop in northeastern Wisconsin doesn't benefit from grounding if its service is only 480V???


edited to korrekt speling misteaks
Dan

 

[dsteves]

Dan, A TVSS does not require a connection to earth ground to operate. I am not sure if you mean this when you state "line surges" and perhaps you meant contact with a higher voltage line.

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

 

[jim dungar]

The Code requires that single components of 250.52(A)(5) or 250.52(A)(6) must either be <= 25 ohms, or be augmented by using one or more of the elements in 250.52(A)(2) through 250.52(A)(7) in order to reach the 25 ohm maximum value.

... you have to buy another rod or pipe and pound it in the ground no less than 6 feet from the first and test again, and repeat the process until you're under 25 ohms on the cheap seats.

Dan,
250.56 says measure the resistance to ground, if it is >25ohms add ONE additional ground electrode. It never requires more than 2 electrodes.

And if you are using the metal of the building (250.52(A)(2)), a ground ring (250.52(A)(4)), or a concrete-encased electrode (250.52(A)(3)) then there is no NEC requirement for any specific resistance value.

 

[dsteves]

Jim, You're right on the one additional. I'll go back and edit that.

The absence of the spec for the others, as I understand it, is the expectation that those elements outperform rod and pipe electrodes, and where used will give an inherently low resistance. I'll stand by that part For example, we'll have a 2200 foot ground ring and about 3000 lineal feet of CEE. I don't anticipate any problem hitting the 25 ohm mark, even in a drought season. Rho of concrete is lower than rho of soil. 250.52(A)(3) specs 20 feet or more of rod; that by itself is more contact area than two 8 foot rods. A retired Hungarian friend figures the rods' tips are always in water. I'm not sure I agree with that. The #4/0 copper ring alone is expected in our case to be < 0.050 ohm WRT earth according to soil tests.

Thanks for the clarification.

Dan

 

[coulter]

250.4(A)(1) states the requirement for limiting the voltage during lightning strikes, etc. ...

The "etc" part is the most interesting. Line surge for example. Can someone tell me: Just what is a line surge and how would a grounded system mitigate one?

carl


Edited cause I'm a slow typist

 

[iwire]

For mitigation of lightning and line surges, yes - absolutely.

Absolutley....not.

 

[Pierre C Belarge]

Jim, You're right on the one additional. I'll go back and edit that.
The #4/0 copper ring alone is expected in our case to be < 0.050 ohm WRT earth according to soil tests.

Thanks for the clarification.

Dan

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.