LIghtning - Grounding question from Mike Holt

[tom baker]

Mike Holt asked me to post the following to see if
Grounding is important to for reducing overvoltage of electrical wiring and metal parts of electrical system [250.4(A)(1) and (2)]. What I don?t know is how to calculate the needed ground resistance for a grounding electrode system. What bothers me about grounding to reduce overvoltage from lightning is that lightning is a high-frequency event and I?ve never seen this taking into consideration when ground resistance is discussed.

Which works best for a 25k ? 50K lightning event operating at a frequency of 5-10 kHz?
1. Ten feet of 6 AWG to an eight-foot ground rod having a contact resistance of 25 ohms.
2. Twenty-fifty feet of 3/0 AWG to a counterpoise consisting of three ten-foot ground rods have a combined contact resistance to the earth of 5 ohms.
3. Fifty feet of 250,000 kcmil copper to the utility primary grounding system which as practically zero ohms (because of the thousands/millions of connections of the primary neutral connection to the earth).

Thanks, Tom

 

[bob]

Which works best for a 25k ? 50K lightning event operating at a frequency of 5-10 kHz?
1. Ten feet of 6 AWG to an eight-foot ground rod having a contact resistance of 25 ohms.
2. Twenty-fifty feet of 3/0 AWG to a counterpoise consisting of three ten-foot ground rods have a combined contact resistance to the earth of 5 ohms.
3.Fifty feet of 250,000 kcmil copper to the utility primary grounding system which as practically zero ohms (because of the thousands/millions of connections of the primary neutral connection to the earth).

Thanks, Tom

Tom
The time I've seen associated with lightning strikes is 20 to 50 microseconds.
Assuming the Xfactor in the conductor tables is calculated by the formula
X = 2 x pi x F x L where F = 60 cycles an L = conductor inductance
L per 1000 ft for the following wire sizes are:

#6= 16.9 uh #3/0 = 13.3 uh #250 = 12.9 uh #500 = 12.1 uh 750 = 11.7 uh

X in OHMS/1000 for the same conductors at an impulse of 20 microseconds

#6= 51 #3/0 = 40 #250 = 39 # 500 = 37 # 750 = 35. As you can see there is not much difference in the impedance of the conductors.

I've checked utility grounding sustems and found them to be very high.
Some measured as high as 1000 ohms.

 

[dereckbc]

Tom I think your criteria is wrong and based on false assumptions.

1. Lightning is a much higher frequency event than what you have lined out. It is in the Mhs range, not Khz range. IEEE test determine use a number of KVA rise in .08-microseconds. For example 50 KVA rise in .08-microseconds. 8 microseconds = 12.5 Mhz

2. I have never seen a MGN lower than 3 or 4 ohm's, and seen many above 50-ohms

Lastly I would say for a residential application, it doesn't make a bit of diference what the GES impedance is as long as a GEC of any length is involved regardless of its size.

For example lets say we have two GEC's both 10-feet in length. One is a #4 AWG, and the other is 4/0 AWG. The single free-air resistance is 260 and 230 respectively. So what difference does the GES impedance makes with the road block of the GEC. Besides the measured impedance of the GES is meaningless since it is made at very low power frequencies and not HF or RF.

 

[bphgravity]

I agree with the other two comments and would like to add a few thoughts.

Article 250 grounding must be put into perspective. The grounding as required by Article 250 is NOT lightning protection. While 250.4(A)(1) implies grounding of electrical systems will protect against surging events, including lightning, it doesn't actually state that. The exact phrasing is "limit the voltage". Now we can all assume based on our understanding of electrical fundamentals that limiting the voltage on the system is a good thing, but the NEC does not require us to know that, nor does it require us to know how much voltage is bad, nor does it require us to know the BEST way to mitigate the damage of surging events. As soon as you pass 250.4, the rest of the article goes back to being minimum prescriptive code that permits a single rod electrode with a ground resistance of 25 ohms, connected by a #6 AWG grounding electrode conductor to serve as the grounding electrode system for ANY service to ANY building regardless to its size, capacity, or amount of equipment served, IF that electrode is the only one present.

Does that comply with 250.4(A)(1)? The answer is yes. The minimum required grounding as required in Part III of Article 250 must at a minimum comply with 250.4(A)(1) as there is nothing else in the article to indicate that you must go beyond the minimum requirement of Article 250, Part III to obtain a pre-determined level of performance. This is not the purpose of the code.

So why does 250.50 require us to BOND all the electrodes that are present at each building to form the grounding electrode system? Most assume it is to reduce the impedance of the system, as other sections like 250.56 seem to imply. Some assume it is to provide a greater level of lightning protection; however Article 250 grounding is NOT lightning protection. The answer again is 250.4(A)(1), to "limit the voltage imposed."

As the surging event is imposed on the electrical system, all parts and components of the system and building connected to the earth will begin to experience ground potential rise. With all electrodes bonded together to form the grounding electrode system, any and all equipment that has also been effectively grounded (bonded) will have a potential rise equal to that of the grounding system and all other properly grounded (bonded) equipment. If one electrode or piece of equipment is missed, the potential between those objects and other grounded objects around them will rise to a point that could be both hazardous to human life and damaging to property.

A good analogy would be river blocked by a damn. During a flood, the waters on the blocked side of the damn begin to rise to a higher level than the waters on the other side of the damn. The water may rise so high that it “spills” over the damn causing a huge rush of water to the other side. Now put that into an electrical perspective. The damn can be viewed as a lack of bonding between two objects connected to the same earth. During the surging event (the flood), the potential that is created between those two objects increases rapidly. This will create a flow of hazardous current (the spill over) between the two objects.

So as you can see, grounding of the electrical system in compliance with Article 250 will indeed limit the voltage imposed on the system. From a remote point, the voltage rise on the system may exceed several thousand volts; however no hazardous current flows as no potential between all near objects is created. So it doesn't matter what the earth impedance of the grounding system is.

This is all 250.4 is telling us. If you want REAL lightning protection, go read the NFPA 780.

 

[Pierre C Belarge]

Bryan
I agree.

Your post is well written (great anology)!!!

 

[ghostbuster]

Tom I think your criteria is wrong and based on false assumptions.

1. Lightning is a much higher frequency event than what you have lined out. It is in the Mhs range, not Khz range. IEEE test determine use a number of KVA rise in .08-microseconds. For example 50 KVA rise in .08-microseconds. 8 microseconds = 12.5 Mhz

Agree

Typical lightning impulse test waveforms for power transformers are 1 microsecond rise time as per IEEE and 40 or 50 microsecond decay time to 50% of peak value.That would put it in the Mhz. range for sure.

 

[wwhitney]

As the surging event is imposed on the electrical system, all parts and components of the system and building connected to the earth will begin to experience ground potential rise. With all electrodes bonded together to form the grounding electrode system, any and all equipment that has also been effectively grounded (bonded) will have a potential rise equal to that of the grounding system and all other properly grounded (bonded) equipment.

From this point of view, is it important that the conductors making up the GES and the conductors making up the EGC system be separate and interconnected only at the service? If so how does 250.54 fit into this picture?

Thanks, Wayne

 

[coulter]

Bryan -
Yes, this was well written explanation of the code. There is one section I don't understand your point, and a couple of places I don't agree with the physics.

... If you want REAL lightning protection, go read the NFPA 780.

First, I absolutely agree with this.

... Does that comply with 250.4(A)(1)? The answer is yes. The minimum required grounding as required in Part III of Article 250 must at a minimum comply with 250.4(A)(1) as there is nothing else in the article to indicate that you must go beyond the minimum requirement of Article 250, Part III to obtain a pre-determined level of performance. This is not the purpose of the code. ...(emphasis by me)

I read this paragraph several times and I am not sure what you are saying is, "...not the purpose of the code." Could you elaborate on this a bit?

...As the surging event is imposed on the electrical system, all parts and components of the system and building connected to the earth will begin to experience ground potential rise. ...

I'm having trouble with the physics here. In particular, this "surge event" - is not very well defined. IEEE and NFPA 70 are conspicuously silent.

I see four different events contributing to a "surge". I suspect only in one will the system react as the code suggests.

1. A close ground lightning strike: (I don't have a definition on "close") My analsys suggests the system will react as you have outlined.

2. A direct strike lightning strike to the non-current carrying, bonded, grounded parts: To quote mr Mike, "You're toast on a direct strike." I don't particularly agree with mr Mike on this one, particularly for industrial applications, but I do agree with your NFPS 780 statement.

3. Dropping an HV line across the MV feeding a service transformer or a lightning strike on the HV or MV feeding a service transformer: I suggest this is the surge that 250.4 is intending. My analysis suggests that a ground rod, ground mat, 1 ohm to ground even, does not do anything to "limit the voltage". For example, dropping a 138kV line on the 13.8 feeder to a residential service transformer will raise the customer side to 2400V. The utility equipment is fine, but it sure eats all of the equipment in the house. There are methods available to mitigate this "surge" but not only do we not avail ourselves, we still collectively think this magic ground will "limit the voltage" caused by a "surge".

4. A switching transient: Not really in the catagory as a surge, the rise time is very fast, but possibly still within the 250.4 intent. I don't see where a ground mitigates this any more than Item 3.

I'm quite willing to listen to other views, but the last time I asked about this question, I got a lot of answers on a lot of subjects, a lot of opinion, and a lot of "this is what I learned", very little physics, no reference to peer reviewed papers, and none that answered the specific questions.

... So as you can see, grounding of the electrical system in compliance with Article 250 will indeed limit the voltage imposed on the system. ...

Nope, don't see that. Surge suppression would, but not necessarily grounding. The physics is just not there.

... From a remote point, the voltage rise on the system may exceed several thousand volts; however no hazardous current flows as no potential between all near objects is created. ...

Nope, don't see this one. As I mentioned earlier, if the MV feeder goes up X10, the customer side goes up X10.

Article 250 is poor, and some sections do little to promote safety. But to paraphrase some of our inspectors and journemen, "You're not going to confuse me with reasoning and physics - If it was good enough for Peter and Paul, it's good enough for me" (This is susposed to be light humor - But I am aware sometimes I fail)

Oh yeah - just a reminder - I have some exceptions, but it still is a good analysis.

carl

 

[tom baker]

So why does 250.50 require us to BOND all the electrodes that are present at each building to form the grounding electrode system? Most assume it is to reduce the impedance of the system, as other sections like 250.56 seem to imply. Some assume it is to provide a greater level of lightning protection; however Article 250 grounding is NOT lightning protection. The answer again is 250.4(A)(1), to "limit the voltage imposed."

Bryan: After instructing many grounding classes and reading IEEE Green Book, I came to the conclusion that:
1. Bond everything together to create an equal potential ground plane.
2. Don't get too concerned with 25 ohms, 5 ohms or 1 ohm. Bond all together and use a UFER ground, forget the ground rods.
3. 99% of what we do in grounding and bonding should be on the bonding side. But thats not the question being asked by Mr Holt.

Great comment on limit the voltage imposed.

 

[ghostbuster]

Here is some additional info on typical lightning strike waveform frequencies and shapes for calculating voltage rise/drops etc. in the grounding systems.

http://ecmweb.com/power_quality/electric_putting_microscope_2/

 

[bob]

Bryan -

I read this paragraph several times and I am not sure what you are saying is, "...not the purpose of the code." Could you elaborate on this a bit?

Coulter
I believe Bryan is saying the the NEC does not deal in depth with lightning.
The National Lightning Institute does put out specification for a lightning
system installation.

3. Dropping an HV line across the MV feeding a service transformer or a lightning strike on the HV or MV feeding a service transformer: I suggest this is the surge that 250.4 is intending. My analysis suggests that a ground rod, ground mat, 1 ohm to ground even, does not do anything to "limit the voltage". For example, dropping a 138kV line on the 13.8 feeder to a residential service transformer will raise the customer side to 2400V. The utility equipment is fine, but it sure eats all of the equipment in the house. There are methods available to mitigate this "surge" but not only do we not avail ourselves, we still collectively think this magic ground will "limit the voltage" caused by a "surge".

It is my understand the this section addresses the problem of a high voltage
circuit making contact with ground(dirt) a raising the ground potential. The system being all bonded to the ground rod, will experience a voltage rise
but it will all be at the same potential.

4. A switching transient: Not really in the category as a surge, the rise time is very fast, but possibly still within the 250.4 intent. I don't see where a ground mitigates this any more than Item 3.

It doesn't but this is not lightning.

Nope, don't see that. Surge suppression would, but not necessarily grounding. The physics is just not there.
Nope, don't see this one. As I mentioned earlier, if the MV feeder goes up X10, the customer side goes up X10.

Correct but this is a different problem. The question is about lightning striking
the ground(dirt) or some structure.

 

[brian john]

I contacted Lyncole regarding Mike's question but have not received a response to date, I did receive the attached and thought it might be of interest.

I have no affiliation with Lyncole and attach this purely for informational purposes.

 

[coulter]

... The question is about lightning striking
the ground(dirt) or some structure.

Good thing you cleared me up on that. I must have missed that phrase when I read Tom's OP.

carl

 

[marshallf3]

I've been a ham radio hobbyist for some years, and with the rise and fall times one can see from a lightning strike you could almost consider it to be RF current.

With this in mind we must remember the "Skin Effect" :

http://en.wikipedia.org/wiki/Skin_effect
http://www.mos.org/sln/toe/skineffect.html
http://www.google.com/search?hl=en&q="skin+effect"&btnG=Google+Search

In essence a stranded wire or several wires in parallel will be more effective carying lightning surge current than would be a single solid conductor.