Is a remote earth ground a good idea or a bad idea?

[lshaff01]

I was reviewing Mike Holt’s video “Grounding - Safety Fundamentals” on the internet and noticed something I was curious about. About 31 minutes in, (Statement 1) Mike shows how an equipment ground (earth ground) is required by the NEC at the service entrance and at a breaker box at a remote building some distance away. He says this is required for lighting protection i.e. to give a path the earth for lighting current to pass.

Later in the video, about 51 minutes in, (Statement 2) Mike shows how extra remote earth grounds connected to equipment is a bad idea. He says if there is a lighting strike near the extra remote ground, it can result in a voltage difference between the extra remote ground and the service entrance ground. This voltage difference can result in a current pulse that can travel from the extra remote ground through the equipment via the equipment ground wire to the service entrance ground rod. This can result in damage to the equipment.

These two statement seem juxtaposed in that: (1) is placing a earth ground at both the service entrance and a remote breaker box a good idea because the NEC says so or (2) is it a bad idea because a lighting strike near the remote ground rod will result in a current pulse that can travel from the remote ground rod through the equipment ground wire to the service entrance ground rod that will damage the remote breaker box and/or the service entrance breaker box?

Please comment. I hope I got my point across. Putting this in words without being able to draw pictures and wave my hands was difficult.

 

[tom baker]

The extra ground rods in statement 2 are auxiliary ground rods and are not required by the NEC, and that information he discusses was based on a study done by the Electric Power Reliability Institute (that I sent Mike Holt). The aux ground rods introduce another path for lightning, and the study said sometimes it helps, sometimes it can make it worse, or sometime makes no difference. I didn't keep a copy of the study but I will see if I can get a copy for my records.
With lightning we want to create a single point ground so there is no difference in potential. That is why we have a intersystem bond at services for telecommunications equipment.

 

[Joe.B]

I think I understand what you're asking here and I've also been slowly wrapping my head around all of this too. There are two separate issues to discuss in this topic. One is "required by code" and "what happens in reality."

The issue with nearby lightning strikes is induced voltage and that things in space are exposed to this induced voltage at a gradient. That means that an electrode at 10 feet from a strike will experience a different voltage potential that an electrode at 20 feet from that strike. If the two are connected together than there is a difference in potential between the two that can cause damage.

The issue that comes to my mind when discussing "required by code" is when you have a service to a building code requires an grounding electrode system at that building. If that service then has a code compliant feeder running to a separate (detached building) with a code compliant subpanel then it's required to have its own grounding electrode system.

Hopefully I'm right so far, if not I'm sure I'll be properly corrected. If so then we now had two separate electrodes (required by code) that have some distance between them and are electrically connected. If I'm understanding the science right a nearby lightning strike could cause an imposed voltage difference between the two electrodes and the conductors that connect them will have voltage and damage may occur.

Did I get that right?

 

[lshaff01]

Boy did you ever get it right. Well written. Sounds to me like your damned if you do and damned if you don't. So what's the right answer

 

[LarryFine]

My take is that statement 1 refers to a building's entrance panel (service or detached building sub-panel), and statement 2 refers to extra electrodes added to branch circuits or added for other individual equipment.

 

[tom baker]

Attached is a page from the EPRI article that Mike Holt based this graphic on. This is what the OP mentions as statement #2. The article is too large to attach and I will see if I can rescan later. click on the graphic to enlarge

 

[romex jockey]

can someone post this video plz?

~RJ~

 

[Joe.B]

Boy did you ever get it right. Well written. Sounds to me like your damned if you do and damned if you don't. So what's the right answer

Let me know if you find out!

 

[Joe.B]

can someone post this video plz?

~RJ~

I don't remember which one it is, but I think it's available here:

Mike Holt Enterprises - NEC Code Clips

Mike Holt worked his way up through the electrical trade from apprentice electrician through electrical contractor, to become one of the most recognized experts in the world as it relates to electrical power installations. He was a Journeyman Electrician, Master Electrician, and Electrical...

www.mikeholt.com

 

[romex jockey]

well here we have the whole 250.32 B vs the old 250.32 B Joe.......~RJ~

 

[Joe.B]

well here we have the whole 250.32 B vs the old 250.32 B Joe.......~RJ~

I've seen that mentioned here a few times but I'm still pretty new to all of this. the "old" allowed neutrals and ground to be bonded at the sub and did not require the additional GES? Something like that?

 

[tom baker]

I've seen that mentioned here a few times but I'm still pretty new to all of this. the "old" allowed neutrals and ground to be bonded at the sub and did not require the additional GES? Something like that

Old was before the 2002 NEC. that code no longer allowed regrounding the neutral at a separate building, so its been 20 years. In the old case the white wire had an imaginary green stripe. In the new case you had a white and green wire. Separate ground and neutral busses
In either the old or new, a grounding electrode system was required.
GES or ground rods for lightning
Green wire for bonding

 

[Joe.B]

can someone post this video plz?

~RJ~

I think it's this one:

 

[romex jockey]

In either the old or new, a grounding electrode system was required.

the dif was a 3 or 4 wire Tom

~RJ~

 

[Joe.B]

At about 25 minutes he starts talking about this.

 

[romex jockey]

I think it's this one:

yes, he dances all around the one fundamental

and is rather good at it

consider, the poco can run a 3 wire (Noodle & Ground) in their MNG system 3' from the next customer

~RJ~

 

[romex jockey]

At about 25 minutes he starts talking about this

At about 25 minutes he starts talking about this.

Yet fails to be specific Joe

we exist in a particular earthing system, that has particular electrical physics

Earthing system - Wikipedia

en.wikipedia.org

~RJ~

 

[Joe.B]

I hear you and I am starting to know enough to know that I really really don't know. The simple answer is "because code says so" but that's never been good enough for me, so I dig in with an open mind and try to understand what I can. From what I've gathered so far the "system" we have to work in has been a series of experiments of mostly trial and error, and choosing the lesser of potential hazards. There are multiple reasons for buildings to have a GES and it seems to me that the codemakers decided that the benefits of certain requirements do enough good that outweighs the edge cases when they cause problems.

 

[Sahib]

If only auxiliary grounds and no bonding, damage due to passage of lightning current likely. If bonding also present, not likely because all points would rise to the same potential on passage of any lightning current and consequently no damaging discharges likely.

 

[wwhitney]

Say you have a house and a detached garage 10' away, supplied by a feeder from the house. The house service requires a GES. The garage requires a GES; say there are no other metallic paths between the two buildings.

So what is a scenario in which safety is greater with the garage GES in place and connected to the feeder EGC, rather than just having the single GES at the house service? E.g. a fault condition, with some postulated impedances of various paths, showing that the presence of the garage GES reduces some hazard from the fault. And which requires that the garage GES be located at the garage, rather than just being additional grounding electrodes that reduce the total GES to remote earth impedance.

If necessary, increase the 10' distance to 100' for a good example.

Cheers, Wayne