Alan:

I've got a wonderful book on lightning and EMP pulse protection for radio and TV broadcast plants that devotes about 90% of the book to getting good grounds in the field.

The one thing above all else it stressed was that earth was only a "good" ground when there was some level of moisture in the earth that contacted the grounding electrodes. In desert and rocky locations, it is nearly universal to specify "ufer" grounds (electrodes encased in concrete) because even when cured, the internal portions of the concrete retain moisture which helps to provide an effective path for electron movement. This moisture is so important, that on "ufer" grounds, if the electrode is less than 4" from the outside edge of the concrete, the current moving through it will cause the moisture to become steam and actually blow out the concrete.

Obtaining good earth grounds in dry/rocky climates is a continual challenge.

Brad

Read the Ufer ground link I posted or google "ground rods" and you'll run into all kinds of notes on moisture & pH factors. As far I can tell, not all dirt is created equally.

Talked to one my favorite electricians this morning and he agrees that many ground rods in our "bad" local dirt are really only cosmetic after a few years.

He also says the local inspectors are finally pushing for Ufer grounds, especially after they did their own local measurements on new construction & 2-3 year old foundations. They think the Ufer maintains or actually improves as it ages.


Lighting systems are where I really started to interact with electricians, and yes, Larry is *way* atypical. That's exactly why I'm trying throw out my half-baked ideas here so he can edumucate me.


I'm trying to walk that fine line of making my stuff actually work without becoming the annoying A/V guy.

We had a home with lightning strike on a tree 50 yards from the house... split the tree like a toothpick and vaporized the underground PVC water line and a buried LV landscape wire as it followed them back to the house...

* Shattered the outside Phone demarc box, vaporized 25' of Cat5 leaving just little blue PVC gobules and scortched marks on the studs until it hit our Panamax KSU4x4. Toasted the 4x4, but protected all downstream phones & modems. I was impressed.

* Blew several light swtches completely *out of the wall*. Had to re-paint around many other switches.

* Wiped out what I thought were low-tech durable things like the toaster, washing machine motor, icemaker, but left almost all A/V & computer gear intact.

* The concussion blew out the main picture window in the Living Room. The wife was in the Kitchen. When she sees a storm coming now she literally hides in a downstairs closet.

* Popped sheetrock screws in wood studs out... where is the conduction path there?

* The crazy part is it took a 2' chunk of concrete out of the foundation as it entered. I never understood how, but maybe your moisture is the key. I'd always thought of concrete as an insulator.

The simple answer is that the heat from lightning boils the water, and the expansion explodes, just like popcorn. A tree splits because the water in the tree turns into steam, which has a much higher volume for the same amount of water molecules.

Ufer grounds are effective because of a combination of the sheer area in contact with the earth, below the frost line, which is typically 2 to 3 feet, and the rebar, which is wire-ried together, and must be bonded with a #4 copper conductor or equivalent.

Driven rods are not consistant in their impedances. Soil conditions do vary with the seasons, and that does have an effect on impedance. Sometimes, rods that can be screwed together end-to-end are driven 50 feet or more for certain applications.

The damage described by Barry points out the importance of EVERY conductive pathway into a building having proper lightning arrestors installed, or bonded, like metallic piping. Plastic underground piping has a wire added to make it traceable.

Cable-TV coax would be much less of a problem if the NEC requirements were enforced. They should be grounded not only to a rod nearest the cable entrance, but that rod should be bonded to the power rod with a #6 copper or equivalent.

It is NOT safe to say that all underground metal piping qualifies as a grounding electrode. To do so, it must be unbroken for at least 10 feet underground (how to verify?), and the connection must be within 5 feet of the pipe's entrance into the building.

However, with the exception of incoming cables, ground loops are usually caused, and cured, within the premises. Ground loops and different-phase issues are related, but are different. Ground loops can occur even with all equipment on one circuit.


"That's all I have to say about that." - Forrest Gump

I have a friend who is a Leiutenant on a small town fire dept.
His best lighning strike story is when a metal clothesline frame took a direct hit and then the bolt jumped on to the underground phone line between the pedestal and the demarc.
He said it TRENCHED the run back open (his words "it looked like the little trail in the Bugs Bunny cartoon", and then the answering machine and phones exploded!!

All applicances and electronics were toasted, and there was a request for a full electrical inspection that came from either the Village or the Fire Marshal.

,

6 grand!!!!????? Ouch...................

Holy Crap.

Yep. that's why it is CHEAP to make sure the power distribution is right to start with! But at least you found a solution and hopefully you didn't have to eat any of it.

Wow, 6K. I'm writing another exclusion in our warranty right now.

good thought....