Newbie needs Advice on Grounded Metal Roof and whether to tie to Main electrical GND

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Walk_the_walk

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Due to neighborhood problems with poor quality electrical power from our electric Coop company, I have managed to get myself tangled into a debate with their head engineer about my grounded metal roof.

I live in central Florida, on a 2 acre parcel, which has been struck by lightning mulltiple times in the past 20 years. Two years ago, after installing a metal roof on my main home and detached garage, and THEN adding aluminum guttering and downspouts, a bolt of lightning struck the peak of the detached garage, traveled down the aluminum downspout, exploding the corner of the foundation, blowing concrete out 15 feet into the yard, and also destroyed the garage door opener and even exploded a small hand gardening tool that happened to be leaning on a peice of rebar protuding from the other corner of the garage. It was pretty stunning. This same lightning strike tripped most of the ground fault receptacles in the house but did not damage any equipment, although it caused charcoal particles in the shower water filter to get knocked loose. Weird.

After that, I contacted the metal roofing manufacturer who denied any problems with metal roofing and lightning... however, he had no answer as to whether adding aluminum guttering created a capacitive array, as I theorized it did. The guttering manufacturer was no help either.

So, I opted to ground the roofs via the metal downspouts from both roofs (house and detached garage each attached to a separate ground rod). So far, 2.5 years later we have had zero problems from storms. What we did was to drive a 10 ft. copper grounding rod in the ground next to the bottom of each metal down spout, then attach a copper bonding plate to each downspout and wire the bonding clamp to the rod with a solid #6 AWG copper wire and appropriate clamp. A quick and inexpensive way to remove the static charge from the metal roof on each structure.

Now, the power company's big Chief know-it-all is saying that according to NEC 2011 (and I think he is referring to 250.50 specifically although he did not say), that I have to have ground rods within 6 feet of one another and that I MUST connect the ground rod from the downspout on the main house roof to the electrical ground for the main electrical system. BTW, our house is frame construction, not metal, and the roof is not a structural element, per se, and the house was built in 1986 to those codes in affect then, and is currently grounded at the metal water faucet on the outside of the house, via the copper water piping built into the slab and the associated rebar network from its original construction. (This is still standard practice for slab-built homes in Florida to the best of my knowledge.)

My concern is this: 1) I do not believe that NEC 2011 applies to the grounding I did 2.5 years ago for the roof, and 2) I do not want to tie (bond) the metal roof copper rod ground and the main house ground together in case there is a evear a future lightning strike, for fear of "backfeeding problems", which we know do happen with lightning strikes, and 3) the metal roof does not qualify as a structural element, nor as something that would ever be "energized" (unless struck by lightning), so I do not see where 250.50 applies anyway.

I took the additional step of measuring the difference in ohms between the two grounds (the roof ground and the main electrical ground at the outside water faucet which are about 15 feet apart), using a Fluke digital multimeter and one leg of a car jumper cable. The resistance was only 0.5 ohms, after subtracting the resistance of the jumper cable wire used in the measurement.

I am willing to add a 10 ft. copper ground rod next to the water faucet and tie the main house electrical system ground to that instead of just relying on the water piping and slab rebar network, but even that is not a requirement.

Sorry about the long post: my question is whether anyone agrees with the electrical company's know-it-all guy, as to whether I should tie my metal roof ground rod back to the main house ground, which are 15 feet apart. My inclination is to NOT do this to prevent backfeeding. Please advise. THANKS!
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
Not bonding them together is a bigger hazard than bonding them together. There will be a huge voltage difference between the two "systems" if they are not bonded. This can create side flashes and additional damage.
See informational note #2 after 250.60. This section has been the same since at least the 99 code.
The reference to the required bonding of the electrodes is to a rule in NFPA 780 for lighting protection systems. I don't think your bonding qualifies as a lighting protection system, so the requirement is not triggered, but it is a good idea to make the bond because of the high voltage difference that will be created when 10s of thousands of amps of lighting current are flowing.
 

WIMaster

Senior Member
Location
Wisconsin
I agree with Don also.
The code requires that ground rods be a MINIMUM of 6' apart not 6'. In WI we are required to drive 2 of them. The NEC states it differently.
If you are going for lightning protection get a copy of NFPA 780 and read it CAREFULLY.
Again you should bond EVERY metal object and grounding system to prevent side flashes as Don said. This is very important! If you have metal gas piping bond it.
The heat of the arc flash is extreme and much worse than that on a good bonding path with current flowing on it.
Bonding everything will also help to dissipate the energy, Think about parallel resistors.
Studies have also shown that having a good lightning protection system in place will REDUCE the chance of being stuck by lightning.
 
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