Lightning protection for metal pole building

woodlot · Jul 14, 2006

I saw a PBS special once that talked about lightning rods. They do more than just direct lightning strikes to the ground.

Lightning is attracted to high places (like roofs) because as the wind blows over the top a charge is placed on it in relation to the ground. This diffrence in potential is what attracts lightning. By placing lightning rods on the roof the charge (and potiental diffrence) is unable to occur. So, in essance, lightning rods act as a lightning deterrant.

More directly to the point, as some already stated, an all metal building with electric service should be bonded directly to the grounding system of that service. I am unsure if additional ground rods would be of any benefit, or if they would would make the electric service grounding system be non code compliant. I'd sure want to talk to an electrician familiar with metal framed buildings before I started pounding ground rods all over the place.

If no electric service to the building, do whatever you want. If it where my building I'd connect the frame to two 8' 1/2' ground rods with a single bare 4awg soild cu wire. Placing the rods 6' apart. It might help direct a lightning strike into the ground.

srs · Jul 21, 2006

Thank you guys for your comments/suggestions. I thought Paddy's explanation was very good, but I'm still confused on this topic. I've ruled out lightning rods but need to do more research. Especially concerning placing ground rods at every corner and connecting the metal skin of the building with the rebar that's in the concrete to ground rods. Again thank you.

texasjohn · Jul 23, 2006

Paddy, let me ask a real life example question.

I just built a 40X60 metal building...

Concrete slab, rebar and reinforcing wire in slab

I beams are welded to metal plates on slab surface that were inserted into the slab while concrete was wet.

Weld plates had 4 rods about 1 foot long welded to each of them and these rods were pressed into the fresh concrete before it set up, leaving the weld plate flush with the concrete floor surface. As far as I know, these weld plates do NOT join to the rebar/reinforcing wire of the slab. Maybe they do if one of the rods happened to hit the rebar/reinforcing wire when the weld plate was inserted.

Roof metal and side metal is all screwed to the metal I beams and purlins, thus I am sure that all sides, roof and metal support strucures are electrically connected.

There is, currently, no electrical service to the building although I am in the planning stages for that.

For lightening protection, as best possible, what should I do?

From this discussion, it seems that I should do the following:
1) chip into the slab until I expose rebar/reinforcing wire and attach copper wire between that and an I beam.
2) carry a ground wire from the pole where the transformer is to the electrical service box in the building and make sure it is attached to an I beam of the building.
3) don't worry about lightening rods per se.

Did I get it right?

hitekcountry · Jul 23, 2006

Paddy

You say grounding won’t hurt, which implies that it wouldn’t mater whether that building was grounded or not. You say all is safe inside, yes that’s true as long as you stay inside, but consider the circumstance where you’re outside the building, lets say walking up to the door to enter and you grab the door knob on the steel door. If the surrounding environment is somewhat electricly charged and if the building is not grounded, you could be providing the last link to ground just prior to a discharge, in which case you could be in big trouble. In this scenario you would want the building to be well grounded such that the grounding system provides the path of least resistance.

midlf · Jul 23, 2006

Your weld plates are not technically connected to the slab re-bar. However as a practical matter it should be good enough. Concrete is contact with earth is considered "wet" and conductive. Thus the need for GFCI's in basements and garages.

TMcD_in_MI · Jul 24, 2006

hitekcountry said:
Paddy

You say grounding won’t hurt, which implies that it wouldn’t mater whether that building was grounded or not. You say all is safe inside, yes that’s true as long as you stay inside, but consider the circumstance where you’re outside the building, lets say walking up to the door to enter and you grab the door knob on the steel door. If the surrounding environment is somewhat electricly charged and if the building is not grounded, you could be providing the last link to ground just prior to a discharge, in which case you could be in big trouble. In this scenario you would want the building to be well grounded such that the grounding system provides the path of least resistance.

I think that in this scenario, you're going to be toast with or without a grounding system. I know that somewhere back in our lives we all learned that electricity follows the path of least resistance, but it's not exactly true. Electrical current can follow many paths at once, splitting itself proportionally so that the path of least resistance gets the most current, but other paths can also get plenty (which is why in your house you can have many different lights on at once, not just the one with the least resistance). When lightning strikes, there is such an incredible amount of current, that even though a lot of it will go through the grounding system, a high resistance side-path can easily carry lethal current too.

With the greatest respect,
Tom

Paddy · Jul 24, 2006

On vacation here so if I get your question correct, How important to tie the slab in to get the full Faraday cage/gausing effect?. Since the the slab is so directly conected with ground, minor requirements.

texasjohn · Jul 26, 2006

Paddy... thanks for the reply... will stop worying about completely bonding the reinforcing rods to the metal skin of the building..

Have a good vacation

OutbackL130 · Jun 13, 2014

I realize this thread is almost 10 years old, but I found it in a google search and just wanted to be clear on something. I recently put up 3 metal carport buildings. They are fully enclosed and set on concrete slabs that I had poured. The larger building has rebar in the concrete, but the rebar isnt in direct contact with the building in any way. The smaller buildings as far as I know have no rebar in the concrete. Will these buildings still create a faraday cage and protect a person inside during a direct lightning strike?

texasjohn · Jun 13, 2014

I don't think that a car port is sufficiently enclosed with metal structure to form a Farady Cage...The wikipedia definition is:

These rooms are spaces that are completely enclosed by one or more layers of a fine metal mesh or perforated sheet metal. The metal layers are grounded to dissipate any electric currents generated from external or internal electromagnetic fields, and thus they block a large amount of the electromagnetic interference.

Faraday cage - Wikipedia, the free encyclopedia