Do Lightning Rods Work...

Cecil has chosen this as a topic to elaborate upon; however, I have a bone to pick w/the concept!

I understand the theory, but…If you know basic wiring, can someone explain how the heck such a thin grounding wire can carry 1MM Volts? Impossible! Why do they have such a thin ground? Wouldn’t the wire be fried, and part of the house it’s on, too?

Maybe I’m missing something…

  • Jinx

Pass the Scooby Snacks…

heh sometimes they do sometimes they don’t

but then a church in Wales UK was burnt down from a lightening strike heh

This is probably off to Comments on Columns, but I’ll throw in…

Even if the ground wire burns out, the current is still going to flow along the path of least resistance, which is likely to be the hot (if not ionized) gas left where the wire was.

Whether the ground wire is adequate depends on both the voltage and the current. In a lightning strike, the voltage is massive but I don’t have numbers handy on the current involved, so I can’t give an off-hand sizing for the wire.

It doesn’t matter how many volts the lightning is. All that matters is the current, which is around 100 amps for lightning IIRC. Some wires are rated for a certain voltage, but that’s how much voltage the insulation can take before it might arc to another nearby conductor. You wouldn’t have that concern with a lightning rod ground wire.

Nope, lighting has a much higher current level, but its duration is very shortlived.

Happening to have my handy-dandy copy of NFPA 780 (the code that Cecil himself quoted…I feel so cool), 1995 edition in front of me, here are the sizes of the conductors that are required:

Copper conductors, structures under 75’ tall:
Air terminal, solid (the pointy thing): 3/8" diam.
Air terminal, tubular: 5/8" diam with 0.033" wall
Main conductor, cable: 17 AWG
Main conductor, solid strip: 0.051" x 1"

Now, I don’t work for NFPA (although I am a member), and I can’t speak for them. I only speak for myself with this statement: NFPA 780 looks like witchcraft to me. I have no clue where they came up with these numbers*, and they don’t say how they did it. Go figure.

  • Most fire codes of today are “disaster engineered.” Oh, crap, those doors were too narrow and people couldn’t fit out. Lets make the door width requirement bigger in the next edition of the code. Hmm, looks like those sprinklers didn’t put enough water on that fire and we lost the building. Next edition of the code, we’ll up the quantity of water needed. If you know the history of fires (Cocoanut Grove, Triangle Shirtwaist, Iriquois Theater, MGM Grand, K-Mart Warehouse, etc), you know where a lot of the code requirements come from. Perhaps they did the same thing with NFPA 780. Then again, maybe not. I’m not in a position to say how they got those numbers.

I may be a quasi-expert in fire science, but this lightning stuff scares me.