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lightning hazards

Megunticook | Posted in Energy, Heating & Insulation on July 9, 2004 01:17am

Over the fourth we had some ferocious thunderstorms that swept through Maine. I was a few miles away from home at the time, but when I returned I found my cable modem and network hub had been fried–fortunately I had unplugged all computer equipment from power and network before leaving. The breaker for one circuit in a room upstairs was also tripped (still haven’t figured that one out).

A house in a neighboring town burned to the ground after lightning apparently struck a nearby tree, ricocheted over to the service entrance, and somehow started a smoldering fire when nobody was home. Poor family returned from the holidays to a smouldering pile.

At the time of the storm I was actually sleeping in a cabin in a bed next to a VHF radio with a roof-mounted aerial. I had disconnected the aerial as a precaution, but then I got nervous about being too near the end of the cable running to the aerial and left the bed during the worst of the storm. Is that paranoid or smart?

All of this has me thinking–what can I do to protect my house and systems from lightning? Electrical work was done by a pro and everything’s properly grounded, and I grounded the panel containing the phone and computer network system.

Should I have surge arrestors? What about a lightning rod on the house? Or at some point are you at God’s mercy?

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  1. junkhound | Jul 09, 2004 04:22pm | #1

    1.  Should I have surge arrestors? In your area, yes - refer to the NASA thunder site for relative need.  Here in Seattle area, almost nobody has lightning rods, not even the power company (they lost 128 transformers in an unusual 1988 lightning storm though, still not cost effective here. .

    e.g. http://thunder.nsstc.nasa.gov/lightning-cgi-bin/otd/OTDSearch.pl?single=1998.197?alat=2?alon=2?months=na?selected=na?file=14??215,96

    2.  What about a lightning rod on the house? Yes in your area, but successful installation means you have a GOOD ground (not just one 8 ft rod dirven into gravel) and the down conductors are the equivalent of 1/2 type L copper pipe with BRAZED (not soldered, the solder melts and spits out and the joint opens for lightning currents) connections. The old 30 degree cone of protection is still often used, but even better (and the only really valid way to determine lightning rod locations) would be to download one of the free electric field modeling tool (Ansoft/Maxwell 2d student version, Quickfield eval program, etc) and draw out and put lightning rods where the drawn model of your house shows high field intensity. "apparently struck a nearby tree" - yoou need to add the trees to your electric field model also. 

    Or at some point are you at God's mercy?  If you do the above correctly, God will already have shown mercy by giving you the brain and previous knowledge to let you protect yourself.  50% of lightning is below 20 kA, 99.999% below 300kA, and the 1/2" type L brazed will handle 300 kA if screw clamped to the house every couple of feet (doesn't fuse, but can break due to magnetic forces if not adequately clamped down). I once put 4 MegAmps thru a 3 foot section of 500 MCM, it broke it.

    Edit PS: If you do go to the trouble of learning and doing an electric field model of your house, e-mail me and I'll help you interpret the results and help you determine the type ground you really need - typical 'pro installed' house has code compliant ground but is likely inadequate for lightning protection, - unless you have a poured basement with welded rebar brought out as the ground, that is probably the best.



    Edited 7/9/2004 9:29 am ET by JUNKHOUND

    1. JohnSprung | Jul 09, 2004 09:53pm | #3

      > 50% of lightning is below 20 kA, 99.999% below 300kA,

      Very interesting.  Do you have any information on durations and voltage gradients?

      -- J.S.

  2. JRuss | Jul 09, 2004 08:56pm | #2

    You are aware of the #2 tin foil hat design I hope. Be safe.

    Never serious, but always right.
  3. DanH | Jul 10, 2004 12:07am | #4

    You should always have surge arresters on any lines entering the house. This includes electric, phone, cable, etc. The phone guys provide semi-effective surge protection on the phone lines, but having an extra protector ahead of any expensive equipment (computers, satellite gear that needs a phone line, etc) is wise.

    For cable (or TV antennas), grounding the cable shield where it enters the building (and grounding the antenna mast) is 90% of the job (though that other 10% can still be a big problem). So it doesn't hurt to have one of those fancy plug strips with a cable arrester on it, at least at your high-value devices. It's also is a good idea to ground the chassis of any cable amps, cable modems, etc that you might have. There are also available arresters that can be placed in an ethernet line to block anything that might get through a cable modem, eg.

    For power lines, it's good to start with a whole-house arrester. These can be installed in or adjacent to the breaker panel, or there is a variety that installs behind the meter (though electric co assistance is obviously needed for that). One thing to keep in mind, though, is that the larger the arrester, the slower it is, so brief pulses can "sneak through" a whole-house unit. So individual arresters on your most valuable equipment are a good idea.

    How much of this you really need depends a lot on where you live. I have what is presumably lightning-caused damage about once every 10-15 years (so far have lost a modem, a firewall box, two ethernet cards, and a rooftop antenna preamp in 28 years). Since the useful life of electronic equipment is generally 5-10 years, that rate doesn't merit extreme measures. But other folks, with an equivalent level of protection to mine, would experience damage several times a year, so obviously they need more protection.

    Keep in mind that there's not a whole lot you can do to protect from a direct hit from a mega-bolt. But a lot of damage is caused by "minor" lightning bolts, or hits (and near-misses) farther down the power/phone/cable line from where you live. These are what you try to protect from.

  4. Don | Jul 10, 2004 02:37am | #5

    Dear Talking Horse: Lightning is a whopping current. Any time you have a whopping current, you have whopping magnetic fields around it, with the resultant whopping inductive coupling to any other conductors that are even remotely parallel to the lightning bolt. I once witnessed a lightning bolt strike NEAR a house. About 5 min later, the FD showed up. A fire started in an electrical box in a wall adjacent to the strike. The bolt apparently induced enough current in the house wiring to cause an arc that started a slow fire.

    Second - don't get near the base of any metal rod or other conductor that can act as a lightning receptor. Just think - the bolt just jumped a space measured in thousands of feet - another few feet won't slow it down. You don't want to be the ground connection for a bolt.

    Don

    The GlassMasterworks - If it scratches, I etch it!

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