[olug] One Point Twenty-One Jigawatts

[Eric Penne]

Lightning rods on houses really don't do that much for protecting your
equipment.  The lightning produces one hell of an EM pulse that can fry
equipment.  It also has enough power to "skip across rooms" in a Jacob's
ladder scenario.  One of the purposes of the lightening rod is to
dissipate the extra charge building up in the ground around the building. 
The charge follows the cabling up to the lightning rods and collects in
the tip (away from ground).  The tip is usually sharp so the charge will
concentrate in the tip and hopefully discharge into the air.  The
discharge is not designed to come all at once (lightening style) but to
come from consistent smaller "sparks" that are sometimes visible.  These
small sparks help get rid of the the charge slowly without building up
large amounts of charge that can cause lightning.

When lightning strikes it is looking for the path of least resistance.  A
properly grounded house with lightening rods will have less damage after a
lightening strike than a poorly grounded house.  Lightning carries a lot
of energy and if it is jumping across a room from outlet to outlet then
the grounding was probably not sufficient and/or there were no lightening
rods on the house.  Lightning can do funny things with metal siding also. 
Arcs jumping from piece to piece until it can't go any farther and jumps
to ground or nearby metal object like gutters and downspounts.  Makes the
whole house glow like in the movies. :)

To try to protect your home from a direct lightening strike, put up a
large metal object (antenna, steel pipe) that is taller than your house in
the yard.  When lightning strikes it will probably jump to the low
resistance metal object than travel through your house.  Your ears will
hurt from the concussion but your house will be safer.

This article in USA Today discusses research using blunt tip lightning
rods instead.
http://www.usatoday.com/weather/resources/basics/2000-05-15-lightn-rod-tests.htm

On a similar note, immunity testing for european electromagnetic
compliance includes ESD (static) tests.  The tests consist of two parts,
contact discharge and air discharge.  Air discharge uses a blunt tip tool
with about 8,000 V on it that when put near non conductive material will
sometimes jump to a nearby conductive object and cause the unit to fail or
shutdown.  This simulates the people walking up to a unit and touching it.

Eric


> On Wed, May 14, 2003 at 12:45:11AM -0500, Vincent wrote:
>> I agree with what everyone said (very good advice), but no one
>> addressed Joe's question.  There is no protection from Lightning that
>> may strike your home, or near enough to enter your house.
>>
>> Surge protectors and UPS will protect you from surges, spikes and
>> brown-outs, but would do no nothing against lightning.  Lightning is
>> just too powerful and will arc across any blown fuse.
>> I was in a house struck by lightning once and saw it arc from one
>> power outlet to another across the room.  If it'll arc 30+ feet, that
>> 1cm fuse won't be a problem and forget about "paths of least
>> resistance" too!
>>
>
> Did the house have lightning rods?  Anyone know if lightning rods on top
>  of houses actually work?
> kent
>
> --
> "I am always doing that which I can not do,
>    in order that I may learn how to do it." --Pablo Picasso
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