[TowerTalk] lightening strike

[Donald Chester]

Bill W4WHW wrote:
"...a point current source in the ground at some
distance between two separated ground points will cause a different voltage at
each ground point caused by the resistance between the voltage source point
and the ground point by Ohms law [E=IR]...lightning prevention
experts recommend that all grounds in a building be bonded together to a
common ground point. This point should also be the entry point for all
services coming into and out of the building. This is known as a single point
ground. The purpose of this is to prevent differential ground voltage rises
during nearby a lightning strike...the goal in an existing building is to minimize
the ground resistance between
separate circuit grounds by bonding them together with large gauge, low
resistance conductors. This is typically either copper strap or large gauge
copper conductors. This means interconnecting the electrical, cable, and phone
grounds together to a single grounding point with a heavy gauge low resistance
conductor."


All true, but there is one other factor to consider. Because of the rapid rise time of a lightning pulse, it is more like RF than it is to DC or 60 Hz a.c., so even large gauge wire copper wire or copper strap, with on extremely low DC resistance, does not guarantee there won't be a potential difference between two grounding points. The lightning pulse generates standing waves along the wire leading to the ground rod, and even in the earth itself, just as RF generates standing waves on an antenna or a feed line, and even though the DC resistance may be very low, the reactance at the predominant pulse frequency will not go away. If anything, the low DC resistance may raise the Q, allowing the pulse to generate even higher voltage between grounding points separated by an appropriate distance.

I'm not sure there is agreement on the dominant frequency in a lightning pulse and this probably varies widely, but I recall reading somewhere that it tends to be about 10 mHz, but I have seen figures elsewhere that dispute that figure. The safest approach is to treat the lightning ground like an RF ground, and use the same precautions you would use to keep the RF out of the equipment. That would include a single-point ground at the service entrance, and keeping the length of the path to ground for every device in the house as close as possible to the same. 

The lightning ground is NOT the same thing as the a.c. power safety ground. A perfectly satisfactory safety ground may still be a lightning hazard, and an excellent lightning ground might not conform to the code requirements for a safety ground.

A good illustration that ground voltage is not necessarily the same at every point on the earth, is the old story about a fault in a power line generating enough voltage gradient in the vicinity of a utility pole ground rod, that a man received a nasty jolt while urinating in the bushes next to the pole.


Don k4kyv