The effects of lightening can be pernicious and insidious.
A lightening strike on AC lines may cause the voltage in the two 120 VAC lines coming into your house to go "high" perhaps to 1,000 volts during a split second transient or to 200-300 volts during a longer surge. Also, the neutral wire to your home may no longer be neutral but might carry several hundred volts for a short period of time. And, the "ground" in your home's system may have a voltage surge.
It has been observed that cows out in the field can get electrocuted by a lightening strike even though that strike was 100-200 yards away. The strike area might see a sudden rise in voltage to, say, 50,000 volts. The ground around presents resistance to the flow of its electricity so that as the voltage from the strike begins to travel outwards from the strike zone the voltage decreases. Now, suppose the poor cows were facing in the direction of the strike. Their front legs might experience a 10,000 volt shock and their hind legs, because of the resistance of the ground, might only experience a shock of 8,000 volts. The 2,000 volt difference between front and hind legs, as it flows through the cow's body, is sufficient to electrocute it. If all 4 of its legs had been very close together the voltage differential would have been reduced, perhaps to the point the cow would not be electrocuted.
So, if you get caught out in an open field during intense lightening activity make sure you only stand on one leg at a time (joke).
This explains why a home's earth ground should only exist at one point. Many homes have an earth ground at their electrical meters but may also have a different grounding point at the entrance for telephone service and a different one at the cable service entrance, etc. They should all have their grounds connected to the one at the meter. |
