On April 12, 2004 03:21 am, Aron Smith wrote: > On Monday 12 April 2004 01:47 am, John Wilson wrote: > > On April 11, 2004 07:39 pm, Marc wrote: > > . > > > > In the vast majority of cases two grounds will simply, as you say, make > > things worse. > > Can't see that electricity is going to follow Ohms law and go the path of > least resistance so 2 grounds would be (r1+r2)/(r1*r2) =Rt > It doesn't quite work like that, though you might think it should. What a second ground (or third etc) would do, unless it's engineered properly, will create a different potential to earth. This can be due to differences in the makeup of the earth in the different locations, conductivity of elements in it and so on.
You can see something of this where you see a fork lightening flash strike the ground and the different "widths" and intensity of the light. Electrically lightening is complex, a mixture of frequencies, voltages and currents. Each of these can seek earth in slightly different ways depending on the combination. Another problem with mutliple grounds, unless properly engineered, is that it sets up the possibility, however slight, of a feedback loop. The lightening surge seeks and finds its best potential to ground and goes into the earth at a single point. This energizes the earth around it. When this energy reaches the second ground rod it is at a different potential than the first and can (will) induce current back up the rod and into the building. One solution to this is to erect a ground field around the building and, perhaps, below the base of the structure. The field itself is earthed much deeper. All ground rods are placed into this field which, in theory, reduces the potential to close to 0 ohms as the field itself is, in turn, earthed much deeper than the rods connecting to it. There remains a problem even with an earth ground field, no matter how well engineered. There are transients involved the most common one being the amount of water in a given part of the field at a given time. The presence of water affects the field and throws off the potential differences in different parts of the field, essentially setting up a second, third, fourth and so on, earth ground potential. We had an example of this many years ago at a small central office near a river. In the spring, during breakup, the river stayed within its banks but under the surface soaked the ground near it. One side of the CO had dry earth around the ground field, the other side, nearest the river, was soaked. The potential difference was so great that it took the CO down without any kind of storm or structural power failure. Welcome to complexity. :-) ttfn John
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