Hello All, An interesting fact: Some areas REQUIRE multiple ground electrodes on the power system. The state of Oregon electrical codes require at least 2 ground rods spaced at least 8 feet apart. The conductor connecting the rods to the system must be continuous and must run from one rod to the other and then to the the service box. The rods must be at least 8 feet long but can be driven at an angle in shallow soil. Extra grounds for individual equipment are not covered by these requirements. _______________________________ Al Allworth On the Beautiful Southern Oregon Coast ________________________________ ----- Original Message ----- From: "Charles R. Patton"
To: ; "Charles R. Patton" Sent: Thursday, July 13, 2000 10:36 AM Subject: Re:ground rods and noisy power supplies I wanted to make one additional comment regarding ground rods in general. It's very important NOT to use multiple rods in an installation. Not only does it add ground current noise as has already been mentioned, but it is a safety issue as well. In particular if you have a lightning strike in the vicinity, the ground currents become enormous, and so the voltage drop across the ground and thereby the voltage across the ground rods can be very large, and depending on the distance between them can be thousands of volts. We're not talking miles here, just feet. So if your equipment has connected grounds, suddenly opposite ends of your equipment are connected to a very powerful, high voltage source which causes large currents to flow, melting cables and of course, frying the attached electronic equipment. And the big point is, not only can it fry equipment, it can electrocute you. If you used multiple ground rods, but the grounds aren't connected, then the problem becomes the high voltages placed on the AC isolation means, i.e., the primary of the AC line transformers suddenly have to withstand huge voltages, again with the same result, breakdown and fried equipment. Within a single installation, use only a single point ground rod, and then provide a ground strapping/connection grid that acts as a reference "plane" for your system. Try to insulate the outlying sensors and bring their grounds back to your central system or else use total galvanic isolation which is a whole another subject in itself. A good idea on the insulated outlying systems is to provide lightning strike protection. A good example is the telephone system. They use carbon blocks separated by a thin mica sheet with holes in it or gas tubes from each line to the ground rod at the entrance to the house. These are very high impedance devices until they breakdown, at which point they can carry large discharge currents. I don't remember now, but I believe Sean-Thomas talked about some seismo equipment inside PVC pipe that took a lightning hit. Not a pretty description! Charles R. Patton __________________________________________________________ Public Seismic Network Mailing List (PSN-L) __________________________________________________________ Public Seismic Network Mailing List (PSN-L)
Larry Cochrane <cochrane@..............>