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In Reply to: RE: No, Victor posted by drwkng on July 3, 2010 at 05:23:53:
how about page 199 of Ott' book on the grounding of sheilds.
How about personal communication with Ott directly?
The AC Power/Earth ground system is an array of long conductors, acting as antennas, picking up all kinds of noise. It is also heavily polluted with noisy AC power currents and their harmonics, from the power system and other equipment -- and completely out of your control. The earth is not a low impedance conductor. The resistance between two earthed points is rarely less than a few ohms and typically tens of ohms. The National Electrical Code allows the AC power ground to earth ground connection to have an impedance as high as 25 ohms.
Further:
If you did try to ground your product through a 6' power cord connected to the AC power outlet on the wall and then another 20' of house wiring to the earth ground, the conductors would have so much impedance (inductance) at RF frequencies that they would not keep the equipment at the same potential as the earth at all.
Let's take an example. At 20 nH per inch (typical inductance of a random wire), the twenty six feet of wire would have a total inductance of 6. 24 uH. At 50 MHz that amounts to an impedance of 1,960 ohms. So you see the futility of trying to ground your product to the earth using a wire. Even 6" of wire would have an impedance of 75 ohms at 100 MHz.
Let's look at the situation another way. The 26' of wire is over a wavelength long at 50 MHz, therefore it will act as a good antenna. If RF current flows between your product and the earth on the ground wire, it will radiate. If no RF current flows on the ground wire, you don't need the wire. Hence, at best the ground wire will be useless. At worst the ground wire will be an effective antenna and radiate. Therefore, you would be better off without it.
se
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Follow Ups
- RE: No, Victor - Steve Eddy 08:51:53 07/03/10 (4)
- as high as 25 ohms.... - jea48 15:48:10 07/03/10 (1)
- RE: as high as 25 ohms.... - Steve Eddy 17:15:09 07/03/10 (0)
- Since you know nothing about this subject, you become an easy prey to a man on a mission - Victor Khomenko 09:47:20 07/03/10 (1)
- More empty hand-waving - Steve Eddy 10:52:09 07/03/10 (0)
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