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From: "Ken Taylor"
Subject: Re: Ground Resistance monitor
Date: Sat, 26 Oct 2002 13:21:05 +1300
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Hang on, if the earth is "rarely if ever" tested, how do you know that "90%
of all homes exceed the 25 ohms recommendation"??
"N. Thornton" wrote in message
> I am a distributor of Residential Meter Base Surge Arrestors. These
> ;collar; devices are installed at the main electrical
> service entrance between the electric meter and the meter can. The
> basic premise is to prevent catastrophic surges from entering the
> house by limiting the amount of voltage allowed through the device and
> shunting the excess voltage to ground. The National Electric Code
> recommends a resistance level of 25 ohms or less at the service
> entrance, however, electrical inspectors rarely if ever test a ground
> as they are only required to visually inspect it. Henceforth, we know
> from practical experience that more than 90% of all homes exceed the
> 25ohm recommendation. Simply put, surge protection devices are
> rendered ineffective if a high resistance level exists at the service
> entrance. For example, in a C-1 impulse test (6kv@3ka) a service
> arrestor that has a let through voltage of 500 volts @ 25ohms will
> allow 1000 volts through @ 100ohms. A consumer can spend hundreds of
> dollars on surge protection products that for the most part will not
> work. For some strange reason I really would like to do something to
> make this situation better. Could there be a simple way to monitor
> the ground resistance and turn off a light source, if the ground
> resistance is too high. Or maybe the LED would change color (green to
> amber) when the resistance was above a preset level. Perhaps the LED
> could turn off if the ground wire became disconnected. How would I do
> this? Let me know.
> In principle is is not hard to measure earth resistnace or impedance.
> Simplest way is to put a 2nd earth spike in and measure R between
> Another way is to run some mains current to earth and see how much the
> V live to earth changes, thus finding out how much the earth rod
> voltage rises. But that should not be done in an occupied building!
> A 3rd way would be to feed (TF isolated) high frequency to the earth
> rod. The mains supply lines will have a relatively low impedance to
> earth at rf, and you cna measure the rods earth R safely this way with
> an isolated low V supply.
> I cant help thinking there is a more elegant way, but cant think of
> Regards, NT
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