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From: email@example.com (N. Thornton)
Subject: Ground Resistance monitor
Date: 25 Oct 2002 10:04:56 -0700
NNTP-Posting-Date: 25 Oct 2002 17:04:56 GMT
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
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