Theory Of Operation - Greenlee CMGRT-100 Manuel D'instructions

Theory of Operation

The CMGRT-100 performs two measurements: resistance and current. It measures resistance by
applying a voltage at 1.689 kHz and indirectly measuring the resulting current. It measures current
indirectly by measuring the electromagnetic field around a conductor, which is proportional to the
current flowing through the conductor.
The following illustration and schematic diagram represent a typical multiple ground rod system.
The ground electrodes are represented by R (R1, R2, R3 ... Rn), and the CMGRT-100 is represented
by V.
The CMGRT-100 induces a voltage (V) into the system and measures the resulting current (I). The
CMGRT-100 then divides the voltage by the current to derive the resistance (V/I = R). The equation for
the resistance of Rx is shown below.
The equation shown above is the standard equation for finding the resistance of one leg of a parallel
circuit. However, the CMGRT-100 uses a simplified equation (V/I = Rx) since the rightmost section of
1
Σ
the formula ( ) is a very small amount of resistance if "n" is sufficiently large. This is shown
1
n
i=1 Ri
mathematically in the example below.
Example: A grid with 101 ground electrodes, each with a resistance of 25 Ω.
The measured resistance, Rx, would equal the resistance of Rx in series with the parallel resistance
of the other 100 ground rods:
Rx = 25 Ω +
Note, however, that most grounded systems have more than 100 ground electrodes. Therefore, the
parallel resistance is negligible. It is practical to simplify the equation and the calculation to V/I = Rx.
8
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where, usually
Rx = 25 Ω + 0.25 Ω
I
V
Rx R1 R2
Rx = 25.25 Ω
Rn-1 Rn