16
3.
ME AS UR E ME NT P R INC IP L E
The schematic diagram below illustrates the general case of measurement of the
resistance of a loop comprising:
■
The earth electrode Rx;
■
The earth;
■
Several earth electrodes of resistance R
i
;
■
A guard wire looping all of these earths, introducing an inductive component.
The clamp combines two functions in the measurement head:
■
The generator winding of the clamp emits an alternating voltage having a
constant level E.
■
The receiver winding (current measurement) senses I=E/Z
loop
.
Knowing
E
imposed by the generator and
I
measured, the value Z
loop
can be
deduced and displayed on the device. The
Advanced
mode makes it possible to
distinguish the resistive and inductive parts and to refer the impedance to the
network frequency.
More generally, this principle can be used to search for a defective earth. This is
because the loop resistance comprises:
■
Rx (the value sought);
■
Z
earth
(a value that is normally very low, less than 1Ω);
■
R
1
//R
2
...// R
n
(a negligible value: case of multiple earths in parallel);
■
Z
guard wire
(a value that is normally very low, less than 1Ω).
■
R
loop
=Rx+Z
earth
+(R//R...//R )+Z
guard wire;
As an approximation, Z
loop
can therefore be treated as equivalent to R
x
.
If this value is very high, an inspection of this earth electrode is strongly
recommended.
Z
earth
Z
guard wire
Fig. 7
