Amprobe DGC-1000A Ground-Resistance-Tester - Manual (Page 15)

12
circuit is shown in Figure A. If R
1
, R
2
, R
3
, ... R
n
are combined as R
eq
,
then only R
g
and R
eq
are left in the circuit (refer to Figure B). If a
constant voltage is applied to the circuit, the following equation will be
derived.
V
I
R R
g eq
 
where
Req
1
1
, i 1, 2, ......, n
Ri
 

If R
g
and R
1
, R
2
, ... R
n
are about the same, and n is a large number
(such as 200), then R
eq
will be much less than R
g
and maybe approach
zero.
R
g
>> R
eq
(Req→0)
Example:
If R
g
and R
1
, R
2
, ... R
n
are all 10

, respectively and n = 200. Then R
eq
by calculation equals
R
...
.
e q

  

1
1
10
1
10
1
10
0 05 Ω
V
I
      
R R 10 0.05 10.05 R
g eq g
In this example, we can see that as long as the number of multiple
electrodes is large enough, the equivalent resistance is negligible with
respect to the ground resistance to be measured.