5
．
4-wires Precise Earth Resistance Measurement
For
testing earth resist
ance
, first confirm the
earth
voltage
i.e.
the voltage between C
(H)
and E or
P(s)
and ES must under 20V
.
T
he meter NOISE
symbol
shows
when the earth voltage exceed
s
5V,
which
may
produce
resistivity measurement
error.
If
high earth voltage is found,
interrupt
power
supply
causing earth voltage.
Confirm the
earth
voltage decrease,
then test earth resist
ivity
again.
4-wires measurement: The 4-wires method can eliminate
the
influence
of
contact resistance (usually
caused by rust
) between
probes
, test clip
s,
and/or
meter’s input interface.
The 4-wires method can also eliminate
influence of line resistance.
I
t
is
much
better than 3-wires measurement.
As shown below: Start from
probe
E
, interval
10-30'
, respectively
insert
P
(S)
,
C
(H)
probes
deep into the earth in a straight line, and then
connect testing wires (black, green, yellow, red) from
E
,
ES
,
P(S)
,
C
(H)
interfaces corresponding to measured earth electrode
E
, auxiliary
voltage electrode
P
(S),
auxiliary current electrode
C
(H).
The distance from
probe E
to current electrode C(H),
should at least
ten
times
farther
than the embedded
depth
probe depth (h)
Testing wires
cannot
be w
ound
together
as this will
affect measuring accuracy.
-13-
>10h
