Hioki FT3151 - Measurement Precautions and Tips

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Measurement Precautions and Tips
2.5 Measurement Precautions and Tips
Using the auxiliary earthing rods
For 3-pole measurement, two auxiliary earthing rods are required.
Be sure to drive the rods well into the ground to assure correct
measurement results.
Earth resistance of auxiliary earthing rods
When the earth resistance of the auxiliary earthing rods is not higher
than about 10 k
Ω
, the FT3151 can carry out correct measurement.
However, especially when relatively low earth resistance values are
measured, high earth resistance of the auxiliary earthing rods can
impair measurement sensitivity.
To assure correct measurement results, be sure to check the earth
resistance of the auxiliary earthing rods by setting the range selector
to H and S. If the needle of the galvanometer rests within the green
band of the scale, the auxiliary earth resistance is within 7 k
Ω
.
If check results are unsatisfactory:
• Drive the auxiliary earthing rods deeply into the ground and water
the entire area with a sufcient amount of water. In particular,
watering is usually effective in reducing the contact resistance.
• Change the location of the auxiliary earthing rods. Choose a
location with high humidity.
If the ground is volcanic rock or sand, the supplied auxiliary earthing
rods cannot lower the auxiliary earth resistance sufciently. In such
a case, use a metal pipe or other conductive object with a large
surface and bury it as deep as possible in the ground.
Distance between earthing electrodes
As shown in the gure (a) on the next page, the distance between E
and H(C) is given by
l
m; and the distance between the E and S(P),
by
x
m. If the resistance of the earthing body E is measured while
x
is being varied, the result will be as shown in the gure (b).
Thus, the measurement errors become larger as the auxiliary