KYORITSU 4300 - Manual (Page 17)

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8. Earth resistance measurement
This instrument can measure the earth resistances of distribution
line, internal wiring and electrical appliance.
# DANGER
● Never apply inputs exceeding 300V between the measuring
terminals at earth voltage measurements.
● No voltage should be applied between the measuring terminals
during earth resistance measurements.
8-1 Principle of measurement
This instrument carries out earth resistance measurements based
on the fall-of-potential method. This is a simplified earth resistance
tester and uses the existing earth systems (with a sufficiently
low earth resistance) such as buried metal pipes like main water
pipe, common earth for commercial power supply and a lightning
electrode on buildings and performs measurements based on the
2-pole measurement method.
The AC constant current
“ I” is applied between the
measurement object “ Rx”
(earth electrode) and the
existing electrode “ re” to
obtain the earth resistance
value “ Rx + re” and find out
the voltage “ V” between the
E and H+S(C+P) terminals.
See Fig. 8-1.
Rx + re = V / I
The resistance “
re” of the existing earth electrode with which
H+S(C+P) terminal is connected is added to the true resistance
“ Rx” of the measured object, and displayed as the measured result.
Re (measured value) = Rx + re
If the value of “ re” is already known, deduct it from the measured
value “ Re” to determine “Rx” value.
Rx (true resistance value) = Re – re
H+S (C+P)
terminal
E
terminal
Voltmeter
Earthing system
under test
Earth electrode
Constant current
generator
Secondary Primary
I→
Fig. 8-1