Hioki 3155-01, Instruction Manual

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5.9 Low Resistance Measurement Mode (ME Equipment)
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5.9.1 Low Resistance Measurement Mode
Prescribed current
Voltmeter
R0
I
R
1
R
2
E
0
R
3
R
4
Resistance measuring circuit
R1 to R4 are the measuring lead and connector resistance.
E
5.9 Low Resistance Measurement Mode (ME Equipment)
Under IEC 60601-1 (1988-12) + am1 (1991-11) + am2 (1995-03), measurement
of resistance of the protective earth conductor is to be made at 25 AAC of
current. The 3155, however, makes this measurement simply using the DC
four-terminal method (at 100 mADC max.).
This measurement is made for Class
equipment only.
Measurement is made by touching either the optional 9461 PIN-TYPE LEADS
or 9287 CLIP-TYPE LEADS to the test point. Measurement is also possible
using RS-232C interfacing with the 3157 AC GROUNDING HiTESTER (max.
31A AC, conforming to various standards), available separately.
Four-terminal method
When measuring low levels of resistance, ordinarily the resistance of the
measuring lead and the connector are included, which results in errors. By
using the four-terminal method, this error can be eliminated.
The resistance measuring circuit measures the drop in voltage at the
prescribed current, and calculates the resistance. As shown in the figure,
however, the input resistance at the voltmeter is extremely high, and all of
current I flows into the measured resistance R
0
. Therefore the drop in voltage
from R
2
to R
3
becomes 0, the voltage E and the voltage drop E
0
at both ends of
the measured resistance R
0
become equal, and the resistance is measured
without the influence of R
1
to R
4
.
(R1 to R4 indicate the measuring lead and connector resistance.)