Hioki RM3545, Instruction Manual, Page: 328 / 364

Appendix 7 Unstable Measured Values
A16
(8) Using Low-Power Resistance Measurement
Low-power resistance measurement employs a smaller measurement current than normal
measurements. Therefore, measurements are more susceptible to the effects of external
electrical noise and thermal EMF.
Measurement should be conducted as far as possible from devices emitting electric or
magnetic fields such as power cords, fluorescent lights, solenoid valves and PC displays. If
electrical noise ingress is a problem, see "Appendix 8 Mitigating Noise" (p.A19).
If thermal EMF is a problem, use the RM3545’s OVC function. If OVC cannot be used for
reasons such as tact time limitations, use a low-thermal EMF material such as copper for
wiring, and protect against airflow on connecting parts (measurement target or connectors).
(9) Measuring Transformers and Motors
If noise enters an unconnected terminal of a transformer or if motor rotor moves, measure-
ments may be unstable due to induced voltage on the measured winding.
The effects of noise can be reduced by shorting transformers’ empty terminals. Exercise
care not to induce motor oscillation.
(10)Measuring Large Transformers
When measuring measurement targets with a large inductance component and a high Q
value, such as large transformers, measured values may be unstable.
The RM3545 depends on constant current flow through the measurement target. To obtain
stability in a constant-current source with a large inductance, response time is sacrificed. If
you find that resistance values are scattered when measuring large transformers, please
consider the above or contact your local Hioki distributor for further assistance.
(11)Effects of cable configuration
To cancel thermal EMF, the RM3545 periodically reverses the polarity of the measurement
current (via its OVC function). Additionally, it only applies the current during measurement
to limit heat generation. Rapid fluctuations in this measurement current trigger correspond-
ing fluctuations in the magnetic field, inducing the following voltage in the voltage detection
line between SENSE A and SENSE B:
To avoid the effects of this voltage, the RM3545 waits for a fixed period of time after the
measurement current changes before acquiring the voltage between SENSE A and SENSE
B.
It is necessary to exercise caution when there are metallic objects present near the mea-
surement cable or measurement target. When the measurement current fluctuates, an
eddy current will be induced in such objects (see Fig. 8). This induced current is character-
ized by a sawtooth-shaped waveform and affects the voltage detection line between
SENSE A and SENSE B for an extended period of time (see Fig. 9-b). The eddy current
gradually decays due to the resistance of the metal plate, so its effect is more pronounced
the faster the measurement speed.
dt
dI
l
S
l
I
S
dt
d
dt
d
v
⋅ = 





= =
μ
μ
φ
d d
dt
μ
S
dt
μ
S dI
dt