Hioki MEMORY HiLOGGER LR8450 Instruction Manual page 282

Common-mode noise countermeasures
Inverter or commercial
power supply
Device under measurement
Thermocouple
Z
Coupled impedance
3
Voltage produced by earth current (common-mode voltage)
Zearth
Connect the chassis GND on the
signal side to a suitable ground with
adequately low grounding resistance.
Blocking external noise
• Keep signal lines away from noise sources.
Keep input signal lines (thermocouples) away from wires that could serve as sources of noise
(power lines, etc.).
Alternatively, install devices so that wires are kept as far apart as possible, for example by
running wires through separate ducts.
• Use shielded twisted-pair wire.
One effective way to counteract common-mode noise is to use shielded twisted-pair wires for
input signal lines (thermocouples).
Twisted-pair wires prevent electromagnetic induction, while shielded wires prevent electrostatic
induction.
Ground shielded wires at the signal source.
For more information about shielded twisted-pair wires for thermocouples, contact the
thermocouple manufacturer.
Electromagnetic
induction noise
Electrostatic
induction noise
r
Enm'
1
Normal-mode voltage
r
2
Ground line
Ecm
Zearth
Ground impedance
iearth
Either connect to a ground line with low resistance or
bypass the grounding electrodes. This will limit the
common-mode voltage and reduce noise current (i
The effect on measured values will be reduced since
normal-mode voltage is limited.
Noise Countermeasures
Measurement module
Vin
i =0 i = 0
2 1
Z Z
2 1
Zearth
11
, i ).
1 2
279