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52120A
Operators Manual
Potential Measurement Errors
When you make current measurements, there are some common measurement errors that
can have an unwanted effect on the output of the Product.
Leakage Paths
Current leakage paths are a problem with the calibration and measurement of very low
currents. This type of error is usually not a problem because of the high current range of
the Product. Unless there is very high humidity or equipment insulation is dirty leakage
path errors can be ignored.
Common-Mode Current
When referenced to local common or ground, there is a common-mode signal on the two
lines of a two-wire cable. These signals on these two lines are in-phase and have equal
amplitudes. The current flows through the signal conductors to ground through inter-
winding capacitance in transformers. Common-mode current does not flow through a
measurement device sense transducer. It does introduce an error if the device and its
electronic circuits are not perfectly balanced about earth potential.
Unless measurement devices and sources are fully isolated, common-mode current errors
have an effect on all measurements to some degree. A handheld measurement device with
no connection to an external power supply or earth is mostly immune to common-mode
current. This is because there is nowhere for the common-mode current to flow. Current
transformers are also immune to common-mode current. It is always good procedure to
attach the output LO connection to a ground to make some reference for guard shields.
Thermal sense devices have some immunity to common-mode current. Although the heat
source is electrically disconnected from the thermocouple, common-mode current can be
found in and around the device through capacitance.
There is a simple check for common-mode interference when the Product is used with a
stand-alone measurement device. Connect and then disconnect the output LO terminal
to/from the earth terminal of the Product while you monitor the measured value. Change
in the measured value is probably caused by common-mode interference.
To prevent damage to the Product, do not connect the LO
terminal to earth if the output is connected to an external circuit
that could be energized.
To decrease the effects of common-mode interference, connect one side of the signal
two-wire system to ground. The effectiveness of common-mode chokes increase as the
common-mode interference signal frequency increases. Figure 2-1 shows the
recommended ground and common mode choke configuration to decrease common-mode
interference if it is a problem.
2-4
W Caution
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