Calibrating Instruments
Setting the Loop and Line Impedance Output 4
4-13
ehq025.bmp
Figure 4-9. Line Impedance Calibration Display
Line and Loop Residual Impedance
While calibrating loop and line impedance functions, the Calibrator uses its ac line power
circuit as part of the measurement. The impedance of this branch circuit can cause a
significant amount of uncertainty in calibration. Therefore, it is important to ensure the
mains circuit the Calibrator is powered from has the lowest impedance.
Residual impedance is that impedance sensed at the power input socket of the Calibrator.
Residual line impedance is the impedance found between the line (L) and neutral (N)
wires, while loop impedance is the impedance between the line (L) and ground (PE)
wires.
Note
The actual residual impedance of the Calibrator and the power circuit to
which it is connected is used in the calibration process for the loop and line
impedance calibration. Therefore, this impedance sets the lowest
impedance value and becomes the “zero” point for calibrating UUTs.
To ensure the best quality and stability of calibration:
• Use only the power cord supplied with the Calibrator.
• Ensure the power outlet makes good contact with the Calibrator’s power cord.
• Have qualified service personnel ensure the branch circuit wire connections to the
power outlet are tight.
• Never use extension cords or power strips to power the Calibrator.
• Because residual impedance can be affected by other electrical devices attached to
the same branch circuit as the Calibrator, move these devices to other circuits.
Note
During a loop residual impedance test, internal test current flowing
between the L and N mains wires is approximately 2.5 A. Both line and loop
residual impedance tests can be performed on a protected network.
Selecting the Residual Impedance Correction Mode
To make accurate line and loop impedance calibrations, the Calibrator employs three
methods of residual impedance compensation present at the power inlet. Without
compensation, the actual residual impedance is excluded and only the selected resistance
is displayed. First, the residual impedance can be added to the calibration value manually.
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