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Loop Impedance Testing
Principle of Measurement – Hi-I
Hi-I Line-Earth loop measurement takes place in several steps:
a) The tester measures an unloaded half-cycle of the mains.
b) The tester switches in load RL.This causes the mains circuit to be slightly attenuated due
to the effective source impedance.
c) The tester measures a second half-cycle with the mains still loaded.
d) The load is removed.
The resistive part of ZI is calculated from the unloaded and loaded half-cycle measurements. Each
pair of measurements is taken adjacently to minimize noise and several such pairs may be used
for best accuracy. The inductive part of ZI is calculated from the unloaded and loaded half-cycle
durations. Any inductance causes a measurable lag in the loaded half-cycle.
Note: When testing on a supply with inductance exceeding 1mH or phase angle exceeding 30°
accuracy may be reduced.
RL
