METREL MI 3298, Instruction Manual With Exercises

Page: 59 / 232

Power Network Application Trainer
– Earthing / Grounding network impedance measurement
59

4
– pole method is useful for measuring low resistance,

Test leads should be separated and not placed close and parallel to each other
(eliminating mutual inductance),
2.3.2.4 Selective (Iron Clamp) Measurement
– single frequency
and sweep mode up to 1.5 kHz
This measurement is applicable for measuring selective earth resistances of individual
earthing points in a complex earthing system (multiple grounds in parallel). The earthing
electrodes do not need to be disconnected during measurement.
Figure 38-2: Selective (Iron Clamp) measurement
Notes:

AC test current I
e
is injected through the test probe H (fixed or sweep mode; 55
Hz … 1.5 kHz)

Impedance of probe H should be as low as possible (to inject higher test current),

Voltage drop is measured between probe S and ES,

Selective current I
c
is measured through the selected earthing electrode Z
e1
by
current clamps

Selected impedance Z
e1
is determined by the voltage measured on the S
– ES
terminals and current I
c
measured by the current clamps.
2.3.2.5 2 Clamps Measurement
– single frequency and sweep
mode up to 329 Hz
This measurement method is used for measuring earth impedances of grounding rods,
cables, under- earth connections, etc. The measuring method needs a closed loop to be
able to generate test currents. It is especially suitable for use in urban areas because
there is usually no possibility to place the test probes.