METREL MI 3298, Instruction Manual With Exercises

Page: 103 / 232

Power Network Application Trainer
– Earthing / Grounding network impedance measurement
103
Note:
Since the limit value selected, there is no PASS/FAIL indication. Measured impedance
value is much under the expected one. Additional, MI 3290 generate four alarms, which
could indicate the problematic measurement:
The measurement range of the instrument is exceeded.
Low current through Flex clamps. Result may be impaired.
High electrical noise was detected during measurement. Results may be impaired.
Expected result for foot number 4:
To get total object impedance, we need to calculate it from the individual values. Four
foots pylon could be presented as parallel connection of four impedances, so the
calculated object impedance is:
Foot Nm. 1 17,8
Ω
Foot Nm. 2 390
Ω
Foot Nm. 3 283
Ω
Foot Nm. 4 5,33
Ω
Total impedance 3,99
Ω
Table 26-
2: Pylon foot’s impedance
Measuring pylon resistance is time consuming procedure; since for each measurement
we need to move flex clamps to the next foot. Performing measurements in real
environment using all four flex clamps, enables additional specific foot resistance
analyse and detecting wrong (bad) earthing.