METREL MI 3298, Instruction Manual With Exercises

Page: 42 / 232

Power Network Application Trainer
– Earthing / Grounding network impedance measurement
42
2 Earthing and Grounding resistance measurements
2.1 Basic theory about earthing
The basic rule to ensure safety on any object (residential/industrial) is to provide proper
and good grounding. Without or not proper grounding people lives could be threatened
and equipment damaged. Correct earthing of exposed conductive parts of the object
assures that the voltage on them stays below dangerous level in case of a fault. If fault
happens a fault current will flow through the earthing electrode. Metrel experts propose
regular testing to ensure proper operating conditions.
Earth resistance (impedance) value is normally rated in few ohms. We suggest
checking the applicable standards and national regulations before performing
measurements.
How earthing (earthing electrode) is done, depends on national regulation, building
regulations and standards. Typically, is done from:

Strips, tubes

underground earth loop

metal strip or cable sunk into the building concrete

metal plates
in different form, like: star, ring shape
d, network meshes, ….
Earth resistance is not concentrated in one point but it is distributed around whole
electrode. If a faulty current flow through the earth electrode, then a typical distribution
of the voltage occurs around it (voltage funnel). Maximum voltage drop is concentrated
around the earth electrode.
Earthing system measurement is normally done after installation to check if the
measured result meets the design criteria. Metrel also propose to perform periodically
measurements to ensure the earthing system is in proper condition (effect of corrosion,
changes in the soil resistivity etc.)
MI 3290 Earth Analyzer measuring methods are designed according relevant standards
and support different measuring methods:
Measuring method According to standard
2
– pole
3
– pole
4
– pole
EN 61557
– 5 [Resistance to earth]
IEEE Std 81
– 2012 [Two-point method, Three-point
method, Fall-of-potential method]
2 Clamps
IEEE Std 81
– 2012 [Resistance measurements by
clamp-on stake-less method]