METREL MI 3298, Instruction Manual With Exercises

Page: 69 / 232

Power Network Application Trainer
– Earthing / Grounding network impedance measurement
69
3 Earth Potential
Correct earthing of exposed conductive parts of the object assures that the voltage on
them stays below dangerous level in case of a fault (short-circuit current, lightning
strike). If fault happens a short-circuit current will flow through the object and equipment
into the earthing electrode. The earth resistance is non-zero, so current injected into the
earth at the grounding electrode produces a potential rise related to a distant reference
point.
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.
A typical voltage distribution occurs around the electrode, so called “voltage funnel”.
Fault currents close to power distribution objects (substations, distribution towers,
plants) could be very high, up to 200 kA. This can result in dangerous step and contact
voltages. If there are underground metal connections (intended or unknown) the voltage
funnel can get atypical forms and high voltages can occur far from the point of failure.
Therefore, the voltage distribution in case of a fault around these objects should be
carefully analysed.
Figure 50-2: Dangerous voltages on a faulty earthing system
where:
U
S
…. Step Voltage in case of a fault current
U
C
.... Contact or Touch Voltage in case of a fault current
U
F
.... Fault voltage