METREL MI 3298, Instruction Manual With Exercises

Page: 70 / 232

Power Network Application Trainer
– Earthing / Grounding network impedance measurement
70
Maximum allowed time / touch voltage / body current relations, according to EN
50522:2011 standard:
Fault duration
(s)
Permissible body
current I
B
(mA)
Permissible touch
voltage U
Tp
(V)
0,05 900 716
0,10 750 654
0,20 600 537
0,50 200 220
1,00 80 117
2,00 60 96
5,00 51 86
10,00 50 85
Table 11-2: Permissible body current I
B
and calculated values of the permissible touch voltage U
Tp
depending on the fault duration t
f
3.1 Ground Potential Rise, Step Voltage, Contact Voltage
3.1.1 Ground Potential Rise
Ground potential rise (GPR) is defined according the IEEE Std 81-2012 as the
maximum electrical potential that a ground electrode (grid, system etc) could achieve
related to a distant grounding point (potential of remote earth).
Under normal conditions, the grounded electrical equipment operates at near zero
ground potential. In case of ground fault part of fault current flows through earthing
system into the earth and causes the rise of the grid potential related to remote earth.
Analyse of ground potential rise is important in the design phase of electrical
substations because the high potential may be a hazard to people or equipment.
Potential gradient (voltage change over distance) may be so high that a person could be
injured due to the voltage between two feet’s (Step voltage) or between the ground on
which the person is standing and a metal object (Contact voltage).
In case, that there is other infrastructure located close to substation (under ground or
above ground such as: telephone wires, rails, fences, water supply…), could also be
energized because of ground potential in the substation. This transferred potential could
be also hazard to people and equipment outside the substation (many hundreds of
meters away from the fault location). Many factors determine the level of hazard: level
of fault current, soil type (specific resistance, which relates to the actual weather
conditions like moisture and temperature) and protection time to interrupt a fault
(standard value is 330 ms).
Ground potential rise measurements are supported in Metrel instruments MI3290 and
MI3295S & MI3295M using 3
– pole wiring method.
MI3290 Earth Analyzer: Current generator and volt-meter in one instrument: 40V AC, up
to 220mA, 55 to 329 Hz; 0.0 mV … 49.99 V