Gx
Series
EN - 154
13.13. MEASUREMENT OF EARTH RESISTANCE IN TN SYSTEMS
Purpose of the test
Check that the measured value of earth resistance is lower than the maximum limit
calculated on the basis of the maximum allowable contact voltage
Utp
for the system.
In accordance with the requirements of standard EN50522 (for USA, Germany and Extra
Europe countries refer to the respective guidelines) the maximum allowable contact
voltage is dependent on the time duration of the fault according to the following Table 9
Fault duration [s]
Allowed contact voltage Utp [V]
10 85
5.00 86
2.00 96
1.00 117
0.50 220
0.20 537
0.10 654
0.05 716
Table 9 Maximum allowable values for contact voltage
Allowable values
The maximum earth resistance is calculated using the following relation:
g
tp
t
I
U
R
where:
Utp
= maximum allowable contact voltage in the system on the basis of the value
of Utp (the values not included in Table 9 are obtained by linear
interpolation) according to the duration time of the fault (value provided by
the Energy distribution board)
Ig
= maximum fault current in the system (value provided by the Energy
distribution board)
On the instrument, it is possible to select the value of the time duration of the fault in the
range between
0.04s
and
10s
and the value of the fault current in the range between
1A
and
9999A.
