28
Ω
L - PE
L-PE
L-N
230
V
50.0H z
ATT:ON
!
For this example the max permissible value is 1667 Ω (RCD =30mA and contact voltage limit
of 50 V). The instruments reads 12.74 Ω, thus the condition RA ≤ 50/Ia is respected.
However, considering that the RCD is essential for protection, it must be tested (Please refer
to RCD TESTS section).
TN System
For TN systems the earth fault loop impedance is the sum of the following impedances.
●Impedance of the power transformer secondary winding.
●Impedance of the phase conductor from the power transformer to the location of the fault.
●Impedance of the protective conductor from the fault location to the power transformer.
The figure below shows (dotted line) the Fault loop impedance for TN systems.
Fig.23
Fig.24
