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.
In accordance with the international standard IEC 60364 for a TT system the
following condition shall be fulfilled for each circuit.
RA must be ≤ 50 / Ia
where;
RA is the sum of the resistances of the local earth system R and the protective
conductor connecting it to the exposed conductor part. 50V is the maximum
voltage limit (it may be 25V in certain circumstances).
Ia is the value of current that causes automatic disconnection of the protective
device within 5 seconds.
Fig. 19
Fig. 20
LOOP/PSC/PFC
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