KYORITSU 6050 - Manual (Page 16)

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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.
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/la
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).
la is the value of current that causes automatic disconnection of the
protective device within 5 seconds.
When the protective device is a residual current device (RCD), Ia is the rated
residual operating current I⊿n.
For example in a TT system protected by an RCD the maximum RA values
are as follows;
Fig.10
Rated residual operating current I⊿n mA
10
5000
2500
30
1667
833
100
500
250 300
167
83
500
100
50
1000
50
25 RA (at 50V)Ω
RA (at 25V)Ω