KYORITSU 4118A Manual De Instrucciones página 38

The figure below shows in marked line the Fault loop impedance for TN
system.
According to the international Standard IEC 60364 for TT system the
following condition shall be fulfilled for each circuit:
Where:
◦ RA is the sum of the resistances of the local earth system R and
protection conductor connecting it to the exposed conductive part.
◦ 50 is the max contact voltage limit (it could be 25V in particular cases)
◦ Ia is the current causing the automatic disconnection of the protective
device within 5 s.
When the protection device is a residual current device (RCD), Ia is the rated
residual operating current I
For instance in a TT system protected by a RCD the max RA values are:
Rated residual operating current I ⊿ n.
RA (at 50V)
RA (at 25V)
Note:
● The loop tester models 4118A measure the fault loop impedance
that is a value normally a little bit higher of RA.
But, if the electrical installation is protected considering the loop
impedance value, also the RA formula will be fulfilled.
Fig.7
＜
RA 50/Ia
n.
⊿
10 30 100
5000 1667 500
2500 833 250
— 15 —
300
500 1000
167
100 50
83
50 25
mA
Ω
Ω