When the protective device is a residual device (RCD), Ia is the rated residual
operating current IΔn. For example in a TT system protected by an RCD the
maximum RA values are as follows:
Rated residual
operating
current IΔn mA
10 30 100 300 500 1000
Ra (at 50V) Ω
5000 1667 500 167 100 50
Ra (at 25V) Ω
2500 833 250 83 50 25
For this example the maximum value is 1667Ω, the loop tester reads
12.74Ω and consequently the condition RA ≤ 50/Ia is met. It is also important
to test the operation of the RCD using the RCD function in accordance with
BS7671.
The following condition shall be fulfilled for each circuit.
Zs ≤ Uo/Ia where Zs is the earth fault loop impedance voltage is the nominal
voltage between phase and earth and Ia is the current that causes the
automatic disconnection of the protective device within the time stated in the
following table.
Uo (Volts) T (seconds)
120 0.8
230 0.4
400 0.2
>400 0.1
Fig. 21
LOOP/PSC/PFC
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