KYORITSU 6016 - Manual (Page 30)

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According to the International Standard IEC 60364, for TT systems the characteristics of
the protective device and the circuit resistance shall fulfill the following requirements:
Ra x Ia ≤ 50V Where:
Ra is the sum of the resistances in Ω of the local earth system and the protective
conductor for the exposed conductive parts.
50 is the maximum safety touch voltage limit (it can be 25V in particular cases like
construction sites, agricultural premises, etc.).
Ia is the current causing the automatic disconnection of the protective device within the
maximum disconnecting times required by IEC 60364-41:
- 200 ms for final circuits not exceeding 32A (at 230 / 400V AC)
- 1000 ms for distribution circuits and circuits over 32A (at 230 / 400V AC)
The compliance with the above rules shall be verified by:
1) Measurement of the resistance Ra of the local earth system by Loop tester or Earth
tester.
2) Verification of the characteristics and/or the effectiveness of the RCD associated
protective device.
Generally in TT systems, RCDs shall be used as protective device and in this case, Ia
is the rated residual operating current I△n. For instance in a TT system protected by a
RCD the max Ra values are:
Rated residual operating current I△n 30 100 300 500 1000 (mA)
RA (with touch voltage of 50V) 1667 500 167 100 50 (Ω)
RA (with touch voltage of 25V) 833 250 83 50 25 (Ω)
Shown below is a practical example of verification of the protection by RCD in a TT
system according to the international Standard IEC 60364.