KYORITSU 6016 - Manual (Page 32)

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According to the International Standard IEC 60364, for TN system the characteristics of
the protective device and the circuit impedance shall fulfill the following requirement:
Zs x Ia ≤ Uo Where:
Zs is the Fault loop impedance in ohm.
Uo is the nominal voltage between phase to earth (typically 230V AC for both single
phase and three phase circuits).
Ia is the current causing the automatic disconnection of the protective device within the
maximum disconnecting times required by IEC 60364-41 that are:
- 400 ms for final circuits not exceeding 32A (at 230 / 400V AC)
- 5 s 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 fault loop impedance Zs by Loop tester.
2) Verification of the characteristics and/or the effectiveness of the associated protective
device. This verification shall be made:
- for circuit-breakers and fuses, by visual inspection (i.e. short time or instantaneous
tripping setting for circuit-breakers, current rating and type for fuses);
- for RCDs, by visual inspection and test using RCD testers recommending that the
disconnecting times mentioned above are met (Please see RCD TEST section).
For instance in a TN system with nominal mains voltage Uo = 230 V protected by
General purpose gG fuses or MCBs (Miniature Current Breakers) required by IEC 898 /
EN 60898, the Ia and max Zs values could be: