KYORITSU 4140 - Manual (Page 16)

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Forthisexamplethemaxpermissiblevalueis1667Ω(RCD=30mA
andcontactvoltagelimitof50V).Theinstrumentsreads12.74Ω,
thus the condition RA ≤ 50/Ia is respected. However, considering
thattheRCDisessentialforprotection,itmustbetested(Please
refertoRCDTESTSsection).
TN System
For TN systems the earth fault loop impedance is the sum of the
following impedances.
●Impedanceofthepowertransformersecondarywinding.
●Impedanceofthephaseconductorfromthepowertransformerto
the location of the fault.
●Impedanceoftheprotectiveconductorfromthefaultlocationto
the power transformer.
The figure below shows (dotted line) the Fault loop impedance for
TN systems.
Fig.7-3
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
ACforbothsinglephaseandthreephasecircuits).
Ia is the current causing the automatic disconnection of the
protective device within The maximum disconnecting times required
byIEC60364-41thatare: