Manual

Forthisexamplethemaxpermissiblevalueis1667Ω(RCD=30mA

and contactvoltagelimit of 50 V).The instruments reads 12.74Ω,

thus the condition RA ≤ 50/Ia is respected. However, considering

that theRCD is essential for protection, itmustbe tested (Please

refertoRCDTESTSsection).

TN System

For TN systems the earth fault loop impedance is the sum of the

following impedances.

●Impedanceofthepowertransformersecondarywinding.

●Impedanceofthephaseconductorfromthepowertransformerto

the location of the fault.

●Impedance of the protective conductor from the fault location to

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

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: