Manual

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16
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Fig.7-4
MaxvalueofZsforthisexampleis1.44Ω(MCB16A,characteristic
C),theInstrumentreads1.14Ω(or202AonFaultcurrentrange)it
meansthattheConditionZsxIa≤Uoisrespected.
InfacttheZsof1.14Ωislessthan1.44Ω(ortheFaultcurrentof
202 A is more than Ia of 160A).
In other words, in case of fault between phase and earth, the wall
socket tested in This example is protected because the MCB will
trip within the disconnection time required.
7.2 Principles of the measurement of line impedance and PSC
The methodformeasuring Line – neutral impedance and line-line
impedance is exactly the same as for earth fault loop impedance
measurement with the exception that the measurement is carried
out between line and neutral or line and line.
Prospective short circuit or fault current at any point within an
electrical installation is the current that would flow in the circuit if no
circuit protection operated and a complete (very low impedance)
short circuit occurred. The value of this fault current is determined
by the supply voltage and the impedance of the path taken by the
fault current. Measurement of prospective short circuit current can
be used to check that the protective devices within the system will
operate within safety limits and in accordance with the safe design
of the installation. The breaking current capacity of any installed
protective device should be always higher than the prospective
short circuit current.