Kewtech KT64, Инструкция по эксплуатации, Страница: 29 / 44

26
The maximum value of Zs for this example is 2 . 10
Ω
( 16 amp gG fuse, 0 . 4
seconds). The loop tester reads 1 . 14
Ω
and consequently the condition Zs ≤
Uo/Ia is met.
9 . 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.
Fig. 23
LOOP/PSC/PFC