Kewtech KT64, Инструкция по эксплуатации, Страница: 25 / 44

22
CAUTION
Never turn the Rotary switch whilst the test button is depressed as this
may damage the instrument. Never touch the circuit, test lead tips or
the appliance under test during insulation testing.
Always release the test button first after testing before removing the
test leads from the circuit. This is to ensure that charges stored in the
circuit capacitance have been totally discharged.
Note:
If the reading measured greater than 2000 M
Ω
( 200 M
Ω
at 250 V) the
over range reading > will be displayed.
9 . 1 Principles of Measurement of fault loop impedance and PFC
If an electrical installation is protected by over-current protective devices
including circuit breakers or fuses, the earth loop impedance should be
measured.
In the event of a fault the earth fault loop impedance should be low enough (and
the prospective fault current high enough) to allow automatic disconnection
of the electrical supply by the circuit protection device within a prescribed
time interval. Every circuit must be tested to ensure that the earth fault loop
impedance value does not exceed that specified or appropriate for the over-
current protective device installed in the circuit. The KT
64 takes a current
from the supply and measures the difference between the unloaded and
loaded supply voltages. From this difference it is possible to calculate the
loop resistance.
For a TT system the earth fault loop impedance is the sum of the following
impedances;
▲
Impedance of the power transformer secondary winding.
▲
Impedance of the phase conductor resistance from the power transformer
to the location of the fault.
▲
The impedance of the protective conductor from the fault location to the
earth system.
▲
Resistance of the local earth system (R).
▲
Resistance of the power transformer earth system (Ro).
The figure below shows (dotted line) the Fault loop impedance for TT systems.
9 LOOP/PSC/PFC