T-305E User Manual Version 3.5 | 17
6.
Fault pre-location
Warning:
•
Cable fault location must be performed by properly trained personnel. A minimum of
two people should be present during the test, to ensure safety.
•
The operators must be at least 0.5 meters away from the HV connections during testing.
•
Before switching between different modes of operation, the user must completely
discharge the T-305E and switch it off.
•
When the test is complete, discharge the cable and surge generator manually using the
discharge rod.
T-305E allows several different methods of fault location. Where a triggered TDR unit is
required, the instructions assume the use of the Kehui T-906 which can be connected directly
to the TRIGGER/SIGNAL output port of the T-305E.
6.1
DC flashover method
The DC Flashover method requires a DC output from the T-305E which is increased until the
fault breaks down, the resulting current flow will trigger the T-906 fault locator.
This method can only be used when the fault resistance is very high, as otherwise, most of
the voltage drop is across T-
305E’s internal resistance
and the voltage at the fault point will
be too low to cause a break-down. In this case, the reading on the kV voltmeter will not
increase and there is a significant buzzing sound from the instrument.
Approximately 20% of cable faults can be located using the DC Flashover methods and most
faults that occur during preventive maintenance tests belong to this category.
The waveform obtained by the DC Flashover method is relatively easy to interpret. However,
the resistance of the fault point will decrease after several discharges, until the method can
no longer be applied. Therefore, the DC Flashover method mus
t
be used sparingly.
Operational procedure:
i.
T
urn the
MODE
to the
DC
position.
ii.
Power on the T-305E, the green
STOP
button lights up.
iii.
Turn the voltage
ADJUST
knob to the zero position, to enable the
START
control.
iv.
Press the
START
button, which then lights-up and the green
STOP
light turns off. The
sound of the HV protective device inside the instrument can be heard.
v.
Turn the
ADJUST
knob slowly, the HV voltmeter will display the voltage applied across
the fault point. When the voltage is high enough, the fault will break down,
accompanied by the capacitor discharge. Repeat the charge and discharge process, the
pointer of the kV meter swings back and forth periodically. Stop the voltage adjustment
and check the T-906 unit to confirm that it has triggered. The resulting waveform can
be used to measure the distance to the fault.
When adjusting the voltage, if the HV voltmeter pointer goes up slowly or remains static
and there is a loud buzzing sound from the instrument, it indicates that the DC Flashover
method is not suitable. The voltage
ADJUST
knob should be immediately returned to
the zero position. Under these circumstances a different mode must be used.
vi.
Once the process is completed, discharge the cable and surge generator manually using
the discharge rod (see section 5.1 Using the discharge rod)
