26
S&C Instruction Sheet 695-510
Appendix A
S&C Vista SD Underground Distribution Switchgear
was designed to allow dc testing of the cables with the
other ways of the gear energized. After testing, the dc test
equipment should be used to discharge any stored charge on
the cable. The dc test voltages and dc cable thumping voltages
should not exceed the voltages given in Table 2.
WARNING
When testing cables connected to energized switchgear,
proper isolation of the power-frequency source from the
dc test source must be maintained . Follow the recom-
mendations provided by the manufacturer of the dc test
equipment or fault-locating equipment . Follow the user’s
operating and safety procedures for grounding the cable,
connecting the dc test source, isolating the dc test source
(in case of flashover), ungrounding the cable, applying the
dc test source, discharging the cable, and regrounding the
cable .
Table 2. Maximum Cable-Testing and Cable-Thumping
Dc Withstand Voltages
Vista SD Switchgear Rating, kV
Dc Cable-
Test
Voltage, kV
Dc Cable-
Thumping
Voltage,
kV
①
IEC
IEEE
Impulse
(BIL)
12
17 .5
95
34
17
24
29
125
40
20
①
The dc cable-thumping voltage is 50% of the dc cable test voltage
because voltage doubling will occur at the open end of the cable, which
is assumed to be a unit of Vista SD Underground Distribution Switchgear .
If the open end of the cable is grounded, the dc cable-thumping voltage
applied to the cable and switchgear can be increased to the dc cable-test
voltage .
Very Low Frequency (VLF) Cable Testing
IEEE Standard 400.2, “IEEE Guide for Field Testing of Shielded
Power Cable Systems Using Very Low Frequency (VLF) (less
than 1 Hz),” addresses the application of 0.01- to 1-Hz high-
voltage ac excitation as one means for evaluating a shielded
power cable system during an acceptance test or a maintenance
test. The cable system must be taken out of service for this
testing.
An acceptance test is a field test made after installation of
the power cable system, including terminations and joints,
but before the cable system is placed in normal service. A
maintenance test is a field test made during the operating life
of a power cable system to detect deterioration and to check
serviceability of the system.
VLF cable testing may subject the S&C Vista SD Under-
ground Distribution Switchgear to the ac test voltage when
the cables are attached to the switchgear. S&C recommends
that the Vista SD Underground Distribution Switchgear be
completely de-energized and disconnected from all power
sources when performing VLF cable testing. However, Vista
SD switchgear has been designed to allow VLF testing of the
cables with the other ways of the gear energized, if necessary.
Upon completion of the VLF cable testing or an interrup-
tion in the testing, the test set must be turned off to discharge
the cable circuit and test set. The cable system must then be
grounded.
The VLF sinusoidal waveform test voltage applied to the
S&C Vista SD Underground Distribution Switchgear must not
exceed the voltages listed in Table 3 on page 27.