26 S&C Instruction Sheet 695-505
Appendix A
Vista SD Underground Distribution Switchgear has
been 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 4.
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 recommendations 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.
Failure to follow these recommendations
can result in a flashover, injury, and equipment
damage.
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 accep-
tance 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 installa-
tion 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 deteriora-
tion and to check serviceability of the system.
VLF cable testing may subject the Vista SD Underground
Distribution Switchgear to the ac test voltage when the
cables are attached to the switchgear. S&C recommends
the Vista SD 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 inter-
ruption in the testing, the test set must be turned off to
discharge the cable circuit and test set. Then, the cable
system must be grounded.
The VLF sinusoidal waveform test voltage applied to the
Vista SD switchgear must not exceed the voltages listed
in Table 5 on page 27.
Table 4. 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 volt-
age 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.
