Megger VLF SIN-45 - Sheath Test and Sheath Fault Pinpointing

Operating Modes
35
5.2 Sheath Test and Sheath Fault Pinpointing
To detect sheath faults, the VLF SIN-45 test system operates in test mode with a DC
voltage of up to 20 kV (negative polarity), which also permits cables with a thicker outer
sheath (such as cables rated 230 kV) to be tested.
If a voltage breakdown occurs during the course of a sheath test or the measured
leakage current indicates that there is a sheath fault, fault pinpointing can be started
directly after the test.
During sheath fault pinpointing, DC pulses with an adjustable pulse rate are coupled into
the screen of the faulty cable.
With each coupled pulse, the current flowing into the earth forms a voltage gradient
around the point of escape (the fault position in the sheath), the centre of which can be
located precisely by means of an earth fault locator and earth spikes (step voltage
method).
To start a test, the submenu of the test operating modes must be opened using the
menu item and from this, the menu item must be called up.
Sheath fault pinpointing can be started directly from the main menu using the menu
item .
The phases (sheath testing only) and the voltage range are automatically queried when
entering the operating mode, however − like all other settings − they can be adjusted
until the actual start of the test or pinpointing.
The following test parameters can be set:
Button /
Menu item
Description
Only adjustable for sheath tests
The phase selection (see page 28) must be carried out in accordance with
the actual type of connection.
If to save time several sheaths are to be tested at the same time, the
phase selection menu also makes it possible to select more than one
phase.
Thereafter, the phase selection menu must be closed using the function
button .
The set voltage range limits the maximum voltage that can be set during
the test or pinpointing.
In terms of the relevant standards (such as the VDE 0276), which may
however differ from the local regulations or standards, the following
guidelines are specified:
• PVC cable
≤3 kV
• PE medium voltage cable
≤5 kV
• PE high voltage cable
≤10 kV
Introduction
Selecting the operating
mode
Setting the test
parameters