Last saved 4/16/2019
T:\DOCUMENTS\Manuals\VLF\MANvlf50CMF_Rf_2k15.doc
Very Low Frequency (VLF) AC High Voltage Testing - Operators Manual Addendum
Instrument Summary
This instrument is a Very Low Frequency AC high voltage test system – a VLF hipot. It is an AC hipot whose frequency
output is 0.1 Hz or lower, compared to the 50/60 Hz of a conventional AC hipot. The lower the frequency, the lower the
current and power it takes to test a capacitive load like a cable. At 0.1Hz, it takes 600 times less current and power,
compared to 60Hz, to apply an AC voltage to a cable or some other high capacitance load.
At 0.1Hz frequency output, one full cycle of the sine wave takes 10 seconds, 20 seconds at 0.05Hz, and 50 seconds at
0.02Hz. Even though each cycle of the waveform can take 10 seconds or longer, it is still alternating current – AC – and is
sinusoidal.
This is not a DC hipot and it does not operate in the same manner. When the voltage output is raised, the voltage will
climb to its peak and then descend back to zero. The output polarity of the set reverses (a solenoid switch can be heard)
and then climbs again in the opposite polarity. This rise and fall of the voltage is obvious on the voltmeter and is normal.
The current meter will also rise and fall as the cable charges and discharges every half cycle. Unlike DC, there are no
leakage currents to read. This is a go/no-go, or pass/fail, AC stress test. The load under test either withstands the test
voltage and passes, or it breaks down and fails.
Important note: Use the VLF test set properly. Test at the proper test voltage and time duration. Abbreviated tests or
tests at reduced voltage levels should not be performed. An improper test may aggravate cable defects that may cause
cable failure later in service. The proper test voltage and time duration are essential to allow the VLF to do its job.
Helpful Notes to Users
Peak Voltage: The Voltmeter on the VLF test set measures the Peak voltage. It is the peak output voltage that is used
during VLF testing. If your test specification indicates a test voltage in RMS, multiply by 1.414 to obtain the peak value for
the test. The IEEE400.2 chart indicating test voltages for different cable ratings shows both rms and peak. All other AC
hipoting using 50/60 Hz instruments is measured by the rms voltage, not VLF testing, so make sure this issue is
addressed.
Output Frequency: All VLF sets are rated by the uF of load they can test at a given frequency. The lower the frequency,
the higher the capacitance, or the longer the cable, can be tested. Always use the highest frequency possible. Testing
should be performed at 0.1Hz, unless the capacitance of the load is too high for the uF rating of the set at 0.1 Hz. It may
be necessary to reduce the frequency to 0.05, 0.02, or 0.01 Hz depending on the capacitance of the load. The load
capacitance can be measured using the capacitance meter of the VLF or be calculated, if the capacitance per foot or
meter is known. Example: At 0.1Hz, 1 – 2 miles of cable can be tested, depending on size.
Test Duration: The recommended test duration is 30 minutes as a minimum. In the body of this manual are reprints from
the IEEE400.2 standard discussing this issue and additional information on the subject. The longer the test duration the
better, as minor defects that triggered into partial discharge due to the test voltage will have time to grow through to failure
during the test. The duration indicated in the standard is based on using a frequency of 0.1Hz. If lower frequencies are
used, the test time must be extended. Rather than 30+ minutes at 0.1Hz, perhaps 60 minutes should be used at 0.05 and
0.02Hz.
Cable Failure Indication: When the cable fails, the thermal Overload on the front panel will pop up. Also, the Voltage and
Current meters will read erratically as the cable arcs. It may take several cycles of the voltage rising to the arc-over
voltage of the fault to cause the thermal overload to warm up enough to trip.
Know Your Test Equipment – Read the Operators Manual
