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General information:

VLF stands for Very Low Frequency. VLF is generally considered to be 0.1 Hz and lower, its major application is for testing solid dielectric cable, transformers, motors, generators, switchgear and other electrical apparatus.
A VLF hipot is just an AC output tester but with an output frequency of 0.1 Hz or lower rather than 50/60Hz. Although the frequency is very low, it is still an alternating current with polarity reversals every half cycle. At 0.1 Hz output, rather than 60Hz, it takes 600 times less current and power to apply an AC voltage to a capacitive load, like a long cable.

Features:

Adapts microprocessor controlled, can realize voltage set-up/down, measuring and protecting automatically. Manual intervention is allowable even during the auto voltage boosting.
Portable, easy to move.
Large LCD can display waveform clearly, and test report can be printed. Built-in printer.
The data of current, voltage, and waveform can be sampled at HV side. 
A high voltage output protective resistor is provided in the voltage boost body in the design and this eliminates the need of additional protective resistor connected outside.
Closed-loop negative feedback circuit is adopted. No capacity rising during outputting.
Over voltage and current protection: When the output is beyond the limitation, the control unit will shut itself down within 20ms.

Specifications:
Peak voltage: 80kV
Output frequency: 0.1Hz, 0.05Hz and 0.02 Hz (selectable)
Fuse: 20A
Maximum load capacity: 0.5μF at 0.1Hz; 1.0μF at 0.05Hz; 2.5μF at 0.02Hz
Power supply: 220V ±10%, 50Hz ±5% (If using a portable generator, make sure the output voltage and frequency are stable. Power >3kW. Power supply at 110V 60Hz is optional)
Output voltage accuracy: peak voltage instability ≤1%; frequency instability <=3%.
Voltage waveform distortion: <=5%.
Working environment:  indoor or outdoor; -10ºC-+50ºC; 85%RH.
Weight: Control unit - 4kg; HV unit I: 25kg; HV unit II: 45kg.
Remarks: Capacitance of test object cannot be larger than the maximum value of the test instrument. If capacitance of test object is smaller than 0.05μF, the hipot should be equipped with an auxiliary device (supplied by Unique m&e at your option).