4
Using the MegaPulse Impulse Tester
The impulse test involves high voltage and caution should be exercised when using the tester. The
RETURN
lead is
referenced to building ground when properly connected. However, both the
OUTPUT
and
RETURN
leads must
always be treated as Hazardous whenever the power switch of the MegaPulse is in the ON position.
The MegaPulse impulse tester generates the impulse waveform only; it does not determine Passing or Failing results.
It is the operator’s responsibility to monitor the output waveform and determine Passing or Failing results. In
monitoring the impulse waveform, consider the following points: The Impulse waveform is high voltage and high
frequency (short duration). Always ensure that the measuring instrument (usually an oscilloscope with a high-voltage
probe) is rated for the voltage involved, and that the frequency response of the instrument and probe can measure the
output waveform of the MegaPulse Impulse Tester. A measuring instrument or probe with a low frequency response
will result in erroneous readings that could be mis-read.
Pressing the POLARITY switch on the front panel can change the polarity of the output waveform. The polarity is
Normal when the
NOR
indicator is lit. In this case, the high voltage will appear on the
OUTPUT
as a positive pulse
relative to the
RETURN
jack. When the polarity switch is in the Reverse position (
REV
indicator is lit), the high
voltage will appear on the
OUTPUT
as a negative pulse relative to the
RETURN
jack. The polarity switch only
operates when the
CHARGE
LED is lit, i.e. the output is not charged.
Note that the peak amplitude of the measured output waveform is proportional to the voltage that is read on the front
panel of the MegaPulse, but it will always be somewhat lower. This is because the meter on the MegaPulse is
measuring the voltage on the main impulse storage capacitor. This voltage will intentionally dissipate to some extent
before reaching the output leads. Therefore, it is important to measure the peak amplitude of the output waveform
and adjust the output of the MegaPulse accordingly.
Determination of Passing and Failing results can prove difficult. To obtain the most accurate results, it is generally
necessary to perform multiple impulse tests on a few different test samples (that have adequate insulation to pass the
impulse test). Take note of the impulse waveshape, amplitude, and duration. Also note how much variance there is
in the waveshape from test to test. Also (if possible), perform impulse testing on some test samples that are known to
have inadequate (or damaged) insulation. Take note of the impulse waveshape, amplitude, and duration, when an
insulation breakdown occurs.
