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2.4 Beware of Residual High Voltages
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WARNING
In DC withstanding voltage testing and insulation resistance testing,
the test lead wire, test probe, and DUT are charged to a high voltage. The tester is
equipped with a discharge circuit, but some time is nonetheless required to
discharge them after the output is cut off. There is a danger of electric shock during
discharge. To avoid electric shock, take the utmost care to ensure that the DUT,
test lead wire, probe, and highly charged parts around the output terminal are not
touched. If it is necessary to touch them, be sure to confirm both (1) and (2):
(1) The analog voltmeter indicates “zero.”
(2) The DANGER lamp has gone out.
As soon as the output is cut off, the tester’s discharge circuit starts forced
discharging. Do not disconnect the DUT during a test or prior to completion of
discharging.
Discharge time
The length of the discharge time varies according to the properties of the DUT.
Discharge is conducted at a resistance of approximately 2kΩ in DC withstanding
voltage testing, and at 10kΩ in insulation resistance testing.
When no DUT is connected, the tester itself requires the following lengths of time
to reduce the internal capacitor voltage to 30V.
If the DUT is disconnected during a test or before the completion of discharging,
assuming that the DUT has a capacity of 0.01µF and a parallel resistance of 100MΩ,
approximately 5.3 seconds at 5kV and approximately 3.5 seconds at 1kV are
required for the DUT to discharge to 30V.
When the approximate time constant of the DUT is known, the time required for
discharging to 30V after the output is cut off is calculated as the time constant
times the value given above.
2.5 Dangerous States of Failed Tester
Typical possible dangerous states of the tester are as shown below and in which
cases the most dangerous situation that “the high test voltage remains
delivered and won't be turned off!” may occur. When this situation has occurred,
immediately turn OFF the power switch and disconnect the AC power cable from
the AC line receptacle.
•The DANGER lamp does not go out despite you have pressed the STOP
switch.
•The DANGER lamp does not light up despite the pointer of the analog
voltmeter is deflected indicating that the output voltage is being delivered.
