AC voltage or DC voltage to check insulation 1

[144x]

I want to check an annuncitor insulation level.
but I'm not sure whether I should apply 2KV AC or 2KV DC
between case and ground.when I apply 2 KV AC, about 6 mA
is drawn from the source.is it normal?
what are the standards to refer?
thanx.

 

[SphincterBoy]

I think you are referring to a 'HI-POT' test which is DC.

Check with ANSI standards or the manufacturer.

 

[electricpete]

If you use a dc test you will measure a current which is representative of the resistive leakage current (after waiting suitable time for the capacitive and absorption components to die away).

If you use an ac test you will get a higher current which is predominantly capacitive component. You won't be able to discern the resistive component. The current magnitude will not be a useful indicator of insulation leakage current (unless you also measure phase). All you know is that the equipment withstood the voltage without breaking down.

 

[scottf]

There is a few sides to this issue.

First "Hi-Pot" just stands for high potential and can can be either AC or DC.

AC is generally regarded as the most stringent when testing insulation systems. One reason is that the ocsilating wave form (changing of polarities) stresses the insulation high than DC does. The second is that applying 2 kV AC is really 2 kV rms, which means the peak voltage is around 2.8 kV. or roughly a 40% higher peak value than 2 kV DC.

For hi-pot testing, you're normally trying to confirm that the insulation will hold the voltage and not neccesarily measuring the current flow, although there is normally a relay to trip off the voltage if the current gets too high, indicating insulation breakdown. The amount of current that flows under normal conditions is a function of the type of insulation.

If you truely want to measure the integrety of the insulation, you need to measure the insulation power factor, sometimes also called dissipation factor or tan delta. This basis the measurement on the angle between the resistive and reactive component of the insulation. The lower the better. For oil-paper isulation, typical criteria is less than 0.5%. Values vary by insulation type, with dry-type inssulation normally being a bit higher.

 

[jbartos]

Suggestion: If the annunciator is important, e.g. safety related, it is better to follow more official standards, e.g.
MIL-STD-705C METHODS. Visit

http://www.katolight.com/pac.html

for more info. Notice, that the annunciator panel will be tested under the dielectric test: 302.1b- High Potential Test.
==============================================================
MIL-STD-705C
METHOD 302.1b
HIGH POTENTIAL TEST
CAUTION: OBSERVE SAFETY REGULATIONS. The voltages used in this method are dangerous to human life. Contact with the leads or the windings under test may cause severe, and possibly fatal, shock. Arrange the high voltage leads so that
they are not in a position to be accidentally touched. Keep clear of all energized parts. Always reduce the test voltage to zero and ground the winding under test before making any mechanical or electrical adjustments on the equipment. When grounding windings which have been tested, always connect the
connection wire to ground first, and then to the winding. Never perform this test without at least one other person assisting. The generator frame shall be securely grounded to the building ground or earth ground.
302.1.1 General. The generator insulation materials must be capable of withstanding, without damage, voltages higher than rated for definite periods of time to preclude damage during transient loading and short circuit conditions.
This test should be performed after the equipment is completely assembled so that damage to the insulation which might have occurred during manufacture and assembly will be detected.
302.1.2 Apparatus. A variable alternating current, high voltage, current limited power source shall be required.
302.1.3 Procedure.
302.1.3.1 Preparation for test.
a. Adequately ground the high potential apparatus to a solid electrical ground in accordance with instructions accompanying the apparatus. Also, enerator set and all windings of the connect the main frame of the g
generator, not under test, to the same ground.
b. Unless rectifiers and semiconductors are specifically designed to withstand the specified test voltage they should be removed or disconnected from the circuit along with any radio suppression capacitors prior to conducting the tests.
c. Raise, or remove, brushes from commutator and slip rings, if applicable.
d. Isolate the generator power output windings, the generator field windings, and the exciter armature and field windings if a part of the set. This is accomplished by disconnecting the various windings from the associated control circuits or other connections. Static excitation systems feeding
the field excitation from the ac generator shall be tested with all rectifiers and capacitors disconnectd, unless such rectifiers and capacitors are designed to withstand the specified test voltage.
e. Connect the high voltage lead from the test apparatus to the winding or circuit under test. All other windings and circuits must be securely connected to ground.
302.1.3.2 Test.
a. Turn on the test apparatus in accordance with the manufacturer's instructions, after making sure that the initially applied voltage will not be greater than 600 volts.
b. The test voltage shall then be raised approximately uniformly to the required value. This increase shall be accomplished in not less than 10 seconds nor more than 30 seconds. Unless otherwise specified in the procurement document, the maximum test voltage for power output windings
shall be equal to 1,000 volts plus twice the highest rated voltage of the generator. The test voltage for field windings shall be equal to 10 times the exciter ceiling voltage but in no case less than 1,500 volts nor more than 3,500 volts. Record on the data sheet the maximum voltage reached
during this method (see figure 302.1-I).
c. Apply the maximum voltage for 1 minute. Record the voltage applied, including any breakdown in insulation observed.
d. After 1 minute the voltage shall be reduced gradually to the voltage initially applied. This reduction shall not be accomplished in less than 5 seconds.
e. Turn off the test apparatus.
CAUTION: Ground the high voltage lead of the test apparatus
to make sure that no charge remains on the windings which have been under test.
f. Remove the high voltage lead from the winding and proceed with the tests of the remaining circuits to be tested. Make sure that all the circuits not under test are securely grounded.
g. After test is completed, reconnect all circuits to original configuration.

 

[144x]

jbartos ,thanx for your detailed response
you deserve many stars.