Confirming SCREEN or CORE Connections
to Validate HPAC Testing

The HPAC test is typically performed between all primary windings connected, and all secondary windings. A DCR test can be used to confirm that the connections are good, before the HPAC test is executed, to ensure that any HPAC PASS is valid. HPAC can also be used to verify the isolation between windings and a screen or a core. However, If the screen or core is not properly connected during HPAC test, the AT5600 could give a false PASS, as the unit would (correctly) measure a very low current as there is no connection to the screen or core.

As the screen (or core connections) are to a single node, it is hard to confirm that the screen or core is connected, as you cannot perform a DCR test as you can with a standard winding with 2 connections.

How can users confirm that the screen or core connection is good before performing HPAC?
Especially as the HPAC test does not allow a lower limit to be set on the test.

Even if the Screen or core is well isolated from the windings, there will still be a characteristic capacitance between the Screen (or core) and the windings.
We can use a CAP test to measure this, which will allow upper and lower limits.
In the case of a bad connection, there will be no capacitance.
In the case of a good connection, there will be a measurable stable capacitance.

Please follow the recommended test conditions for Frequency and test voltage when using the C test on the test program. (See user manual 7.12 C TEST for these)

• 1. Initially set the test frequency to 10kHz and 5V as these signals usually give a repeatable capacitance value.
• 2. Set the High and Low terminals of the C test.
  In this example, we used the primary and secondary as the high terminals and used the core as the low terminal. The limits can be set quite wide, as a misconnection of Screen or Core should result in a large error
• 3. Save and download the test program on the AT5600 using the AT Editor.

It is recommended for users to perform Open and Short compensation before starting any test.
The Open and Short compensation is required for best performance on the Interwinding Capacitance test on the AT5600.

Check the result on C test and see if you get a repeatable measurement.

You may need to adjust the frequency of the C test parameters on the test program based on the capacitance range that is measured.

Any changes made on the test program would require performing COMPENSATION after downloading the test program on the AT5600.

Once the CORE is disconnected, the result for the C test indicates FAIL as the measured value goes well below the minimum limit (here, from 66 pF to 413 fF). This proves that the following HPAC is invalid.
This shows that the C test can be used to confirm when there is a bad connection on the screen or core even though the HPAC test is a PASS.