Confirming SCREEN or CORE Connection Integrity

How can I be sure my screen or core connections are good before a Hi-Pot test?


The HPAC test is typically performed between all primary windings connected together, to all secondary windings.
A DC R 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 DC R test as you can through a standard winding with 2 connections.

Question ?

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.

Answer !

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 capacitance test to measure this, which will allow upper and lower limits to be set.
In the case of a bad / missing connection, there will be no capacitance.
In the case of a good connection, there will be a measurable stable characteristic capacitance.

Worked Example

To verify the integrity of the connection BEFORE the HPAC test, an Inter-winding Capacitance Test (C TEST) can be added on the test program.
Here we will demonstrate this on an E-Core Laminate Transformer – TCT40-01E07AB.
This is a two winding transformer with a core connection.
The core is also connected to a test node as the test program will include HPAC test between the windings and the core.

AT Editor Schematic
AT Editor Schematic

A - Add CAP Test Before HPAC

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 10 kHz and 5 V 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.


B - Perform Open And Short Compensation

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 Inter-winding Capacitance test on the AT series.

C - Run Test Program And Check C Test Result Repeatability

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 re-performing compensation after downloading the test program.

Run test program and Check C Test Result
Run test program and Check C Test Result

D - Run Test With Core Disconnected To Prove That Bad Connections Are Detected

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.

Run test with core disconnected
Run test with core disconnected