How to Choose DC hipot test kit for transformers?

8) FAQ - Frequently Asked Questions.

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8.1) I tested the same part with two ATs, both units read differently for the HPAC/HPDC

Unit 1 -   3.2 uA
Unit 2 - 10.0 uA

The ATs do not read the same small current, what should I do?

Answer:

As with any electrical  measurement, as the signal tends to zero, the possible  error in the reading will increase.
HI-pot measurements, by their very nature, are measuring very small currents.
An ideal transformer would give a HIPOT current reading of zero.
It is a common misconception to think of these measurements as you would a more typical DCR reading of say 2 ohms, where you expect stability and repeatability.

However, since the current is very low, you should expect that such very small readings would be susceptible to the inherent noise floor of the unit.
This could give you a range of readings for different test equipment, but that should still be well below the requirement requirement or your specified limits.

A bad isolation (e.g., dead short, weak enamel between windings etc.) would give you excessive current readings greater than your limits or even a hard flash over / trip for genuine bad parts.

Some test equipment manufacturers choose to return results below a set number (say 20uA) as a hard &#;0.00&#;, but we have chosen to always report a number as proof of measurement, even if such small results may be subject to a large (eg + / - 100%) error tolerance.

It is therefore normal to get slightly different HPAC/HPDC on different AT Units in cases where the part under test is a good isolator, and the current measurement is inherently very small. 

8.2) My AT gives error codes even if I reduce the HIPOT requested voltage. 

What are the troubleshooting steps to fix this problem?

Answer:

There are things that could be causing the TRIP on the AT.

1, Fixture plate used - the fixture itself could be contaminated, or does not have enough clearance for the voltage stand off required. (a good rule of thumb is at least 1mm minimum per V standoff)
2, A Genuine bad part.
3, Large natural capacitance of the part (for larger transformers - in these cases increase the RAMP UP time).

The trip mechanism on the AT during the HIPOT test engages when a dead short or an excessive current has been measured.
If the trip happens in multiple parts, that you suspect are actually "good", then you should check the fixture being used.
You can quickly test the fixture and unit by the following two tests.

1, Running  the program (ideally 4-5 times)  and directly from the AT Editor, with the fixture fitted but NO UUT fitted.

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You will expect to get failure on the measurement tests (eg, R, LS, RLS, Z, MAGI etc.) as no part is present.
However, an HPAC or HPDC tests you have programmed should pass
If the HPAC/HPDC tests fail with no UUT fitted, then the fixture is not properly isolating the nodes.

Examine the fixture for 
A, signs of contamination on the top surface.
B, the separation of the wiring within the fixture - wires should not cross over each other, as this can result in &#;cross-talk&#;

2, Run the program on the unit with NO fixture, and NO UTT connected.

As above this will test the hipot integrity of the unit
You will expect to get failure on the measurement tests (eg, R, LS, RLS, Z, MAGI etc) as no part is present.
However, an HPAC or HPDC tests you have programmed should pas
If you have a HPAC or HPDC failure with no fixture connected, then please contact us.

8.3) My AT Screen suddenly flickers during the HIPOT test, is my unit faulty?

Answer:

Any high voltage test (especially with larger signals as high  as 3- 5kV), can result in EMI emissions through the air.  
Due to the relative closeness of your UUT to the AT screen, the large screen of the AT could pick-up this energy.

This can cause the screen to flicker during these tests, but is only a visual effect and has no effect on the digital operation of the unit.
As long as the test is still controlled, maintained, and the AT is able to finish the test, this is expected behavior and should not be cause for concern..

Such a test applies a voltage to the DUT that is much higher than normal operating voltage; typically V AC plus twice the normal operating voltage. For a household appliance designed to operate at 120 or 240V AC, the test voltage is usually about to V AC.

A DC hipot test can usually be substituted for an AC hipot test. The best voltage for a DC hipot is normally higher than the AC test voltage by a factor of 1.414. A product that would be tested at V AC would be tested at V DC.

For double-insulated products, the required test voltages may be much higher, such as VAC or even VAC for a 120 VAC power tool. The voltage is applied between the operating circuits and the chassis or ground &#; the parts of a product that a consumer might touch or come in contact with.

Refer to Figure 5 for typical AC hipot test setup. The setup for a DC hipot test would be identical.

The purpose of the test is to make sure consumers do not receive an electrical shock when they use the product. This typically is caused by a breakdown of the electrical insulation. The test also detects possible defects in design and workmanship that cause components and conductors to be too closely spaced. The danger is that air gaps between conductors or circuit components may become clogged with dust, dirt, and other contaminants over time in typical user environments. If the design spacing is inadequate, a shock hazard can occur after a period of use. By subjecting the product to a very high voltage, the hipot test overstresses the product to the point that arcing may occur if the spacing is too close. If the product passes the hipot test, it is very unlikely to cause an electrical shock in normal use.

Withstanding a very high voltage means that a large margin of protection exists for the consumer. Regulatory agencies usually require a stringent hipot test as a product &#;type test&#; before releasing the product for sale to the public and another less demanding test to be used on the production line. As a rule, testing laboratories consider the hipot test to be the most important safeguard for the consumer. They may accept &#;design&#; or &#;type&#; tests for other types of tests, but always require hipot tests for 100% of the units in a production line.

Contact us to discuss your requirements of DC hipot test kit for capacitors. Our experienced sales team can help you identify the options that best suit your needs.