Five of Your Power Safety Questions Answered

September 4, 2018
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Our recent webinar, Stress-Free Electrical Safety for Power Designers, generated a lot of interest and several questions. Testing the grounding and insulation were key areas about which the attendees wanted more information, and here are the top five questions and answers about these topics.

My system works in a dirty environment. Can I make repeated Hi-Pot tests for example every month to make sure that the isolation barriers are still ok?

Contaminants such as dust, solvents and oils should have the PCB finished in such a way as to avoid such contaminants from affecting circuits. Very often boards and the components that are mounted on them will be coated with a protective film, or conformal coating. This should eliminate the need for such a test.

Does humidity have an influence on the values for leakage and creepage distances?

It is important to specify the acceptable range of humidity applicable to the application. Almost all Vicor DC-DC converter products are not hermetically sealed so THB (temperature/humidity/bias) related effects are high up on the list of any power designer’s concerns. Conformal coating on boards is one way avoid the possibility of tracking on the substrate. Such coatings are designed as a barrier to visible and invisible moisture content in the ambient air.

What is the impact of altitude on Hi-Pot testing?

If an interconnect, either on a substrate such as FR4 or alumina or a wired assembly, gets its high voltage conductors exposed to partial atmospheres, then the Paschen Effect will cause corona discharge at relatively low voltage. Agencies familiar with space flight applications will often resort to placing exposed conductor assemblies in special potting compounds to prevent partial pressure exposure of those assemblies.

Paschen_curves

Paschen Curves: Click to Enlarge
By KrishnavedalaOwn work, CC BY-SA 4.0, Link

What’s the difference between the ground bond test and ground continuity test?

These are two tests that both establish that there is low impedance in a ground system. It’s the test methods that are different. The Ground Bond test should establish a large current in the return between the system return and the current injection point. The ratio of the voltage difference between the current injection point and the zero voltage reference to the amount of ground current is ground bond impedance i.e. Z = V/I.

During continuity testing, no attempt is made to establish a large current. A multimeter is used in this test to establish ground continuity.

Should the grounding bonding test not be carried out before the Hi-Pot test to ensure that the ground connection is there?

The sequence of the tests is immaterial. The ground bonding continuity test is used to verify that very low impedance is afforded to ground in the return network being used. Hi-Pot testing is carried out to establish the effective insulation of nodes that are not intended to conduct current between themselves.

 

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