insulating varnish

[Tmoose]

I vaguely recall many of the insulating varnishes sold in spray cans had a rating of ~1000 volts/mil.

Today a Google and ENG-TIPS searches came up with essentially NO rating for any spray varnish.

Does someone know a product with a rating.

Looking for some insurance during a 3KV hipot test.

Thanks

Dan T

 

[CarlPugh]

The rule of thumb for insulation is when the voltage is doubled, the insulation thickness has to be three times as thick.
Where there is a volts/mil rating for insulation, the rating is usually for a specific thickness of insulation under specific test conditions. (shape of electrode, temperature, voltage rise time, time voltage applied, frequency....)
The volts/mil rating is useful for comparing different insulations. It is practically useless when designing a product.
The best procedure is to test several varnishes under your operating conditions. Most varnishes have high insulation.
Years ago we tested some varnish/paint by driving two nails in dry wood. Painted the wood and then hipotted between the nails.
Black paint (varnish also probably) should be tested before being used. Some paint uses carbon black for coloring and is conductive.

 

[rmw]

Some varnishes are conductive and are used in generator end windings to reduce the potential for PD.

Now, OT but how did the nails in the wood thing work? What about the moisture in the wood as a conductive path? Even the dryest of woods contains a small percentage of moisture. Wouldn't something like Bakelite have been better for this test?

rmw

 

[BJC]

Do a search on Glyptol.
GE use to sell it but it has been sold. The company that makes it now is in Chelsea MAss,
I am sure they have voltage ratings on it. It use to be on everything GE made.

 

[CarlPugh]

The nails in wood gave repeatable results. The arc was through the air. (Except when paint with carbon black was used)
It has been some time since the tests were performed, but the way I remember it, is that we painted the dry wood and then drove nails into the wood.
Probably the moisture was uniform throughout the wood and the wood acted as a resistor between the nails.
Wood was used instead of bakelite because it was easier to set up a test. My belief is that dry wood and bakelite will give similar results.
Has anyone tried bakelite and dry wood?

 

[budw]

BJC

Good old Red Glyptol. Also known as High Voltage Corona Dope.