Hi Mr. Stewart-Hay,

Yours and Mr. Burke's advices are greatly appreciated. Sorry for the confusion. I believe my explanation was not clear enough. Please let me clarify that:

You understood me correctly the first time. The spools were from the same run. However the ones were stored in the warehouse with higher humidity failed and the ones were kept in an area with lower humidity did not show any failure. Please keep in mind that on the first day all passed with a good results. This is the part we don't understand. If the storage of the enameled wire (magnet wire) should not have any effect of the wire why do we got different results?

About your questions please see below the answers.

1. Before I discuss this issue with you we have not given any attention to the humidity in the production. Of course since then we have put everything on a tighter control. I am sure that all the spools were produced approximately with the same circumstances.

2. We did not have any readings at that time. However, the humidity couldn't be too much different since the weather was very much the same.

3. Sorry, it must be a misunderstanding. We had over four tons of production that all produced at the same situation. The spools, which kept in the warehouse with a higher humidity and in an area with a lower humidity, they were all randomly chosen from the same batch of production.

As it was mentioned on my initial writing " The first day of production all the tests passed with the best results. After two weeks sitting in the warehouse we had a dramatic failure. The break down voltage (BDV) dropped on all of the spools."

Many thanks for your help,

Hello Again Mr Rahati;

To answer your question, I suggest your whole run of magnet wire was contaminated with moisture (water) from the air as Mr. Burke explained. There may have been a number of the initial spools that were not contaminated but after the liquid enamel in the holding tank was exposed to high humidity air for say more than 2 hours, the problem started. If the liquid enamel was exposed to the atmosphere for an extended period of time before it was poured into the holding tank, the problem would start immediately.

Refilling the already contaminated enamel in the holding tank with new liquid enamel would not correct the problem.

Once the moisture contaminated the enamel, it is not clear to me as to how, after curing, the contaminated enamel would react when stored for a period of time in a high humidity environment Perhaps somebody else has had experience with this. Obviously there is supposed to be no change to the dielectric strength of the cured, non-contaminated enamel when it is stored in a high humidity environment.

If you want more technical insight, I really recommend that you have a serious, face to face talk with the chemistry people who work for your enamel manufacturer.

Thank you.

Hello Mr. Stewart-Hay,

Many thanks for your prompt answer. You helped me to understand the issue. First we were confused and looking for a solution in our stockroom. Now with your advice and Mr. Burke's getting to this conclusion that we need to focus on our production and eliminate the project for a humidity controlled stockroom. In addition, if I understood you correctly there is no requirements for the shelf life of the magnet wire. (of course this is not includes the special magnet wires like self bodings, etc..)



Best regards

I tried to find some information about the enamel and I could not find that number on the Elantas web site. I may have misunderstood when you were calling the enamel PE - we used to refer to oleoresinous enamel as PE (plain enamel).

Oleoresinous enamel has been being used for about 2,000 years which means about 1900 years prior to film coated magnet wire was invented. PE (plain enamel) is the simplest enamel made. I have found references where samples of the wire were stored in a high humidity test cell and then samples were tested weekly. The first ones taken from the test cell tested lower than when first put into the test cell. Every additional week left in the cell the lower the breakdown voltage. I'm not sure why but obviously storing in a bad environment is not good for the wire.


Whenever you have a problem with a purchased product you should call your supplier, tell them your problem and ask why? I assume that they still keep samples of each batch of enamel and should be able to pull a sample and evaluate it. Magnet wire can be damaged by physical stressors, electrical stressors, and environmental stressors. An environmental stressor would include storing it in a warehouse that has an "open air" environment.

Mr. Peter J. Stewart-Hay & Mr. Richard Burke

Thanks a lote for your valuable tips.
I heared termoplastic materials (for example polyesters) are continuing to cure/downgrade even when sitting on the shelf. Is it possible? and How this phenomenon occurs? If this phenomenon occures on finished wire (enamelled wire) spools in the high humidity place, Whether there will be a voltage drop ?

Best regards

Peter, I have a lot of weak point and chemistry is a big one so ---- I don't remember if it's thermoset or thermoplastic that will continue to cure but I think what might happen is that there is incomplete cross-linking (curing) but at the time the wire was pulled from the machine it looked good and met all properties. If then it continues to cure while sitting on the shelf and since the wire tested good when it was not properly cured, now when you start making coils you may have cracks and fractures in the insulating film. It doesn't matter much if the wire goes from machine to being an encapsulated coil or used in an oil filled transformer because the oil and encapsulating material are both insulating.

I am still not sure if he is talking about polyester enamel or plain enamel which means oleoresinous material, probably the oldest type of paint or varnish in the world.