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High voltage circuits should also be conformal coated. The failure mechanism is totaly differant, but--. A low-level control board has circuits that are susceptable to leakage currents from salt air and road mist. A 144 volt board could develope an arc-trace across the board, following along the contamination. This would be not only destructive, but could be a fire. FR-4 is only "fire-retardent"! Most coatings are rated for about 1KV per .001" thick coating. This would be even more important for an AC controller at 300 volts. I think Digikey sells a 12 oz. can for about $20.00 They aren't the last choice.
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