creepage and clearance for transformers

IIRC... (There might be errors. Please check.)

Creepage is spacing on a surface that collects pollution. Clearance is spacing in air. This pops up because of a difference in air breakdown and the breakdown of accumulated pollution.

Clearance applies to the transformer terminals and any internal air spacings. But if the terminals are on a block that collects pollution, then creepage might be considered.

However I don't think creepage applies inside the transformer.. Transformers are often bundled up or sealed. It's all up to the insulators and air gaps to block breakdown.

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Creepage applies equally inside of a transformer and is the primary reason for 'margins' on either side of windings and for sleeving applied over terminations, splices, and such. 'Enamel' insulation on windings does not count as safety insulation. Under many regulations, you can get away without margins if you use 'triple insulated' wire, however you still may need sleeving and/or special bobbins to provide proper creepage and clearance near terminations. Paul Mathews