What is a transformer construction according to clause 26.2.4.1 of IEC 61558-1?

In transformer specifications or tests, you will often find sentences like

Construction according to clause 26.2.4.1 of IEC 61558-1

This clause refers to hermetically sealed construction by impregnation potting(even if only parts of the transformer are potted), i.e. the transformer is (partially) filled with epoxy or similar potting materials in order to prevent moisture or dust from influencing its performance.

This usually means:

• the transformer can be smaller since the safety regulations (IEC61558-1) requires  less creepage distances compared to non-potted transformers
• the transformer manufacturer has to perform additional testsaccording to IEC61558-1 (compared to non-potted transformers) to prove that the potting material provides sufficient isolation
• the transformer is typically more expensive than an equivalent non-potted transformer since it needs to be potted in the factory and additional safety tests need to be performed on transformer specimens.

Also see What is a transformer construction according to clause 19.12.3 of IEC 61558-1?