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

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

Construction according to clause 19.12.3 of IEC 61558-1

This clause refers to isolated winding wire construction, i.e. a way of making a transformer where the isolation of the winding wires themselves is the only isolation.

This usually means:

• that the winding wires will have multiple layers of isolation (usually two or three to satisfy safety requirements from IEC61558-1)
• that there is no additional isolation foil since the isolation of the wound wires is sufficient to fulfil the safety requirements
• that manufacturer will need to perform additional safety tests for each and every transformer (100% production test) to prove that the transformer is safe.

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