Yellow voltmeters on 20 V AC gauge, red at center in 600 V ~
The 600 V ~ uses IP2x sockets which avoids the use of insulating gloves
I connect the left voltmeter to 12 V ~ transformer output
My toroidal transformer has 2 windings 12 V ~ and a 230 V ~
Bottom in red 2 safety sockets for 230 V ~, top 2 * 12 V ~
I use one of the 2 coils 12 V as primary
It can be seen that the second winding 12 V serves as a secondary winding
The very small deviation (0.05 V), due to the losses, makes it possible to distinguish the primary!
The 230 V winding is also a secondary winding: 220 V! Reversibility validated