# How can a transformer convert the voltage of the same wire before and after connection?

Today I discovered a very strange thing, this transformer in the picture below, has 3 phases as input, and the output varies as you can see in the picture, the weird thing is that when I measure the voltage across every two phases before connecting them to the transformer I get 380v, but when I do so after connecting them, I get different values, the very same wire goes from high voltage to 0 voltage, is this correct? I couldn't believe it until I touched the wire and didn't get a shock! How does this work?

simulate this circuit – Schematic created using CircuitLab

• I get different values - how different - please state before measurements and after measurements. May 2, 2020 at 15:56
• After connecting the wires, I get the numbers written on transformer, 0, 220, 380. May 2, 2020 at 16:00
• That is NOT a 3-phase transformer. It is a single-phase transformer with three windings -- a primary that can vary anywhere from 220 to 550 V (+/- 20V) and two secondaries. May 2, 2020 at 16:08
• @Dave Tweed, can you please explain this to me in details? May 2, 2020 at 16:13
• Touching the wire to verify 0V is not a recommended method to see if there's still 380V on it. May 2, 2020 at 17:38

After connecting the wires, I get the numbers written on transformer, 0, 220, 380.

It's a single phase transformer.

The primary winding also works like an auto transformer.

When 380 V is applied between terminals '0' and '380', the voltages between '0' and the other terminals will read 20, 20, 220, 280, 440, 500 and 550 V.