Raytech TR–Mark III 250V Instruction Manual page 28

And now let us have a look how tap changer control works in this case.
Touch on tap changer display as described in chapter '7.1.5 More Taps', and you will find
yourself in front of the same screen.
The main difference is only the moment you switch to tertiary winding system.
Of course you can also assign tertiary tap changer to be free.
Just be aware, that TR-Mark III 250V automatically sets the primary tap changer to be free,
when you switch back to secondary winding system on the X-Side.
Instruction Manual TR-Mark III 250V Version 1.10 Preliminary Page 28 of 57