Raytech TR-MARK III Instruction Manual page 31

And now let's have a look how tap changer control works in this case.
Touch tap changer display as described in chapter '8.1.5 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 too assign tertiary tap changer to be free. It is the same thing as with the
secondary winding system.
Just be aware, that when you switch back to measure primary to secondary winding system,
the tertiary tap changer is no longer relevant. TR- MARK III then automatically defines the
primary tap changer to be free.
90100.1.02 Instruction Manual TR-MARK III Page 31 of 70