The off-circuit tap-changer is of rather simple design, giving connection to a selected tap in the winding. As the name says it is designed only to be operated only when the transformer is de-energised.
The contact pressure may occur to be retained by some kind of spring arrangement and then some vibration is possible.
In off-circuit tap-changers operating on the same tap position for years the contact resistance then may slowly increase due to local degradation and oxidation of material in the contact point. Heating will take place resulting in a build up of pyrolytic formed carbon, which will increase the contact resistance further, and also reduce the cooling.
Ultimately a runaway situation is reached and the transformer will probably trip on gas actuated protection or worse – a step short circuit occurs.
To avoid this it is vital that the tap changer is operated, off circuit, through its complete range a few times during regular routine maintenance to wipe the contact surfaces clean before returning it back to the selected tapping.