Contact Replacement; Tightening Torque - ABB UC series Maintenance Manual

1.2.3 Contact replacement

On the rating plate of the on-load tap-changer the estimated
contact life of the breaking contacts in the diverter switch at
rated load is stated.
The contacts will withstand a very large number of switching
operations. For normal power transformers the number of
operations of the diverter switch is approximately 20 per day,
which means that replacement of the contacts is not normally
necessary during the life of the transformer. (In case of on-
load tap-changers on furnace transformers, the frequency of
operations may be considerably higher).
CAUTION
The number of operations must in no case exceed
500 000, due to weakening spring tension of the
contacts.

1.3 Tightening torque

The following tightening torques are recommended:
For metallic screw joints:
For non-metallic screw joints:
if not otherwise stated in this guide.
12 Maintenance guide UCG | 1ZSE 5492-124 en, Rev 6
M6 .......... 10 Nm ±10 %
M8 .......... 24.5 Nm ±10 %
M10 ......... 49 Nm ±10 %
M12 ......... 84 Nm ±10 %
M10 ......... 9 Nm ±10 %
M12 ......... 13 Nm ±10 %
M16 ......... 22 Nm ±10 %