Siemens SIPROTEC 7SS523 V3.2 Manual page 315

Repair
Insert the module again into the housing. Make sure that the module is fixed
correctly and firmly contacted with the rear-side plug connector.
Re-establish the electrical connections with the power supply, and fasten the front
panel again.
After fastening the front panel, switch the device on. If the auxiliary voltage failure
continues to be indicated, there must be a failure or short-circuit in the internal
power supply. The complete bay unit should be sent to the factory.
Figure 8-9 Location of the fine-wire fuse on the power supply module (SAF) of the bay unit
7SS523
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7SS52 V4 Manual
C53000-G1176-C182-1