This is a switch-mode PSU, so the mains is rectified to around 400 volts DC inside.

Extra care is required to avoid touching this dangerous part of the PCB.

Fault:

Sparking at back of unit.

Suspected cause:

Faulty power connector or EMC capacitors.

Examination:

The power inlet was loose in the PCB, because of repeated cable insertions and no attachment to the case. Thus the strain goes direct to the PCB joints.

Action:

Resoldered power connector joints.

Result:

No sparking.

Status:

Fixed. Awaiting re-assembly and PAT test.