Siemens Emotion (Straton 422 tube): arcing only above a tube-current threshold
What happened
On a Siemens Emotion with a Straton 422 tube, arcing was reported in the case starting around 80 kV, but only once tube current reached about 95 mA; below that, exposures ran clean. Service-mode HV tests without tube current (dummy plugs fitted) passed up to 145 kV, so the fault appeared only when real current flowed through the tube.
What was checked
- High-voltage test in service mode with the tube current path disconnected and dummy plugs fitted — passed up to 145 kV as reported, but that setup carries no tube current.
- Exposures at several gantry positions to check for position-dependent HV faults — no dependence found.
- Inspection of the HV tank/transformer for oil leakage and bubbling or gurgling sounds — nothing reported.
- Comparison of arc onset against tube current versus tube voltage — arcs followed the current threshold, not the voltage.
Cause
Probable, by indirect signs: the tube insert itself, because the arcing depended on tube current rather than on tube voltage.
The cause is probable and was not fully confirmed.
What was done
The engineers' criterion, reported in the case: if arcing depends on tube current, suspect the tube flask; if it depends on voltage, look at the oil in the emitter and the HV tank/transformer. Here the current dependence plus clean tests without current pointed to the tube as the probable culprit; no parts were changed within the case, so the outcome remained probable.
Safety
Before touching the tube, HV cables or tank, power the system down and allow the high-voltage generator to fully discharge — it can hold a lethal stored charge — and complete electrical safety and radiation checks before the scanner returns to use.
Lesson: Arcs that track tube current point to the tube insert, while arcs that track tube voltage point to the HV oil path or tank, so map the fault against both axes before ordering a tube.
Covered in the course: CT Service — Level 2: Fault-Finding →
Written from an anonymised case shared by service engineers. It is not a manufacturer instruction and grants no authorisation. Independent note; names are trademarks of their owners.