TL;DR
Output voltage distortion above 5 percent THD can reset sensitive pharma instrumentation. Check the output filter capacitors and inverter IGBT driver boards.
What you might see
- sensitive loads resetting unexpectedly
- output voltage waveform distorted on power analyzer
- output THD above 5 percent
- capacitor bank fault alarm
Likely causes
Failed output filter capacitor reducing filtering effectiveness
IGBT driver board fault causing asymmetric switching
Nonlinear load with high crest factor overdriving the filter design
Loose output bus connection causing impedance discontinuity
Required tools
- Power quality analyzer (THD measurement capable)
- PPE: category-rated arc flash gear, insulated gloves
- LOTO kit (for de-energized work only)
Safety first
- UPS output work requires arc-flash PPE and a qualified electrical engineer. The output bus is energized even when the UPS is in bypass.
- Pharma critical loads (PLCs, analyzers, DCS) can be damaged by sustained high distortion. Document and act on distortion findings promptly.
Procedure
- 1
Connect a power quality analyzer to the UPS output bus. Measure THD on all three phases.[1]
- 2
Record the load current crest factor and compare to the UPS design specification. High crest factor from switching power supplies raises output distortion.[1]
- 3
Review the fault log for any capacitor bank fault or IGBT trip events.
- 4
Inspect the output bus connections for looseness. Loose connections add impedance and worsen distortion.
Warning: Output bus inspection requires opening an energized cabinet. Use full arc-flash PPE and an Energized Electrical Work Permit. - 5
If THD is above specification with a normal load profile, suspect a failed output capacitor. Capacitor replacement requires a field service engineer.
- 6
If the load crest factor is high, consider adding a harmonic filter or distributing the load to reduce impact on a single phase.
- 7
Log the measurement and schedule a follow-up measurement after any corrective action.
Sources
Eaton Eaton 9395 / 93PM UPS System (Industrial) general technical documentation, Eaton
Eaton 9395 / 93PM UPS general inverter output quality and filter maintenance procedures (general)
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