Summary
Partial discharge (PD) inside energized equipment can generate ozone (O₃) in electrical distribution rooms. Because PD is intermittent and many rooms are low‑dilution spaces, short‑term spikes can occur and persist long enough to exceed occupational ceiling and short‑term limits. In a recent anonymized assessment, spot checks indicated an elevated condition, and multi‑week logging documented sustained ppm‑level ozone with peaks around approximately 3 ppm (>50% of IDLH conditions).

What is PD and why it creates ozone?
PD occurs when the electrical field partially bridges insulation, ionizing nearby air without forming a full arc. Those reactions split oxygen molecules; free atoms then combine to form ozone. In confined rooms with limited make‑up air, ozone can accumulate near switchgear, bus ducts, cable terminations, and other high‑stress points, particularly during high load, switching, or certain humidity conditions.

What we observed in an anonymized case

• Screening day: Among many electrical rooms at a high‑load site, most were at or near background; the PD‑affected room showed a brief elevated condition during a five‑minute check (on the order of a few tenths of a ppm).
• Trend logging: Over roughly four weeks, the same room averaged around a few‑tenths of a ppm with short‑term peaks approaching approximately 3 ppm. The final week showed extended periods above the OSHA 8‑hour PEL of 0.1 ppm. These patterns illustrate why one‑time spot checks can under‑characterize risk.

How that compares to common limits (U.S.)
Referencing typical U.S. values: OSHA PEL 0.1 ppm (8‑hr TWA); Cal/OSHA STEL 0.3 ppm (15‑min TWA); NIOSH REL ceiling 0.1 ppm; ACGIH TLV 0.2 ppm for work ≤2 hours; IDLH 5 ppm. Short spikes above the ceiling/STEL, even when full‑shift averages look modest, warrant immediate controls.

Why these exposures are easy to miss

• Intermittent source: PD is load‑ and condition‑dependent.
• Transient peaks: Short job steps can coincide with high spikes that an 8‑hour TWA won’t show.
• Room dynamics: Closed doors or minimal exhaust can trap ozone at breathing height.
• Sensory cues are unreliable: Odor and irritation don’t scale linearly with concentration.

Where to look first

• Medium‑ and low‑voltage switchgear rooms, substations, UPS/switch rooms, MCCs.
• Rooms with known or suspected PD, insulation degradation, or arcing history.
• Spaces with recurring “fresh/metallic” odor or irritation reports that clear on exit.
• Areas showing inside‑to‑outside gradients on a quick screen.

How to confirm (practical monitoring)

1. Screen broadly with a calibrated direct‑read ozone instrument; take paired readings inside the room and immediately outside.
2. Log over time in any room with a detectable signal; one to two weeks captures operational and environmental variability.
3. Compare to applicable OELs (PEL/REL/STEL/TLV); consider ceiling/15‑minute exceedances and 8‑hour TWA.
4. Correlate with operations: load, switching events, maintenance windows, humidity, door‑open time.
5. If personnel need to enter regularly, supplement with task‑based or personal sampling to evaluate 8‑hour compliance.

Control strategy (source → pathway → receptor)
Source control

• Confirm and remediate PD: address arcing/corona, insulation damage, cable terminations, and cleanliness; consider continuous PD monitoring.

Pathway/ventilation

• If PD cannot be immediately abated, evaluate airflow and add local exhaust or increase air changes to dilute/remove ozone.

Administrative controls

• Manage access while troubleshooting; post temporary signage; schedule intrusive work during low‑load periods.

PPE (last line)

• For short investigative entries in elevated areas, use respirators appropriate for ozone within the limits of your respiratory protection program, with medical clearance, fit testing, and training.

Verification

• Continue direct‑read monitoring during and after mitigation; re‑check after repairs, load changes, or seasonal humidity shifts; keep ozone in the IH/IAQ screening matrix for rooms with PD history.

Checklist for EHS Teams
□  Do we have high‑load electrical rooms with suspected PD or arcing history?
□  Have we performed inside/outside direct‑read screening?
□  Have we deployed a data logger for at least a week where any signal was found?
□  Are we evaluating against ceiling/STEL and TWA criteria?
□  Do we have interim controls (access, ventilation, PPE) while PD is addressed?
□  Are we verifying and documenting effectiveness after repairs?

Closing
PD can quietly turn a routine electrical room into an ozone exposure hotspot. The most effective approach is to find the source, measure intelligently over time, prioritize source and ventilation controls, and verify. Facilities with significant electrical demand should add ozone checks to electrical‑room assessments and act on short‑term exceedances promptly.