Ozone from Partial Discharge in Electrical Rooms: What EHS&S Teams Should Check Now

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.

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.
• 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.

How That Compares to Common Limits (U.S.)

Standard	Limit
OSHA PEL	0.1 ppm (8-hr TWA)
Cal/OSHA STEL	0.3 ppm (15-min TWA)
NIOSH REL	0.1 ppm ceiling
ACGIH TLV	0.2 ppm for work of 2 hours or less
IDLH	5 ppm

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

Control Strategy

Source control: Confirm and remediate PD; address arcing/corona, insulation damage, cable terminations, and cleanliness.

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.

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.