Sulfur hexafluoride (SF₆) is widely used in high-voltage switchgear because it's chemically stable, electrically insulating, and non-flammable under normal operating conditions.
But that changes when electrical faults occur.
Arcing, switching operations, or prolonged partial discharge can break down SF₆ molecules, producing hazardous decomposition products that pose risks to both maintenance personnel and equipment.
When technicians see SO₂ or other byproduct readings on an SF₆ gas analyzer, those numbers tell you what has happened inside the equipment.
This guide explains the most common SF₆ decomposition products, why they matter, and how to interpret analyzer results using IEC 60480 as a practical framework.
How SF₆ Decomposes During Arcing and Partial Discharge
Although SF₆ is highly stable, the intense energy produced during electrical faults can split the molecule into reactive sulfur and fluorine fragments.
This typically occurs during:
- Internal arcing
- Circuit breaker switching operations
- Corona discharge
- Partial discharge
- Spark events
Temperatures inside an electrical arc can exceed several thousand degrees Celsius, easily breaking the SF₆ molecule apart. As these fragments cool, they react with trace amounts of oxygen, moisture, metals, insulation materials, and other substances inside the gas compartment, producing a range of gaseous and solid decomposition products of sulfur hexafluoride.
The severity of decomposition depends on the energy involved:
- Low-energy partial discharge generally produces smaller quantities of byproducts over time.
- High-energy arcing faults create much larger concentrations and a wider range of hazardous compounds.
Moisture plays a significant role as well. Water vapor accelerates chemical reactions that produce corrosive acids, making moisture control an important part of long-term SF₆ gas quality.
The Main SF₆ Decomposition Products (and What They Mean)
Sulfur Dioxide (SO₂)
Sulfur dioxide is one of the most common decomposition products detected during SF₆ analysis.
Its presence often indicates that electrical decomposition has occurred and that moisture has reacted with intermediate compounds inside the equipment. Elevated SO₂ levels don't automatically mean the gas must be discarded, but they do indicate that further investigation is necessary.
Because SO₂ is relatively easy to detect, it is one of the primary indicators monitored in routine gas analysis.
Thionyl Fluoride (SOF₂)
SOF₂ forms as an intermediate product during SF₆ decomposition.
While it may not remain stable for long, it reacts readily with moisture to produce sulfur dioxide (SO₂) and hydrofluoric acid (HF). Its presence often indicates that decomposition reactions are actively occurring inside the equipment.
Sulfuryl Fluoride (SO₂F₂)
SO₂F₂ is generally associated with higher-energy decomposition events than SOF₂.
Higher concentrations may indicate more severe electrical faults or prolonged arcing, providing additional insight into the condition of the equipment.
Hydrogen Fluoride (HF)
Hydrogen fluoride is one of the most hazardous decomposition products formed when fluorine reacts with moisture.
HF is highly corrosive and can:
- Cause severe skin burns
- Damage respiratory tissue
- Corrode metal components
- Attack glass and ceramic surfaces
Any detectable HF requires careful handling procedures and appropriate personal protective equipment.
Disulfur Decafluoride (S₂F₁₀)
Disulfur decafluoride is an extremely toxic compound that can form under certain severe arcing conditions.
Although it is generally present only in very small quantities, its potential toxicity underscores the need for appropriate safety procedures when handling faulted SF₆ equipment.
Solid Decomposition Products
Not all decomposition products remain in gas form.
Electrical faults can produce solid metal fluorides such as:
- Aluminum fluoride (AlF₃)
- Copper fluoride (CuF₂)
- Tungsten fluoride compounds
These often appear as white or gray powder inside switchgear and can contribute to insulation degradation, contamination, and corrosion over time.
Why These Byproducts Are Dangerous
The hazards extend beyond the gas itself.
Some decomposition products present immediate health risks during maintenance, while others gradually damage critical equipment components.
Potential worker hazards include:
- Respiratory irritation
- Chemical burns
- Eye injuries
- Pulmonary damage following significant HF exposure
From an equipment perspective, decomposition products can:
- Corrode internal contacts
- Damage O-rings and seals
- Degrade insulating materials
- Shorten equipment life
There's also an environmental consideration. According to the U.S. EPA's Inventory of U.S. Greenhouse Gas Emissions and Sinks (1990–2024), SF₆ remains one of the most potent greenhouse gases used in industry, with a 100-year Global Warming Potential approximately 23,500 times greater than CO₂. Preventing leaks and properly managing contaminated gas supports both worker safety and emissions compliance.
What Your SF₆ Analyzer Is Actually Telling You
A modern gas analyzer provides far more than a simple pass/fail result. Each measurement contributes to the overall assessment of gas condition.
SF₆ Purity
IEC 60480 specifies that gas intended for reuse should typically maintain an SF₆ purity of 97% or higher. Lower purity may indicate contamination or dilution and should be evaluated alongside the remaining measurements.
Moisture
Moisture accelerates decomposition reactions and promotes the formation of corrosive compounds such as HF. Monitoring moisture helps identify conditions that could continue degrading gas quality over time.
SO₂ Concentration
SO₂ provides valuable information about the severity of decomposition.
SO₂ (ppmv) | Typical Interpretation* |
<2 | Normal operating condition |
2–12 | Investigate equipment condition |
>12 | Corrective action required |
*These values provide a practical reference based on IEC 60480 guidance. Gas reuse decisions should always consider the complete set of analytical results rather than any single measurement.
HF
Any detectable HF deserves attention because it indicates corrosive decomposition has occurred. Personnel should follow appropriate handling procedures before opening or servicing the equipment.
Carbon Monoxide (CO)
Some analyzers also measure carbon monoxide.
Elevated CO may indicate degradation of solid insulation materials such as epoxy resin or PTFE, providing additional information about internal equipment condition.
Safe Response When Decomposition Products Are Detected
When decomposition products are identified, maintenance activities should proceed cautiously.
Recommended best practices include:
- Isolate and de-energize the equipment.
- Ventilate the work area where appropriate.
- Wear suitable PPE, including respiratory protection, gloves, eye protection, and protective clothing.
- Recover contaminated gas using equipment designed for decomposition byproducts.
- Filter and reclaim gas where appropriate.
- Reanalyze the gas before determining whether it meets reuse requirements under IEC 60480.
Even relatively low levels of decomposition products can justify additional inspection, depending on equipment history and operating conditions.
Compliance Considerations
Safe SF₆ handling is supported by several international standards and regulations, including:
- IEC 60480: Specifications for the reuse of SF₆
- IEC 62271-4: High-voltage switchgear handling procedures
- IEEE C37 series: Switchgear maintenance guidance
- EPA Greenhouse Gas Reporting Program (Subpart DD): SF₆ emissions reporting requirements
Together, these standards help utilities maintain reliable equipment while protecting workers and minimizing environmental impact.
SF₆ is stable under normal operating conditions. The concern is what happens when electrical faults create decomposition products that affect personnel safety and equipment reliability.
Understanding what compounds such as SO₂, SOF₂, HF, and other byproducts indicate allows technicians to make informed maintenance decisions, identify developing equipment issues earlier, and determine the appropriate next steps for gas handling.
Rather than relying on a single measurement, evaluating purity, moisture, and decomposition products together provides the clearest picture of overall gas condition.
Analyze Gas with Confidence Using DILO's Multi-Analyzer SF₆
Reliable decisions begin with reliable measurements.
Our SF₆ Gas Analyzer measures gas purity, moisture, and decomposition products simultaneously — giving technicians what they need to assess gas quality in accordance with IEC 60480 before reuse or maintenance.
When servicing faulted equipment, pairing gas analysis with DILO's working protection kit helps protect personnel from hazardous decomposition byproducts.
For more on gas quality management, see our guide to checking SF₆ quality before starting any gas handling operation.