Oxygen Displacement in Enclosed Switchgear: O₂ and SF₆ Monitoring at Thermal Power Plant Substations

SF₆ gas is heavier than air, and when it leaks from a switchgear cell or gas-insulated switchgear (GIS) bay it displaces oxygen in the lower part of the room. For personnel, this isn't a "chemical leak with a smell" — it's an invisible asphyxiation hazard: O₂ concentration can drop below 19.5% before the SF₆ pressure sensor inside the cell itself even trips.

At thermal power plant substations and house-load switchgear (auxiliary distribution), enclosed switchgear bays are the standard design. It isn't enough to monitor only the gas purity inside the breaker — you need a dual loop: the quality of the insulating medium in the equipment, and the composition of the room atmosphere where maintenance staff work.

Why SF₆ is dangerous beyond being a greenhouse gas

Risk	Mechanism	Consequence
Reduced dielectric strength	SF₆ pressure drop in the cell	Failure to interrupt, arc flashover
Personnel asphyxiation	SF₆ pools near the floor, displaces O₂	Fainting, respiratory arrest
Environmental penalty	Leak to atmosphere (GWP 23,500)	EU F-Gas regulatory sanctions

The first two risks require different measurements. A portable sample analysis from the cell (Rapidox SF6 6100) answers "is the gas inside the breaker within spec?" Fixed atmospheric monitoring (Rapidox SF6 Fixed Detection System) answers "is there a leak into the enclosed space where a person might be standing?"

Oxygen limits in enclosed electrical rooms

Based on occupational safety practice and CIGRE recommendations for SF₆-equipped rooms:

• O₂ ≥ 19.5% — safe working zone;
• O₂ 16–19.5% — elevated-risk zone, entry only with PPE;
• O₂ < 16% — entry prohibited until forced ventilation.

SF₆ has no smell and no colour. The only reliable approach is continuous or periodic O₂ monitoring combined with SF₆ sensors in the room air.

The multichannel Rapidox 6100 QUAD and Rapidox 6100 Clean Air Pumpback measure O₂, humidity and, where needed, SF₆ in a single cycle — convenient for rounds at house-load switchgear rooms where traditional SF₆ cells and newer SF₆-free dry-air cabinets coexist.

Fixed vs portable monitoring

Fixed system at a thermal power plant substation

The Rapidox SF6 Fixed Detection System is installed in:

• 6–110 kV switchgear bays at house-load distribution and generator outgoing feeders;
• enclosed GIS corridors;
• SF₆ compressor station rooms (where centralized filling exists).

Sensors are mounted low, near the floor — where SF₆ settles. Integration with the plant SCADA via Modbus/Ethernet warns the dispatcher before a maintenance crew enters the bay.

Portable monitoring during scheduled work

Before opening a cell or working inside a cabinet, an O₂ measurement with a portable analyzer is mandatory. Sequence:

1. Check O₂ in the room and near the floor of the cabinet.
2. If O₂ > 19.5%, take an SF₆ sample from the cell (step-by-step guide).
3. After work is complete, re-check the atmosphere before handing the room back.

Link to vibration monitoring of thermal power plant auxiliaries

SF₆ leaks and contact degradation in a breaker are often accompanied by partial discharge — detected by ultrasound (SDT340, SonaVu) during switchgear rounds. In parallel, at the same plant, auxiliary pumps and fans need vibration monitoring — a single chief power engineer's service benefits from a unified round schedule: gas → ultrasound → vibration.

Common mistakes

• Relying only on the cell pressure sensor — pressure drops slowly, while room O₂ can drop faster during a large leak.
• Mounting the SF₆ sensor near the ceiling — gas settles near the floor, creating a false sense of safety.
• Skipping ventilation after isolation — SF₆ can remain in the bay for hours without forced purging.

Scope of application

This material is aimed at the power generation sector: thermal power plants, large power stations, hydroelectric plants, and substations at industrial sites. Petrochemical and oil & gas complexes, with their different equipment and procedures, are outside the scope.

Conclusion

Monitoring SF₆ without monitoring oxygen in enclosed switchgear is an incomplete safety program. Rapidox covers both loops: portable analysis of gas quality inside the cell, and fixed monitoring of atmospheric leaks. KEG TRK supplies Cambridge Sensotec gas analyzers and helps build them into the procedures at power generation facilities.

Request a consultation on selecting the right fixed and portable Rapidox configuration for your substation.