Replacing SF6: C4-FN and C5-FK Mixtures and Clean Air — What Changes in Gas Monitoring

For decades SF6 was the standard for insulation and arc quenching in high-voltage equipment. Now the industry is moving away from it: SF6 is the most potent greenhouse gas, and the European Union's new fluorinated gases regulation (Regulation (EU) 2024/573) phases out its use in new switchgear in stages, starting with medium voltage. Major manufacturers already offer GIS and breakers on alternative gases.

For Kazakhstan this is not a distant prospect: equipment on new gases arrives as part of international companies' projects, and suppliers are gradually reducing SF6 ranges. Substation teams need to understand how the new gases differ and how to monitor them.

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Which gases replace SF6

Solution	Composition	Where it is used
C4-FN-based mixtures	Fluoronitrile (Novec 4710) mixed with CO₂ and O₂	High-voltage GIS and breakers
C5-FK-based mixtures	Fluoroketone (Novec 5110) with technical air	Medium-voltage switchgear
Technical (clean) air	Dried air, N₂ and O₂	Medium- and high-voltage switchgear with vacuum interruption

The key difference: SF6 was a single-component gas, while most alternatives are mixtures. Insulating properties depend on the exact ratio of components, so quality control gets more complex: you measure not only purity and moisture but also the share of each component.

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What changes in gas quality control

For SF6 the standard set is purity (% SF6), moisture (dew point) and decomposition products. For mixtures, add:

• concentration of the main component (C4-FN or C5-FK), usually a few percent, which drives dielectric strength;
• CO₂ and O₂, the carrier components of the mixture;
• CO, one of the indicators of mixture decomposition after arcing;
• moisture, still critical.

The Rapidox 6100 C4-FN & C5-FK Multi-Gas Pumpback measures C4-FN and C5-FK with infrared sensors (0–20%), CO₂ (0–100%), O₂ (0–30%), CO (up to 5000 ppm) and dew point from −60 to +20 °C. Gas return pumps the sample back, which matters because the mixtures are expensive and emissions are undesirable.

For equipment on technical air there is the Rapidox 6100 Clean Air Pumpback: it monitors air composition (O₂, N₂) and moisture and also handles SF6, which is convenient at substations running both generations of equipment.

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What changes in leak detection

A leak detector set up only for SF6 will not necessarily see the new mixture. When choosing an instrument, check the list of detectable gases. The Ion Science SF6 LeakCheck P1 XTL detects both SF6 and C4-FN, so one instrument covers both generations of equipment.

For equipment on technical air a gas leak detector is not applicable: you cannot find an air leak into air by composition. Pressure monitoring and ultrasonic leak detection do the job here: an ultrasonic detector hears the turbulent jet of gas escaping under pressure, whatever its composition.

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Summary table: which instrument for which gas

Gas in equipment	Gas quality	Leak detection
SF6	Rapidox SF6 6100	SF6 LeakCheck P1 XTL, SF6 Leakmate
C4-FN mixtures	Rapidox 6100 C4-FN & C5-FK	SF6 LeakCheck P1 XTL (detects C4-FN)
C5-FK mixtures	Rapidox 6100 C4-FN & C5-FK	Check by gas and instrument
Technical air	Rapidox 6100 Clean Air	Pressure monitoring, ultrasound

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What substation teams should do now

1. Inventory: which devices run on which gas and what is coming in near-term projects.
2. Procurement requirement: buy monitoring instruments for the new gas together with the new equipment, otherwise maintenance will be impossible.
3. Staff training: working with mixtures, sampling and gas return rules.
4. SF6 accounting on existing equipment, which will stay in service for decades. How to organise it is in the article on SF6 leak accounting.

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Summary

Moving from SF6 to C4-FN, C5-FK mixtures and technical air changes diagnostics as well as equipment: instead of one gas, mixtures with each component monitored; instead of one leak detector, an instrument chosen for the gas. Substations will run both generations for a long time, so instruments covering both SF6 and the new gases are a sensible choice.

KEG TRK supplies Rapidox (Cambridge Sensotec) analysers and Ion Science leak detectors in Kazakhstan and helps choose a kit for substations with SF6 and alternative-gas equipment.