SF6 gas analysis measures the quality of the gas in a compartment on site: moisture, SF6 percentage and decomposition products, and is accompanied by leak detection. Each measurement sees a different problem: moisture that can condense, air that reduces dielectric strength or arcs that should not exist. IEC 62271-4 covers gas handling and verification and IEC 60480 the criteria for reusing it.
What SF6 gas analysis is
Sulfur hexafluoride (SF6) insulates and extinguishes the arc in high and extra-high voltage breakers, in GIS substations and in part of medium voltage switchgear. Its properties depend on the gas being clean and dry. Gas analysis measures the quality of the gas in each compartment on site, with portable analyzers; when more detail is needed, a sample is taken for the laboratory.
It is not the same as density monitoring. The density monitor says whether there is enough gas; the analysis says whether that gas is in good condition. Both are checked in circuit breaker testing and in GIS commissioning.
What it measures and what defect it detects
| Measurement | What it indicates | What defect it detects |
|---|---|---|
| Moisture (dew point or ppmv) | Water in the gas | Insufficient drying at filling, saturated adsorbent, moisture ingress through seals |
| SF6 percentage (purity) | How much of the gas is SF6 | Air or other gases from poor filling or evacuation |
| Decomposition products (SO2 and others) | Gas decomposed by arcing or discharges | Switching wear in breakers; internal discharges or arcs in insulation compartments |
| Leaks (detector or gas imaging camera) | Gas loss to the environment | Leaking seals, flanges, valves or density monitors |
Why each one matters:
- Moisture: at low temperature the water can condense on insulators and cause surface flashover. In addition, together with decomposition products it forms acidic, corrosive compounds.
- Purity: air mixed with SF6 reduces dielectric strength and interrupting capability.
- Decomposition products: IEC 60480 distinguishes the by-products that appear in insulation-only equipment, in switching equipment and from internal arcs. That distinction is the key to interpretation: the same value means different things depending on the compartment.
- Leaks: SF6 is a very potent greenhouse gas. A leak is a technical and an environmental problem, and its handling is regulated in many countries.
Which equipment and levels it applies to
| Level | Equipment | Factory | Commissioning | Maintenance |
|---|---|---|---|---|
| LV | Not applicable: SF6 is not used | No | No | No |
| MV | SF6 circuit breakers, gas-insulated switchgear and RMUs | Tightness and filling | Gas condition measurement after on-site filling | Only if the equipment has an access valve |
| HV | Live tank and dead tank breakers, GIS, SF6 instrument transformers | Tightness and filling | Yes, in every compartment filled on site | Yes, by trend and after faults |
| EHV | Multi-chamber breakers, GIS, gas-insulated lines | Tightness and filling | Yes | Yes |
In medium voltage, many switchgear units and RMUs are «sealed pressure systems» in the sense of IEC 62271-1: they require no gas refilling during their expected service life, many have no sampling valve and verification is limited to the pressure indicator. For that equipment the analysis does not apply unless the manufacturer provides for it.
Where it does not apply: air-insulated vacuum breakers, oil-filled equipment and equipment with alternative gases, which need their own analyzers and criteria. For vacuum interrupters, the equivalent test is the vacuum integrity test.
Standards and criteria
- IEC 60376:2018 specifies new, technical grade SF6 for electrical equipment.
- IEC 60480 gives the criteria for reusing SF6 and its mixtures with nitrogen or CF4 after recovery, with tables of maximum contaminants. It refers recovery procedures to IEC 62271-4 and leak-determination methods to IEC 60068-2-17.
- IEC 62271-4 covers the handling of insulating and switching gases at installation, commissioning, normal service, repair and end of life, including on-site verification of gas quality with portable equipment or by sampling for the laboratory. It covers SF6, its mixtures, compressed air and other alternative gases.
- IEC 62271-200 (switchgear from 1 kV to 52 kV) includes measuring the fluid condition after on-site filling, and IEC 62271-203 (GIS above 52 kV) covers tests after on-site installation.
Limits for equipment in service come from the manufacturer and may differ for the breaker chamber and for an insulation compartment. The limits in IEC 60480 are for reusing the gas, not an acceptance criterion for equipment in service. In maintenance, trend counts: moisture that rises year after year, or decomposition products appearing where there were none before.
What can distort the result
- Wet or unpurged hoses and couplings, which add moisture to the sample.
- Dew point reference pressure different from that of the limit.
- Equipment temperature: water adsorbed in solid materials passes into the gas or back into them depending on temperature. Record the temperature and compare under similar conditions.
- Measuring right after filling: moisture needs time to stabilize. Respect the time the manufacturer indicates.
- Analyzer set for another gas or with sensors past their calibration date.
- Releasing gas to the atmosphere during the measurement: besides the environmental impact, it can lower density below the alarm. Use analyzers that return or collect the measured gas.
What to do if the result is bad
- High moisture: the gas is processed with a recovery unit that filters and dries it, the adsorbent is replaced if the manufacturer provides for it, and the measurement is repeated after the stabilization time.
- Low purity: the gas is recovered and reclaimed or replaced, and the cause of the air is found.
- Decomposition products in an insulation compartment: an internal fault is investigated with partial discharge testing and, if needed, inspection. Gas with many by-products is handled with the safety measures of IEC 62271-4.
- Leak: it is located, repaired with the manufacturer, the gas is topped up and the quantities are recorded.
Equipment for this test
This test is done with equipment from recognized manufacturers; these guides explain what each one measures. In your quotation we recommend the test and the equipment your case needs.
- SF6 leak detector: WIKA GIR-10 and GA11 analyzer
- SF6 handling: recovery and filling units
- Vaisala MM70 and DM70: moisture in oil and dew point
SF6 gas analysis with Tiza Group
Our team verifies gas quality, density and tightness in medium, high and extra-high voltage breakers, GIS and switchgear, in substations and industry, in Colombia or elsewhere on request, together with the contact resistance test and the other tests of the circuit breaker. Review the other tests in the primary equipment testing guide, see our circuit breaker testing service or write to us.
References
- IEC 60376:2018, Specification of technical grade sulphur hexafluoride (SF6) and complementary gases to be used in its mixtures for use in electrical equipment.
- IEC 60480:2019, Specifications for the re-use of sulphur hexafluoride (SF6) and its mixtures in electrical equipment.
- IEC 62271-4:2022, High-voltage switchgear and controlgear – Part 4: Handling procedures for gases for insulation and/or switching.
- IEC 62271-203:2022, High-voltage switchgear and controlgear – Part 203: AC gas-insulated metal-enclosed switchgear for rated voltages above 52 kV.
- IEC 62271-200:2021, High-voltage switchgear and controlgear – Part 200: AC metal-enclosed switchgear and controlgear for rated voltages above 1 kV and up to and including 52 kV.
- IEC 62271-1:2017, High-voltage switchgear and controlgear – Part 1: Common specifications for alternating current switchgear and controlgear.
- IEC 60068-2-17, Basic environmental testing procedures – Part 2-17: Tests – Test Q: Sealing.
Frequently asked questions
What does SO2 indicate in an SF6 compartment?
That arcing or discharges decomposed the gas. In a breaker interrupter chamber some amount is expected from switching operations; in an insulation-only compartment, such as a GIS busbar or disconnector, it is a sign of an internal fault that must be investigated.
Why must the SF6 dew point be stated at a specific pressure?
Because the same moisture content gives a different dew point depending on the pressure it refers to. Comparing a value at atmospheric pressure with a limit at service pressure leads to wrong conclusions.
Does an SF6 analyzer work for equipment with alternative gases?
Not necessarily. Mixtures with other gases and technical air have their own parameters and criteria, and IEC 62271-4 treats each gas separately. The analyzer must be suitable for the gas in the equipment.