Power & Utilities
SF₆ Leak Detection for GIS Switchgear: Why Density Monitoring Is Not Enough
SF₆ has a global warming potential roughly 24,000 times that of CO₂, which is why its leakage is regulated as an environmental issue rather than a purely asset-management one. Many maintenance teams assume a density monitor settles the matter — but density monitoring answers "has the gas in the compartment been lost", not "where is it going". This article sets out how the two layers divide the work, how the regulated leak-check interval is calculated, and one widely overlooked compliance lever: installing a leakage detection system doubles that interval.
- ≈24,000 SF6 global warming potential (CO₂ = 1)
- 0.1%/yr Exemption threshold: tested leakage rate must beat this
- 6 kg Below this charge, prescribed leak checks do not apply
- ×2 Leak-check interval doubles with a detection system fitted
Three layers, three different questions
The most common misunderstanding in SF₆ monitoring is treating density monitoring and leak detection as the same thing. They measure entirely different quantities and therefore cannot substitute for one another.
| Method | What it measures | Primary purpose |
|---|---|---|
| Density monitor / pressure gauge | Gas condition inside the compartment | Detects falling density or gas loss and triggers an equipment alarm |
| Fixed SF₆ sensor | SF₆ concentration in ambient air across the monitored area | Confirms gas has reached the environment, triggers an area alarm and supports leakage-rate accounting |
| Portable leak detector | Local SF₆ concentration near the equipment | Pins down the leak: valves, flanges, joints, sealing faces |
When leak checks are required
The EU F-Gas Regulation (EU) 2024/573 has applied since 11 March 2024, with the target of cutting F-gas emissions 79% below 2009 levels by 2030. For switchgear it provides that equipment charged with 6 kg or more of F-gases (SF₆ and alternatives such as fluoronitrile) requires prescribed leak checks unless one of the following exemptions is met.
- Exemption 1: the manufacturer's technical specification demonstrates a tested leakage rate below 0.1% per year, with the equipment labelled accordingly.
- Exemption 2: it is fitted with a pressure or gas density monitoring device with an automatic alert system while in operation.
- Exemption 3: it contains less than 6 kg of the listed fluorinated greenhouse gases.
How the leak-check interval is calculated
Where equipment falls within the scope requiring leak checks, the interval is set by the charge expressed in CO₂-equivalent. For reference, 50 tonnes of CO₂-equivalent corresponds to roughly 2 kg of SF₆.
| Charge (CO₂-equivalent) | Standard interval | With a leakage detection system |
|---|---|---|
| Up to 50 tonnes | Every 12 months | Every 24 months |
| 50 to 500 tonnes | Every 6 months | Every 12 months |
| Over 500 tonnes | Every 3 months | Every 6 months |
Two mechanisms, not one exemption
There is a trap here. The exemption refers to a pressure/density monitoring device; the doubling refers to a leakage detection system. Density monitoring reports the state inside the compartment and does not measure SF₆ in ambient air — so with a density monitor alone you satisfy the exemption but do not earn the doubling.
Read the other way, the rules give a clear investment signal. For equipment already exempt thanks to density monitoring, fixed ambient sensing still has value: it is the step from "exempt" to "actively managing emissions". For substations that cannot claim an exemption and must be checked on schedule, ambient monitoring is a direct return on investment.
Why NDIR infra-red sensing
SF₆ is a strong infra-red absorber with a characteristic band near 10.55 μm. NDIR (non-dispersive infra-red) sensors exploit this, deriving concentration from measured absorption, and are highly selective to SF₆ — with no significant cross-response to other gases present in a substation.
- No consumable: unlike catalytic or electrochemical types, no sensing material is consumed, so long-term continuous operation needs no element replacement.
- Good selectivity: humidity swings and other gases common in substations have little effect on the reading.
- Long service life: suited to sites that must run continuously and are hard to visit frequently.
- Two-channel configuration: ambient concentration monitoring for area alarms, plus post-repair verification once a portable instrument has located the source.
Where the industry is heading
The Regulation phases in restrictions on new equipment using fluorinated gases: from 1 January 2026 for medium-voltage switchgear up to and including 24 kV, from 2028 for high-voltage equipment above 52 kV up to 145 kV, from 2030 for medium voltage above 24 kV up to 52 kV, and from 2032 for specified higher-voltage classes. Separately, from 1 January 2035 only reclaimed or recycled SF₆ may be used for switchgear maintenance.
One common misreading is worth correcting: this does not require existing SF₆ equipment to be retired. The Regulation principally constrains putting new equipment into operation, while GIS and breakers already in service will continue running for years — so the need for leak monitoring does not disappear; the transition period makes it longer.
- Start with the charge: establish the SF₆ charge and CO₂-equivalent band per asset — that determines the check frequency and the compliance workload.
- Confirm exemption status: whether the asset is labelled with a tested leakage rate below 0.1%/yr, or already has density monitoring with automatic alert.
- Use the check interval as a selection input: where the schedule is every three months, the doubling from ambient monitoring delivers the clearest return.
- Outputs must reach existing systems: confirm 4–20 mA / RS485 / digital output can connect to the substation SCADA or environmental platform without retrofitting.
- Records: the Regulation requires relevant records to be kept at least five years, with annual reporting duties; monitoring data needs timestamps and exportable logs.
Frequently asked questions
If a density monitor is fitted, is SF₆ leak detection still needed?
When are prescribed leak checks not required?
Can the leak-check interval be extended?
Why use NDIR rather than another principle for SF₆?
Will existing SF₆ equipment be forced out of service?
Sources
Sources: EU F-Gas Regulation (EU) 2024/573 (applicable from 11 March 2024; 79% emission reduction target by 2030 against 2009) and its switchgear provisions — the 6 kg charge threshold and three exemption conditions, the 12/6/3-month leak-check intervals banded by CO₂-equivalent, and the provision doubling control periods where a leakage detection system is installed; the equivalence of 50 tonnes CO₂-equivalent to about 2 kg SF₆; SF₆ global warming potential of approximately 24,000 (IPCC Sixth Assessment); IEC 62271-1 design leakage-rate targets (≤0.5% per year, ≤0.1% per year for sealed-for-life medium-voltage equipment) and IEC 62271-4 on SF₆ handling procedures; the 2026 / 2028 / 2030 / 2032 phase-in schedule restricting new equipment; and the rule permitting only reclaimed or recycled SF₆ for maintenance from 1 January 2035.