How Acoustic Cameras Detect SF6 Leaks in GIS Switchgear

Why SF6 Leaks Matter: Cost, Compliance and the Climate

Sulfur hexafluoride (SF6) is the insulating and arc-quenching gas that makes gas-insulated switchgear (GIS) compact and reliable. It is also the most potent greenhouse gas in common industrial use: one kilogram of SF6 has the same global warming potential as roughly 23,500 kilograms of CO2, and it stays in the atmosphere for over 3,000 years. A single leaking GIS compartment can emit several kilograms per year — the climate equivalent of driving a car tens of thousands of kilometers.

For a utility, the consequences of an undetected leak are practical before they are environmental. SF6 is expensive, and topping up a leaking bay is a recurring cost that never ends until the leak is found. Density monitors eventually alarm, but by then the compartment has already lost gas, and low gas density degrades the dielectric strength that the insulation depends on. In the worst case, a compartment that empties in service can flash over. Finding leaks early is therefore a maintenance priority, a cost-control measure, and a compliance obligation in one.

"We stopped guessing where our GIS was losing gas. The acoustic camera pinpointed a leaking flange gasket on a 220 kV busbar compartment in under an hour — a leak our sniffer team had walked past three times." — Substation Maintenance Supervisor, Brazil

Can an Acoustic Camera Detect SF6?

Yes — and it does it through a physical principle that works in every GIS hall. SF6 is stored and operated at pressures between 0.4 and 0.7 MPa (4–7 bar) above atmosphere. When that pressurized gas escapes through a pinhole, a cracked gasket, or a loose fitting, it accelerates and becomes turbulent at the leak orifice. Turbulent gas flow generates broadband acoustic noise, with a strong ultrasonic component in the 20–100 kHz range — far above human hearing and above most background noise in a substation.

An acoustic camera uses an array of microphones to detect that ultrasonic emission, localize its origin, and display it as a hotspot overlaid on a live optical image. The operator sees exactly where the leak is, without touching the equipment, without opening the compartment, and without shutting down the bay. That is the key advantage over traditional methods: no outage, no gas handling, no proximity to energized high-voltage compartments.

Detection performance depends on two things: the size of the leak (larger leaks produce louder signals) and the pressure differential (higher pressure produces stronger ultrasound). Typical GIS leaks — a pinhole of 0.5 mm or a degraded flange gasket — are readily detectable with a 136-microphone array at working distances of a few meters to tens of meters. The SW136 with its 0.3–130 m detection range is designed for exactly this distance regime.

The Acoustic Signature of a Pressurized Gas Leak

Not every ultrasonic hotspot in a GIS hall is a gas leak. Corona discharge, partial discharge, and mechanical vibration all produce ultrasound too. The skill is telling them apart — and the spectrum display on a modern acoustic camera makes that possible:

• Gas leak: broadband, continuous hissing noise, strongest when the gas is flowing, present regardless of whether the equipment is energized. It does not pulse with the power frequency. Leak sound is usually white-noise-like across 20–100 kHz with energy spread evenly.
• Corona / PD: pulsed crackle synchronized with the 50/60 Hz cycle (100/120 Hz envelope), concentrated in discrete bursts. This signature is the same one used for partial discharge localization on switchgear.
• Mechanical: tonal or rattling noise from fans, pumps, and loose hardware, generally strongest at low frequencies with discrete harmonics rather than broadband hiss.

The practical rule: a continuous, non-pulsing broadband hiss pinned to a flange, valve, or bushing is a gas leak until proven otherwise. The GSW Series shows the live spectrum alongside the acoustic image, so the operator can confirm the white-noise character of a leak in seconds and distinguish it from PD crackle — an important distinction in GIS, where both problems can exist in the same hall.

Where GIS Leaks: Flanges, Valves and Density Monitors

GIS compartments are sealed, but the seal is made of hundreds of discrete joints. Experienced leak teams know the risk points by heart:

• Flange gaskets between compartment sections — the most common leak location, especially on older O-ring designs exposed to temperature cycling and vibration.
• Filling valves and Schrader valves — valve seats degrade, and dust on the seat prevents a full seal.
• Density monitor / pressure gauge connections — mechanical joints with small sealing areas that are easy to over- or under-torque.
• Bushings and instrument transformer (CT/VT) enclosures — cast-resin and porcelain interfaces see thermal expansion and can crack or loosen.
• Weld seams and castings — pinholes in aluminum castings or poor repair welds leak slowly and are the hardest to find with sniffer methods.
• Gas compartment tie-in points for the SF6 filling skid and gas monitoring systems.

An efficient survey scans these points in order, then sweeps the visible surfaces of each compartment. Because acoustic detection works at a distance, the survey team does not need to climb onto the equipment or reach behind energized busbars — a genuine safety advantage over sniffers and soap-bubble tests.

Acoustic vs Optical Gas Imaging: Complementary, Not Competing

Optical gas imaging (OGI) cameras visualize SF6 directly by detecting its infrared absorption signature, and they have become the industry standard for leak repair verification. They are excellent tools — and they pair naturally with acoustic detection rather than replacing it. Each method has a different sweet spot:

The efficient workflow used by most utilities: acoustic camera screens the whole hall quickly and flags candidate leaks; OGI confirms the suspect joints and supports repair records; sniffer measures the rate. The acoustic step is what makes the overall survey fast — and it is the only one of the three that can be done safely at distance on an energized GIS.

Choosing an Acoustic Camera for SF6 Surveys

For GIS halls where the camera must reach between racks and across compartments, the SW136 is the workhorse: its 0.3–130 m range covers any indoor GIS layout, and the 100 kHz upper bandwidth captures the full leak spectrum. Teams that survey the whole substation in one pass prefer the GSW Series, which combines the acoustic array with a 640×512 thermal imager so a single survey finds SF6 leaks, PD, and hot connections. For smaller distribution installations, the FA611S delivers the same capability in a lighter tablet form factor.

A Field Workflow for SF6 Leak Surveys

1. Check gas records first. Review density-monitor alarms and top-up logs for the last 12 months — compartments that needed frequent topping are the highest-probability leaks.
2. Set up at a safe distance. Position the camera on the walkway or opposite the compartment. Acoustic detection does not require proximity to energized parts.
3. Screen the risk points. Scan flanges, valves, density monitors, and bushing interfaces in order, holding each point for a few seconds to let the array localize.
4. Localize with the array. When a hotspot appears, move the camera until the source is centered; watch it stabilize on the optical image.
5. Confirm with the spectrum. Verify the broadband white-noise character and absence of 100/120 Hz pulsing that would indicate PD instead.
6. Record and tag. Save the image with GPS location, gas compartment name, and date — building a leak map of the GIS hall over time.
7. Hand off to repair. Schedule OGI confirmation and sniffer measurement for the flagged joints; prioritize by leak size and compartment criticality.

Repeat the survey quarterly in halls with known leakage history, or annually where the fleet is stable. The images become a baseline: a joint that was quiet last year and hisses this year is a trend you caught early.

SF6 Compliance in Latin America: What Utilities Face

Latin American utilities operate some of the most modern GIS fleets in the world — large urban 220–500 kV networks in Brazil, Mexico, Colombia, and Chile are dominated by gas-insulated technology. That means a large installed base of SF6 compartments, and a growing regulatory pressure to account for every kilogram of gas. Regional utilities increasingly report SF6 inventories and emissions under national greenhouse-gas reporting frameworks, and many state-owned operators now tie maintenance budgets to leakage-reduction targets.

The practical challenge is the same everywhere: leak detection crews are small, and sniffer-based surveys of a large GIS hall are slow and labor-intensive. An acoustic camera changes the economics — one technician can screen an entire GIS hall in a morning and produce a GPS-tagged leak map that the repair team acts on the same week. That speed matters in hot climates, where temperature cycling accelerates gasket wear, and in coastal substations, where salt and humidity attack sealing surfaces.

Field rule: if your GIS hall topped up any compartment more than once in the past year, you have an active leak — find it before the density monitor alarms, not after.

The Bottom Line

SF6 leaks are costly, dangerous to the environment, and — until recently — frustratingly hard to find. Acoustic cameras change that: they detect the ultrasonic hiss of escaping gas at a safe distance, on energized equipment, without an outage. The GSW Series, SW136, and FA611S from Unitech Tools give a maintenance team the screening speed of a full GIS-hall sweep and the precision to pinpoint a single flange gasket. Add an acoustic SF6 survey to your routine, and the leak that has been costing you top-ups for years becomes a photo, a GPS tag, and a work order.