Phoenix Journal · Extraction
A fusible link, a fire damper and an interlock each do a distinct job around a commercial kitchen extract system. Here is what separates them - and where each belongs.
Fire safety devices explained
Three small components do a great deal of quiet work in and around a commercial kitchen extract system, and they are easy to confuse - so here is what a fusible link, a fire damper and an interlock each actually do.
If you look after a busy kitchen, you will have seen these terms on service reports, insurance schedules and Gas Safe paperwork without always being told how they relate to one another. They sound interchangeable. They are not. A fusible link is a heat-sensitive trigger. A fire damper is a barrier that a trigger closes. An interlock is a control link that stops one system running unless another is proven. Get the distinction clear and the rest of your compliance picture - testing intervals, grease cleaning, gas safety - falls into place much more easily.
This piece keeps the definitions plain and points out the one thing that trips up most kitchen operators: where each device belongs, and where it must never be fitted.
Start with the simplest. A fusible link is a mechanical fuse for heat rather than electricity. Two metal tabs are joined with a soft solder that melts at a set temperature - most commonly 72°C under BS EN 15650, the standard that governs fire dampers for building ventilation. Below that point the link holds firm and keeps something under spring tension in its open position. Reach the rated temperature and the solder gives way, the two halves separate, and whatever the link was restraining is released. It is a one-shot device. Once it has parted, it is replaced, not reset.
A fire damper is the barrier that link releases. It is a steel blade or curtain built into ductwork where the duct passes through a fire-rated wall or floor. In normal running the blade sits open and air flows through freely. When the fusible link fuses, a spring snaps the blade shut and seals the opening, so fire and smoke cannot travel along the duct from one fire compartment into the next. Under BS EN 15650 dampers are classified for integrity (E), insulation (I) and smoke leakage (S) to EN 13501-3, and a typical specification calls for an ES rating of at least 60 minutes. The whole point is to preserve compartmentation - the invisible grid of fire-resisting boxes that a building is divided into so a fire stays where it starts. That relationship between the ventilation network and the building's fire compartments is worth understanding in its own right, and we cover the link between ventilation and fire compartmentation separately.
An interlock is not a fire barrier at all - it is a control condition. In a commercial kitchen the one that matters is the gas interlock required under BS 6173:2020 and IGEM/UP/19. The rule is straightforward: gas-fired catering appliances must not run unless mechanical extraction is proven to be working. The interlock watches the ventilation - usually through a differential air pressure switch that senses the pressure difference across the extract fan - and only then allows the gas solenoid valve to open. Lose airflow, a fan fault, a tripped isolator or a belt failure, and the interlock closes the gas within moments. Many systems add gas proving too, checking the pipework for leaks at start-up before the valve is allowed to open. The purpose is to stop combustion products such as CO₂ and, more dangerously, carbon monoxide building up in a space with no working extraction.
So the three sit in a chain of cause and effect. Heat melts the fusible link; the released link lets the fire damper close; the interlock, entirely separately, keeps gas and ventilation tied together so the kitchen cannot run unventilated in the first place.
The figures that matter
Three thresholds capture how these devices are rated and how often they must be checked in a kitchen setting.
Here is the point that surprises many operators. A fire damper with a fusible link is exactly the wrong device to fit inside a greasy kitchen extract duct - and both Approved Document B and BS 9999 reflect that. Two problems make it unworkable. First, grease deposits coat the blade and its mechanism, so when the link finally fuses the damper cannot close cleanly and fails at the moment it is needed. Second, the heat and grease-laden air in an extract duct can fuse a 72°C link prematurely or unpredictably, shutting the duct during ordinary service. For those reasons fusible-link fire dampers are generally not permitted within kitchen extract ductwork itself. They belong in the wider ventilation and supply-air network, at the fire-compartment lines, not in the grease stream.
That leaves the grease duct's fire risk to be managed a different way - by keeping it clean. TR19® Grease, the BESA specification insurers now lean on, sets minimum cleaning frequencies by usage: every three months for heavy use of 12 to 16 hours a day, every six months for moderate use, and every twelve months for light use. A clean duct removes the fuel that turns a small flare-up into a duct fire, which is the protection a damper cannot reliably give you here.
The fire dampers you do have - on the supply side and at compartment penetrations - carry a legal testing duty. Under BS 9999 a competent person must drop-test every damper on completion of installation and at least annually thereafter. In dust-laden or high-risk settings, and commercial kitchens are named among them, the interval typically shortens to every six months. A drop test is simple in principle: the engineer accesses the damper, releases the fusible link or actuator, confirms the blade travels fully closed and seals, then resets it and logs the result. The Regulatory Reform (Fire Safety) Order 2005 makes this the responsible person's duty, and the Building Safety Act 2022 has sharpened accountability further - untested dampers can invalidate insurance and attract enforcement. Gas interlocks sit under their own regime and are checked as part of Gas Safe servicing, but the principle is the same: a device is only a safeguard if it has been proven to work. If a damper will not reset or has seized, that is a fault to act on, not to note and move past - we explain the usual causes in why is my fire damper stuck.
Put together, the maintenance picture is coherent. Fusible links and dampers are drop-tested on a schedule; the grease duct is cleaned to TR19® because a damper cannot protect it; and the gas interlock is proven at each gas service. Miss any one and the others cannot cover for it.
Questions
The most common rating is 72°C, in line with BS EN 15650 for ventilation fire dampers. A soft solder joining two metal tabs melts at that point, releasing the spring that snaps the damper blade shut. Higher-rated links exist for warmer applications, but 72°C is the standard you will see most often. Once a link has fused it is replaced, never reset.
Generally no. Both Approved Document B and BS 9999 steer against fire dampers in kitchen extract ductwork because grease coats the mechanism and stops it closing when needed, while the hot, greasy air can also fuse the link unexpectedly during normal service. Fire dampers belong in the wider supply and ventilation network at fire-compartment lines. The grease duct's fire risk is instead managed by regular TR19® cleaning.
Under BS 9999 a competent person must drop-test every damper on completion of installation and at least once a year afterwards. In dust-laden or high-risk settings, which include commercial kitchens, the interval typically drops to every six months. The test releases the link or actuator, confirms the blade fully closes and seals, then resets and logs it. Under the Fire Safety Order 2005 this is the responsible person's legal duty.
A gas interlock is a control condition, not a fire barrier. Required under BS 6173 and IGEM/UP/19, it stops gas catering appliances running unless mechanical extraction is proven, usually via a differential air pressure switch across the extract fan. If airflow is lost, the gas solenoid valve closes within moments. A fire damper, by contrast, is a physical steel barrier that seals a duct when heat triggers its fusible link.
Phoenix Duct Clean · by the numbers
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