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Duct cleaning - the terms defined
Fusible links, fire dampers and interlocks come up whenever ductwork and fire safety are discussed - and they often get muddled together. They are three different things doing three different jobs. Defining each clearly - what it is, what it does, and how it differs from the others - clears up the confusion and shows how they fit into keeping a system safe.
The short answer
These three fire-safety terms describe different things. A fusible link is a heat-sensitive device that holds something in place until heat reaches a set temperature, then separates - releasing a fire damper to close, or shutting equipment down, when a fire raises the temperature. A fire damper is a barrier within ductwork that closes to stop fire and smoke spreading through the duct system between fire compartments - often held open by a fusible link and released to close by it. An interlock, in a kitchen context, is a system that links the gas supply to the ventilation, so gas cannot flow unless the extraction is running. So the fusible link triggers, the fire damper blocks, and the interlock links - three distinct roles.
Three terms, three jobs
Fusible links, fire dampers and interlocks are terms that circulate together whenever ventilation, ductwork and fire safety come up - and because they appear in the same contexts and relate to the same systems, they often get muddled. People are unsure which is which, or use the terms loosely, or assume they are variations of the same thing. In fact they are three quite different things, each with its own distinct function in keeping a system safe. Clearing up what each one actually is removes the confusion and makes it possible to talk about them - and understand their roles - accurately.
The value of getting them straight is that they do genuinely different jobs, and understanding a system's fire safety means knowing which does what. A fusible link, a fire damper and an interlock are not interchangeable; each addresses a different aspect of safety, and they relate to each other in specific ways. So rather than a vague sense that these are all fire-safety bits and pieces, it is worth having a clear definition of each - what it is, what it does, and how it differs from the others. With that, the terms stop being a confusing jumble and become an understandable set of distinct safety devices, each with its place.
Fusible link
A fusible link is, at its simplest, a heat-triggered release device. It is made so that at a set temperature - a temperature reached in a fire - part of it melts or separates, releasing whatever it was holding. The word fusible means capable of being melted, and that is exactly the principle: the link holds something in place under normal conditions, but when heat reaches its rated temperature, it fuses apart and lets go. It is a simple, reliable, automatic response to heat, needing no power or signal - the heat of a fire itself triggers it.
What a fusible link does in practice is trigger a safety action when a fire raises the temperature. Most commonly, it holds a fire damper open under normal conditions and releases it to close when a fire heats the link - so the fusible link is the trigger that makes the damper respond to fire. It can also be used to shut down equipment or trigger other safety responses on heat. So the fusible link is not itself a barrier or a control; it is the heat-sensitive trigger that activates one. Understanding it as a release device that responds to heat is the key to seeing how it relates to the fire damper - which is the thing it typically releases.
Fire damper
A fire damper is a barrier within ductwork that closes to stop fire and smoke spreading through the duct system. Buildings are divided into fire compartments - areas separated to contain a fire and stop it spreading - but ductwork passes through the walls between these compartments, creating a route by which fire and smoke could travel from one compartment to another through the ducts. A fire damper sits in the ductwork at these points, and its job is to close in a fire, blocking that route so the ductwork does not become a path for fire to spread. It preserves the compartmentation that the ducts would otherwise breach.
Under normal conditions the fire damper is held open so air flows through the ductwork as intended; in a fire, it closes to become a barrier. This is where the fusible link comes in: a fire damper is often held open by a fusible link, which releases it to close when the heat of a fire reaches the link. So the fusible link and the fire damper work together - the link is the heat-triggered release, the damper is the barrier it releases. Because a fire damper is a safety device that must work when needed, it requires periodic testing to confirm it will close - which is why fire damper testing is part of ductwork fire-safety maintenance. The fire damper, then, is the barrier that blocks fire's path through the ducts, triggered by the fusible link and confirmed by testing.
Interlock
An interlock is a different kind of thing entirely - not about fire spreading through ducts, but about linking two systems so one cannot operate unsafely without the other. In a commercial kitchen, the key example is the gas interlock, which links the gas supply to the ventilation system: it ensures that the gas appliances cannot be used unless the extraction ventilation is running. If the ventilation is not working, the interlock cuts or prevents the gas supply, so gas cooking cannot happen without the extraction that removes the combustion products and makes it safe. The interlock links the two, making the gas dependent on the ventilation.
The purpose of this is safety: gas cooking without adequate ventilation is dangerous, because it can lead to a build-up of combustion products, and the interlock prevents that by making the extraction a precondition of the gas. So the interlock is a safety link between systems, quite distinct from the fusible link and fire damper, which are about stopping fire spreading through ductwork. The interlock addresses a different hazard - unsafe gas use without ventilation - by a different mechanism - linking the two systems. This is why lumping the three terms together is a mistake: the fusible link and fire damper deal with fire in the ducts, while the interlock deals with safe gas operation, and the interlock is not part of the fire-damper story at all. Three terms, genuinely three different jobs.
The takeaway
Fusible links, fire dampers and interlocks are three different fire-safety devices that often get muddled, but each does a distinct job. A fusible link is a heat-triggered release: at a set temperature reached in a fire, it separates, releasing whatever it holds - typically a fire damper - or shutting equipment down. A fire damper is a barrier within ductwork that closes in a fire to stop fire and smoke spreading through the ducts between fire compartments, often held open by a fusible link and released to close by it, and requiring periodic testing to confirm it works.
An interlock is different again - in a kitchen, the gas interlock links the gas supply to the ventilation, so gas appliances cannot be used unless the extraction is running, preventing unsafe gas cooking without ventilation. So the fusible link triggers, the fire damper blocks, and the interlock links - three distinct roles, addressing different hazards by different mechanisms. The fusible link and fire damper work together against fire spreading through ducts; the interlock stands apart, ensuring safe gas operation. Defining each clearly is what turns a confusing jumble of terms into an understandable set of distinct safety devices.
Questions
A heat-triggered release device. At a set temperature reached in a fire, part of it melts or separates, releasing whatever it was holding - typically a fire damper to close, or shutting equipment down. It is a simple, automatic response to heat, needing no power or signal, because the heat of a fire itself triggers it.
A barrier within ductwork that closes in a fire to stop fire and smoke spreading through the duct system between fire compartments. Ductwork passes through compartment walls, creating a route for fire to travel; the fire damper closes to block that route, preserving the compartmentation the ducts would otherwise breach.
A fire damper is often held open by a fusible link under normal conditions, so air flows through the ductwork. When a fire heats the link to its rated temperature, it releases, letting the damper close. So the fusible link is the heat-triggered release, and the fire damper is the barrier it releases.
In a kitchen, a gas interlock links the gas supply to the ventilation, so the gas appliances cannot be used unless the extraction is running. If the ventilation is not working, the interlock cuts or prevents the gas, so gas cooking cannot happen without the extraction that makes it safe.
Because it addresses a different hazard by a different mechanism. The fusible link and fire damper deal with fire spreading through ductwork; the interlock deals with unsafe gas use without ventilation, by linking the gas to the extraction. It is not part of the fire-damper story at all - a genuinely separate safety device.
Yes. A fire damper is a safety device that must work when needed, so it requires periodic testing to confirm it will actually close in a fire. Fire damper testing is part of ductwork fire-safety maintenance, alongside the cleaning that controls grease build-up in the ducts.
Fire dampers in your ductwork need periodic testing to confirm they close - part of the ductwork fire-safety maintenance we can help keep on track alongside the cleaning.
