What Is a Fire Alarm System and How Does It Work?

What Is a Fire Alarm System?

A fire alarm system is a network of devices that detect the early signs of fire — smoke, heat, or carbon monoxide — and alert the occupants of a building so they can evacuate quickly and safely. In most cases, the system also alerts a monitoring centre or the fire brigade directly.

Fire alarm systems range from simple standalone smoke detectors in a domestic kitchen to complex, addressable panel systems covering a multi-storey commercial building. Whatever the size of the property, the goal is the same: give people the maximum possible time to get out before a fire becomes life-threatening.

The Main Components of a Fire Alarm System

Most fire alarm systems share the same core building blocks, even if the technology inside them varies considerably.

• Detectors — Smoke detectors, heat detectors, and multi-sensors. These are the 'eyes and nose' of the system, constantly monitoring the air for signs of combustion.
• Sounders and strobes — Audible alarms and visual alerts (flashing lights) that warn occupants when a detector activates.
• Control panel — The brain of the system. It receives signals from every detector and triggers the appropriate response. On addressable systems, it can pinpoint exactly which device has activated.
• Manual call points — Red break-glass units positioned at exits. Any person who spots a fire can trigger the alarm manually.
• Power supply — Mains electricity plus a backup battery, ensuring the system keeps working during a power cut.

Types of Fire Detector

Not all detectors work the same way, and choosing the wrong type for a location is a common mistake. Here's a quick breakdown:

• Optical smoke detectors — Use an infrared light beam inside a sensing chamber. Ideal for living rooms, hallways, and bedrooms where slow-burning, smouldering fires are most likely. These are the most common type in UK homes.
• Ionisation smoke detectors — Contain a tiny amount of radioactive material that ionises the air inside a sensing chamber. Fast-flaming fires (such as burning paper) disrupt the current and trigger the alarm. Less common in modern installations due to false alarm sensitivity.
• Heat detectors — Respond to a rise in temperature rather than smoke. Perfect for kitchens, garages, and lofts where cooking fumes or dust would trigger a smoke detector. They respond more slowly than smoke detectors but are far more reliable in those locations.
• Multi-sensor detectors — Combine optical and heat sensing for greater accuracy and fewer false alarms. Increasingly popular in homes and offices.
• Carbon monoxide detectors — Specifically detect CO gas, which is produced by faulty boilers, gas cookers, and open fires. Often integrated into combined fire and CO alarm systems.

Conventional vs Addressable Systems

For larger buildings, fire alarm systems come in two main architectures: conventional and addressable.

A conventional system divides a building into zones. When a detector triggers, the panel identifies which zone it's in — but not the exact device. These systems are simpler and less expensive, and are suitable for smaller commercial premises.

An addressable (or 'intelligent') system assigns a unique electronic address to every detector and device. When anything activates, the panel shows the exact location on a map or display. This makes it far easier to respond quickly and reduces the risk of unnecessary full evacuations. Addressable systems are required in larger commercial buildings and are increasingly used in medium-sized premises.

What Does BS 5839 Mean?

BS 5839 is the British Standard for fire detection and alarm systems. It sets out the design, installation, commissioning, and maintenance requirements for both commercial systems (Part 1) and domestic systems (Part 6). Any reputable fire alarm company in London should be working to BS 5839 as a minimum.

The standard also defines Categories and Grades that describe how much of a building is protected and how the system is powered. For example, a Grade D, Category LD2 system — the most commonly required in HMOs — consists of mains-powered interlinked alarms with a battery backup, covering escape routes and high-risk rooms.

Do I Need a Professional Fire Alarm Installer?

For domestic properties, you can legally buy and fit standalone smoke alarms yourself. However, for rented properties (especially HMOs), commercial premises, or any system requiring a certificated panel, you must use a qualified fire alarm engineer. Your insurer may also require proof of professional installation.

At Security Red Alert, all our fire alarm engineers are fully qualified, DBS-checked, and experienced across both domestic and commercial installations throughout London. We provide full certification on completion and offer ongoing servicing contracts to keep your system compliant.