Fire Alarm Grades and Categories: What They Mean and How They Differ

A grade describes how a domestic fire alarm system is built and powered. It covers questions such as whether the alarms are standalone or connected to control equipment, whether they run on the mains or on batteries, and what kind of backup supply they have.

Grades come from BS 5839-6, the British Standard for fire detection and fire alarm systems in domestic premises, which covers homes occupied by a single person or family and sheltered housing. A grade says nothing about which rooms are protected. It is about the equipment, not the coverage.

A category describes the protection a system is intended to give: what it is there to protect, and how much of the building it covers. It is a statement of purpose and extent rather than of equipment.

Homes and non-domestic premises use different sets of categories, set out in different standards. A category says nothing about how the system is powered or what kind of equipment it uses.

The simplest way to separate them is to ask two different questions. How is the system built and powered? That is the grade. What is it intended to protect, and where? That is the category.

For a home, the two are used together. Government guidance for dwellings in England, Approved Document B, describes a system by naming a grade and a category side by side, in the form “Grade A, Category LD2”. Two systems can share a grade but cover different parts of a home, and two systems can share a category while using different equipment.

In BS 5839-6:2019+A1:2020, the grades in use describe the following arrangements. These are summaries in plain English, not the standard’s own wording.

• Grade A: detectors and sounders connected to central control and indicating equipment, with a backup power supply.
• Grade C: separate mains-powered detectors and sounders connected to central control equipment, with backup power.
• Grade D1 and D2: mains-powered alarms, each with its own standby battery. In D1 the standby battery is sealed against tampering; in D2 it can be replaced by the user.
• Grade F1 and F2: battery-powered alarms only. In F1 the battery is sealed against tampering; in F2 it can be replaced by the user.

For homes, BS 5839-6 uses three categories to describe how much of the property is covered by detection:

• LD1: detection throughout the home, including the escape routes and the rooms where a fire could start.
• LD2: detection in the escape routes plus the rooms with the highest fire risk.
• LD3: detection in the escape routes only.

These describe extent of coverage. They do not say which category is appropriate for a particular home, which depends on the property.

Non-domestic premises, such as offices, shops and other workplaces, are covered by BS 5839-1, the code of practice for non-domestic premises. Its current edition is BS 5839-1:2025, which replaced the 2017 edition. It uses categories, and not grades, to describe a system’s purpose.

• M: a manual system, which relies on people to raise the alarm using manual call points rather than on automatic detection.
• L (L1 to L5): systems intended to protect life. L1 to L4 describe a decreasing extent of automatic detection, from the whole building down to escape routes only, and L5 is for systems designed around a specific objective.
• P (P1 and P2): systems intended to protect property. P1 covers the premises as a whole and P2 covers defined parts of it.

This is a summary of purpose only. The detail of each category sits in the standard and can differ between editions, so it should be read from the current edition rather than from a summary like this one.

No grade or category is the right one in general. What suits a building depends on what it is used for, who occupies it, how it is laid out and what the fire risks are. A fire risk assessment of the premises is the starting point for deciding what a system needs to do, and the grade or category terminology is how that decision is then described.

That is also why a summary like this cannot tell you what a particular home or business needs. It can only explain what the terms mean when you meet them.

The final specification of a fire alarm system is specific to the site. It is worked out from the premises, the assessment of its risks and the relevant standard, by someone competent to design it. Reading about grades and categories helps you follow that discussion and ask better questions, but it does not replace it.

A grade describes how a domestic system is built and powered, and a category describes what a system is intended to protect. Homes use both. Non-domestic premises use categories. Mixing the terms up is the most common source of confusion.

If you are planning a system for business premises, see our commercial fire alarm installation services, or get in touch to talk through the system appropriate for the premises.

This guide summarises the following, which should be read directly for the detail of the standards.