Fire systems and the first step in fire design

Comparison between conventional and analogue addressable systems

The conventional system

A conventional system incorporates detectors which are in one of two states: either on or off.

The detectors incorporate the criteria for measuring levels of fire phenomenon and, after reaching the levels defined within the standards, will switch from an Off state to an On state.

Once the On state is reached, a signal is passed to the CIE (Control and Indicating Equipment) which in turn switches ON the alarm sounders.

The analogue/addressable system

A typical Addressable system is wired with a series of loop circuits, which can either contain:

1. Input devices such as detectors and MCP's or
2. Input and output devices including sounders and strobes.

The detectors are usually Analogue type that are checked at short intervals(up to 10 seconds) on what condition they are in.

If the detectors are seeing a fire-like phenomenon and have reached a recognized threshold, the CIE will operate the alarm.

Which system should we choose?

General Conventional systems are used on smaller buildings where the evacuation plan is simple, that is "one out - all out", or total evacuation.

Addressable systems are generally more cost-effective to install in medium to large buildings and also have the additional benefits offered by the Analogue detectors and CIE.

Fire design - the first step

The relevant standard in Britain is BS 5839-1:2002 - the current British Standard for Fire Detection & Alarm Systems in Buildings. Before setting out to design a fire detection system, it is important to understand the responsibilities and steps involved in the design process.

The individual who picks up this role will be expected to signoff the G1 Design certificate and provide all the documentation that will form the foundation of a record that will remain in place for the life of the system.

Ideally, the designer should have some recognised competency in fire engineering, having either attended a recognized training course or have proven past experience.

The importance of design planning cannot be overstated. The purchaser or nominated designer should consult the following parties before the design is finalized:

• The building Control Officer;
• The Local Fire Officer;
• The Building Insurer;
• The Health & Safety Executive;
• The User of Facilities Management Team;
• The Architect;
• Consultant Engineers and/or Building Services Engineers;
• The System Installer.

After speaking with all the interested parties, the responsibilities of the designer will be to complete the following:

• Ensure all legal requirements are met;
• Clarify the level of protection that is required, ex: L1, L2, etc;
• Document the argument and list any "Variations";
• If needed, provide a separate Risk Assessment report;
• Complete the specification & drawings;
• Prepare a "Fire Plan" or "Cause and effect" matrix;
• If possible, witness the final test and handover to the client;
• Sign off the G1 Design certificate shown within BS 5839-1:2002;

Fire detection systems are installed for many reasons and purposes. They may be installed for the protection of life, or for the protection of property, or a mixture of both.

The British Standard encourages the building owner to carry out a Risk Assessment of the property to determine the level of protection required, which can be any of the following:

• Systems for Protecting Property(P) The primary objective in protecting property is to ensure the fire is detected at an early stage, in order for the fire fighters to take action and minimise the damage.
  • Category P2 A lower, but often adequate, standard allowing for automatic detection in specific rooms or areas, which have been highlighted in the Risk Assessment or by the buildings insurer;
  • Category P1 The highest level of property protection in which all areas of the building will be covered with automatic detection.
• Systems for Protecting life The objective of the fire alarm system is to warn occupants of a fire so that they can safely evacuate. In some instances, automatic detectors are not necessary, all that is required is manual "break glass" system with adequate alarm sounders. However, the majority of life protection categories consider the capability of the occupants to escape, the potential speed of the fire and structural protection afforded by the type of building. These categories are defined as follows:
  • Category M A system with no automatic detection for use in a building with no sleeping risk and where the fire can be detected at an early stage before escape routes are affected.
  • Category L5 A level of automatic detection to high risk rooms or much larger areas or buildings where none of the other life risk categories are adequate.
  • Category L4 Allows for the provision of automatic detectors within all escape routes, corridors, and stairways and any other area forming the common escape route.
  • Category L3 In addition to the coverage described in L4, detectors should be provided in all rooms that open onto the main escape route except rooms that open onto a corridor less than 4 metres in length and with fire resistance doors separating the area from other parts of the escape route.
  • Category L2 Protecting all areas detailed under L3 as well as all areas of high risk such as boiler rooms, etc.
  • Category L1 L1 provides automatic detection throughout the building.

To be continued, with the next steps in fire design: Steps 2 and 3, Step 4, and Steps 5, 6 and 7.