35.1 Commentary

In principle, measures to limit false alarms and unwanted fire alarm signals can be divided into eight groups:

Siting and selection of manual call points;
Selection and siting of automatic fire detectors;
Selection of system type;
Protection against electromagnetic interference;
Performance monitoring of newly commissioned systems;
Filtering measures;
System management; and
Regular servicing and maintenance.

35.2 Recommendations

35.2.1 General

● Responsibility:

Applicable to those involved in specification, design, commissioning, verification, and maintenance.

● Nature:

Serve as general guidelines, not definitive rules.

35.2.2 Recommendations for siting and selection of manual call points

The following recommendations are applicable.

a) Protective Covers:

Recommended for new and existing systems with frequent unwanted operation, though not mandatory for existing systems.

b) Siting:

Avoid locations prone to accidental damage; use guards or protective covers if unavoidable.

c) Water Resistance:

Use water-resistant manual call points in moist areas (e.g., external locations, wet industrial areas).

d) Malicious Operation Prevention:

Avoid locating manual call points in accessible public areas or use restricted access and surveillance in such locations (e.g., shopping malls, public houses).

NOTE 1 Examples include external locations, wet areas of industrial buildings, food-processing areas that are subject to periodic washing down and certain kitchens.

NOTE 2 In public car parks, consideration can be given to the use of a suitable emergency voice communication system in lieu of manual call points [see 20.2k)].

35.2.3 Recommendations for selection and siting of automatic fire detectors

a) Aspirating Smoke Detectors: Preferable in dusty environments; ensure filters are certified and maintain them regularly.

b) Necessity Check: Confirm if automatic fire detection is necessary in high unwanted alarm areas to meet system objectives.

c) Heat Detectors: Consider heat detectors first for lower unwanted alarm rates before using other types; ensure they meet system objectives.

d) Heat Detector Temperature: Minimum response temperature should be 29°C above ambient for long-term and 4°C above maximum short-term temperatures.

NOTE 1 BS EN 54-5 specifies six classes of heat detector for use in high ambient temperatures (between 40 °C and 140 °C).

e) Rate of Rise Heat Detectors: Avoid in areas with rapid temperature changes, such as kitchens or loading bays.

f) Detector Guidance: Follow Annex E when selecting point and optical beam smoke detectors or their principles.

g) Optical Beam Detectors: Avoid in areas prone to beam obstruction.

h) Installation: Ensure proper mounting of optical beam detectors.

i) Multi-Sensor Detectors: Can reduce unwanted alarms, but effectiveness depends on detector characteristics and alarm causes.

j) Aspirating Smoke Detection Sensitivity: Be cautious with high sensitivity systems to avoid excessive unwanted alarms; consider using as pre-alarm or early warning.

NOTE 2 Such systems are often used to provide signals intended to be managed as a “pre-alarm warning”(to indicate that an alarm condition is approaching) or as a very early warning (to alert staff of abnormal conditions that are worthy of investigation). Such signals might not be suitable (or intended) to be managed as an alarm signal that triggers alarm devices and evacuation of the building. However, such pre-alarm warnings or very early warning signals can be used to avoid false alarms, unnecessary evacuations and unwanted fire alarm signals to the fire and rescue service. In some applications in which Class A and/or Class B aspirating smoke detection systems are used, a Class C (normal sensitivity) signal is provided which is managed as an alarm condition.

k) Carbon Monoxide Detectors: Avoid in areas with normal carbon monoxide presence, like poorly ventilated kitchens or laboratories.

NOTE 3 Such areas include badly ventilated kitchens, some laboratories and areas in which vehicle or other exhaust fumes occur.

l) Contamination:

Avoid use in locations where CO detectors may be contaminated; follow manufacturer’s guidance.

m) Flame Detectors:

Avoid in areas with non-fire infrared or ultraviolet sources; consult manufacturers for detector sensitivity guidance.

35.2.4 Recommendations for selection of system type

The following recommendations are applicable.

a) Smoke Detectors: Use analogue systems in installations with a high number of smoke detectors unless there are overriding considerations.

NOTE 1 Analogue smoke detection systems are likely to offer better immunity to false alarms than conventional two-state fire detection systems, particularly if signal processing incorporates techniques specifically intended to discriminate between certain unwanted alarms and real fires. Some two-state fire detection systems also employ such signal processing. Many analogue systems also provide a pre-alarm warning (see 3.45), which enables investigation of conditions that, if no action is taken, would lead to an unwanted alarm.

NOTE 2 A high number of smoke detectors might be regarded as more than 100 detectors.

b) High Number of Detectors:

For systems with many automatic fire detectors (excluding heat detectors), consider multi-sensor detectors and measures to minimize unwanted alarms during the design stage.

NOTE 3 At the time of publication of this edition of BS 5839-1, a very high number of detectors might be regarded as more than 1 000 detectors.

35.2.5 Recommendations for protection against electromagnetic interference

The following recommendations are applicable.

a) Minimize Interference:

Follow Clause 28 recommendations to reduce the risk of electromagnetic interference causing unwanted alarms.

b) Design Considerations:

Account for potential sources of electromagnetic radiation, such as mobile phones and high-power transmitters.

c) Special Cases:

For areas with high electromagnetic fields (e.g., radio sites, airports), consult the system manufacturer for suitability and special measures to reduce unwanted alarms. Provide field strength information for existing buildings.

35.2.1 Recommendations for performance monitoring of newly commissioned systems

The following recommendations are applicable.

a) Soak Period:

For systems with over 50 automatic fire detectors, a soak period of at least one week should follow commissioning, defined by the designer. Connection to an ARC should be disabled during this period.

b) Operational Status:

The system should not be considered operational until the soak test is successfully completed. Manual call points should indicate they are not to be used during this time.

c) Completion Criteria:

The soak test is deemed successful if:

No false alarms occur during the period, or
Any false alarms are investigated, causes identified, and measures are taken to prevent recurrence.

NOTE A purchasing specification might require that, on completion of any relevant measures, a further soak test be carried out.

35.2.7 Filtering measures

35.2.7.1 Commentary

1. Design Considerations:

At the design stage or after unsatisfactory performance, measures may be needed to prevent continuous evacuations or calls to the fire service due to false alarms. (see Clause 34)

2. Time-Related Systems:

Automatic Disabling: Automatic fire detectors may be disabled or their sensitivity reduced during times when false alarms are likely due to environmental conditions. This creates a system that alternates between Category M (manual detection) and Category P (automatic detection).
Sensitivity Adjustment:

Sensitivity of detectors can be reduced at certain times, maintaining some level of protection.

3. Staff Alarm Arrangements:

Investigation Period: Allows time to investigate an alarm before triggering fire alarm sounders or summoning the fire service. This can be applied all the time or during specific periods.
Safeguards: Ensures early response to a real fire, including full alarms for manual call points or heat-activated devices, and coincidence detection to cancel delays if multiple detectors activate.

4. Filtering for Fire Service Summoning:

Delayed Summoning: Automatic calling of the fire service can be delayed until after an investigation, though building evacuation should occur immediately. See also Clause 19.

35.2.7.2 Recommendations

The following recommendations are applicable.

1. Adoption Criteria:

● Category P Systems: If an investigation period is planned before calling the fire service, consult insurers.

● High False Alarm Rates: Use filtering if false alarms cannot be controlled by other means.

● System Objectives: Ensure filtering does not compromise life, property, business continuity, or environmental protection.

2. Exclusions:

● Manual Call Points, Heat Detectors, and Sprinklers: Filtering should not apply to signals from these sources.

3. Time-Related Systems:

● Automatic Changes: System should switch to unmanned mode automatically and to manned mode manually.

● Control Options: Can use manual controls or automatic systems; linking to intruder alarms is a better approach.

4. Staff Alarms:

● Conform to Guidelines: Must follow specific guidelines for staff alarms (refer to Clause 19.2.2).

● Immediate Fire Service Notification: Fire service should be called immediately if the investigation period ends without confirming the alarm is false.

5. ARC Filtering:

● Compliance with BS 8591: Filtering of fire service summoning should follow BS 8591 standards and not apply in residential care premises.

6. Sprinkler Systems:

● Coordination Required: Work with sprinkler system installers to prevent false alarms due to water pressure changes; consider time delays for pressure surges.

35.2.7.3 Recommendations for system management

The following recommendations are applicable.

1. Awareness and Communication:

● Ensure building staff and visiting contractors are informed about automatic fire detectors and measures to minimize false alarms.

2. Contractor Responsibilities:

● Tender documents should require contractors to notify their staff about fire detectors and necessary precautions.

3. Temporary Work Precautions:

● For work generating dust or smoke:

a) Permit System: Manage work through a permit to work system.

b) Detector Replacement: Temporarily replace smoke detectors if needed.

c) Screens/Covers: Use screens or temporary covers to protect detectors.

d) Disablement: Disable detectors and communication with ARCs if necessary; ensure proper reinstatement afterward.

NOTE 1 One method of managing such work would be the operation of a permit to work system. Other suitable measures may include the following:

a) temporary replacement of smoke detectors by other types of detector that are less susceptible to the effects of the pollutants in question;

b) provision of a temporary screen between the work and the detectors. The flammability of the screen ought to be considered to ensure that it does not increase the fire risk.;

c) provision of brightly coloured temporary covers over detectors to prevent the ingress of contamination;

d) disablement of automatic fire detectors. On completion of the work, the premises management (see Clause47) should ensure that proper reinstatement of the protection occurs;

e) disablement of communication with any ARC. On completion of the work, the premises management (see Clause 47) should ensure that proper reinstatement of the monitoring occurs.

NOTE 2 Unless the work involving generation of dust etc. is to be carried out throughout a large part of the building, the measures described in c)4) are preferable, as these still permit remote monitoring of fire alarm signals from areas where work is not being carried out.

4. Routine Testing and Maintenance:

● Notify building occupants and ARCs before testing or maintenance that might trigger alarms.

5. Address Building and Environmental Issues:

● Fix defects and issues (e.g., leaks) that could cause false alarms

6. Recording and Action:

● Record all false alarms in the system logbook and take appropriate actions to address them.

35.2.7.4 Recommendations for service and maintenance

Service and maintenance on the system should be carried out by a competent organization in accordance with Section 6 of this standard.

35 Measures to limit false alarms and prevent unwanted fire alarm signals