3.3 Fire detection and alarm systems and procedures Flashcards
A manual alarm system
consists of break-glass units and alarm sounders connected to a control panel, and can only be activated by an individual operating a break glass unit after detecting a fire.
An automatic system
adds smoke and heat detectors to the above, and is designed to raise the alarm to give early warning, whether or not people are present.
BS 5839-1 specifies three categories of system:
M Manual alarm system L Life protection P Property protection
The emphasis of a category L system is
to ensure people are quickly and safely evacuated. This is a ‘people out’ approach.
The emphasis of a category P system is
Category P systems require an effective interface with fire extinguishing systems, as property protection is based on a ‘fire out’ approach.
Information from a fire detector signal is sent to and processed by the fire alarm control panel. Fire detection systems are further classified according to the way the information is sent and processed as either:
conventional monitored systems addressable systems (including addressable analogue).
In a conventional monitored system
the detection points (either smoke or heat) are wired in radial circuits from the control panel. At the end of each circuit, a resistor or semiconductor device creates a known resistance across the circuit, providing a steady-state reference. If a detector is activated by a fire, the steady-state resistance of the circuit to which it is connected will be altered, and the fire alarm will be raised. As each radial circuit from the control panel will have a number of detectors connected to it, identification of the location of a fire is limited to the affected circuit (or Zone).
In an addressable system
detectors are connected in loops rather than radial circuits, and each detector is allocated a unique identification ‘address’. Addressable analogue systems use detectors that constantly relay information on their operating condition to the control panel.
Fire detectors are designed to detect one or more of the four characteristics of fire, namely:
heat smoke combustion gas (such as carbon monoxide) infra-red or ultraviolet radiation.
Point smoke detectors utilise one (or both) of two principles:
Ionisation chamber smoke detectors Optical smoke detectors
(a) In relation to automatic fire detection and alarm systems, outline the basic principles of operation of: (i) heat detectors (ii) smoke detectors 4 marks 4 marks
(a) (i) Heat detectors: A fixed rate heat detector has a sensing element fixed at a particular temperature. By means of a thermocouple, it detects when the predetermined temperature is reached. A rate of rise heat detector detects abnormally fast temperature rises by means of electronic resistors. (ii) Smoke detectors: Smoke detectors are of two main types: ionising and optical or photoelectric. The chamber of an ionising smoke detector has two plates set apart, one charged negatively and the other positively. A small radioactive source is used to ionise the chamber into which smoke may enter in the event of fire. The detector reacts to the change in electrical current caused by the neutralisation of ions by the smoke particles. An optical smoke detector responds either to the obscuration or scattering of focused light rays by smoke particles.
(b) Identify the circumstances in which EACH type of detector would be appropriate. 2 marks
(b) A smoke detector would be inappropriate in situations where smoke, steam or vapour might normally be expected, such as in kitchens. A heat detector might be considered inappropriate in situations where sudden temperature changes might be experienced in normal circumstances or in situations where slow burning materials are involved which would have an effect on the speed of response.
A warehouse that stores stationery products generated six false fire alarms over a three month period while in the process of expanding its premises. On each occasion, the local Fire and Rescue Authority attended the premises. After the last occasion, the Fire and Rescue Authority inspected the warehouse and discovered that the employees had failed to evacuate on all but the first occasion. They also discovered that no testing or maintenance had been carried out on the fire alarm system for five years. Identify the possible causes of the false alarms. 6 marks Identify the actions the warehouse company should take to ensure their employees respond appropriately to fire alarms. 4 marks
(b) Possible cause of the false alarms include: faults due to corrosion wiring defects the wrong choice of detector heads or the wrong positioning of detectors or call points a failure to isolate a zone in the vicinity of hot work dust from maintenance or product spillage activating an optical detector clandestine smoking or horseplay. (c) To ensure appropriate response to the fire alarm, an important primary action to be taken would be to minimise the number of false alarms and the consequent ‘alarm fatigue’. Following this, the employer would need to provide the employees with a comprehensive programme of training, backed up by the use of reminders such as pocket card campaigns and posters. This could be coupled with: the use of a fire warden evacuation system a programme of fire drills where senior management are seen to play a leading part the use of an incentive scheme to reward the department with the best time for evacuation disciplinary action in the case of persistent offenders.
