Fire Alarm System Classifications under BS5839

BS5839 Fire Alarm System Classifications

The British Standard splits fire alarm system designs into different classifications of system.  This article looks at the different types of fire alarm system classifications for Public, Commercial and Industrial buildings (a further article will be written to explain domestic system classifications).

BS5839 Splits fire alarm systems into 3 categories.  These 3 categories are Category P Systems, Category L Systems and Category M Systems.  Please find following a description of these categories.

All the systems described below are expected to have manual call points on all final exits, entrances to stairwells and corridors where persons are not expected to walk more than 45 metres to operate a call point.

Category P Fire Alarm Systems

BS5839 Category P Fire Alarm Systems are designed specifically for protection of property only.  Category P systems are split into 2 classifications P1 or P2.

The main objective of a Category P1 fire alarm system is to provide the earliest possible warning of a fire to minimise the time between ignition and the arrival of the fire fighters. 

The difference between a P1 and a P2 system is that a P1 system is designed to protect the whole building, whereas a P2 system is installed in defined parts of the building only.  These defined parts of the building may be areas with an extraordinary high fire risk or hazard.

BS5839 Category P Fire Alarm System Equipment

To create a category P fire alarm system it is possible to use either conventional, analogue addressable or wirelesss fire alarm system equipment.  We have an extensive range of all these types of systems available. 

For smaller installations a conventional / non-addressable fire alarm system may be more cost effective, whereas for larger installations an analogue addressable system would be more suitable.

In buildings where installations are difficult wireless fire alarm system equipment can lead to cost savings.  These systems can also be used for category P systems.

Category L Fire Alarm Systems

The main objective BS5839 Category L Fire Alarm System is life protection.  These systems are split into 5 classifications L1, L2, L3, L4 and L5.

BS5839 Classification L1 Explanation

A BS5839 L1 classification includes automatic fire detection in all rooms, on all escape routes and in all voids over 800mm in height.  Sounders positioned throughout the building to achieve a minimum of 65dB(A) throughout the building and 75dB(A) at bedhead where there is a sleeping risk.  In areas of high ambient noise sound levels the fire alarm sound levels should be 5dB(A) above the normal noise level although not exceeding 120dB(A).

BS5839 Classification L2 Explanation

A BS5839 Classification L2 System should include automatic fire detection on all escape routes and rooms leading onto escape routes.  An L2 system can also include additional areas deemed as a high risk not included in the escape routes and adjoining rooms, such as boiler houses.  The sounders in the building should be according to the description in the BS5839 Category L1 description as above.

BS5839 Classification L3 System Explanation

A BS5839 Classification L3 system is very similar to a category L2 system in that automatic fire detection should be positioned on escape routes and adjoining rooms, although it does not have to include for additional areas deemed to have a high fire risk.  The sounders in the building should be according to the description in the BS5839 Category L1 description as above.

BS5839 Classification L4 System Explanation

A BS5839 Classification L4 system includes automatic fire detection on escape routes only, and not in the adjoining rooms as per the L2 and L3 classifications.  The sounders in the building should be according to the description in the BS5839 Category L1 description as above.

BS5839 Classification L5 System Explanation

A BS5839 Classification L5 System is designed for buildings that have a particular fire risk identified which warrants some special attention.  For example if there is an area of high fire risk which is considered worthy of having some automatic detection but a manual system is also needed, then this will be termed as L5/M.  The sounders in the building should be according to the description in the BS5839 Category L1 description as above.

Category M Fire Alarm Systems

A BS5839 Category M Fire Alarm System is a manual operation only system which has call points on all exits as well as corridors where persons are not expected to walk any more than 45m to operate one.

Category L Fire Alarm System Equipment Available At The Safety Centre

Have you been informed that you require a BS5839 Category L Fire Alarm System?  If so, The Safety Centre has hundreds of products available suitable for creating BS5839 Category L Fire Alarm Systems.  We have products available to suit all budgets.  We have extensive ranges of equipment from many of the worlds leading fire alarm system manufacturers.  These ranges include fire alarm panels, detectors, manual call points, sounders and beacons all suitable for use on BS5839 Category L fire alarm systems.

BS5839 Part 6 2004 - Fire Alarm System Installations in Dwellings

BS5839 Part 6 2004 - Fire Alarm System Installations in Dwellings

The British Standard Associated with Fire Alarm System Installations in Dwellings is BS5839 Part 6 2004.  This part of the British Standard is split up into 3 categories of systems.  These categories are as follows:-

• LD1 - Alarms in all circulation spaces that form part of escape routes and all areas where a fire might start, but not bathrooms, shower rooms or toilets
• LD2 - Alarms in all circulation spaces that form part of escape routes and rooms or areas that present a high fire risk
• LD3 - Alarms in circulation spaces that form part of escape routes

The category of system that should be installed in a building is often defined by Councils, Insurance Companies, Fire Officers, Building Control and many others.

The above categories indicate the category of system that should be installed.  Further to this information different grades of system may be specified.  

These are as follows:-

• Grade A - A full system with control and indicating equipment installed to BS 5839: Part 1
• Grade B - Detectors and sounders using simpler specified equipment
• Grade C - Detectors and sounders or alarms with central control
• Grade D - Mains powered alarms with an integral stand-by power supply
• Grade E - Mains powered alarms with no stand-by power supply
• Grade F - Battery powered alarms

These grades are used in conjunction with the category type, to indicate what type of system should be installed within the property.  For example if an LD2 Grade A system is requested, this would require a system with alarms in circulation areas that form escape routes and high fire risk areas with the equipment with control and indicating equipment compliant to BS5839 Part 1.

Beam Detectors Installation process

Beam Detectors are a well-established product available in the fire alarm industry.  Commonly beam detectors are used to provide automatic fire detection in large open spaces, such as warehouses, airports and factories.

A beam detector works by having a transmitter and receiver.  The transmitter part of the beam detector fires out an infrared beam which is then received by the receiver.  The beam detector is designed to set a fire alarm system into alarm when a pre-set level of smoke obscures the infrared beam.  Beam detectors are available in two types, these are reflective beam detectors and point to point type beam detectors.  A reflective beam detector works by firing the infrared beam from a transmitter at a reflector positioned at the other end of the area being covered by the beam detector, the beam is then reflected back towards the beam detector unit which also has a receiver built into the enclosure.  A point to point beam detector operates by having a dedicated transmitter located at one side of the area being covered by the beam detector and then having a dedicated receiver at the other end of the protected area.

One major advantage of beam detectors is that they can be located a much greater heights than standard point detectors.  The maximum mointing height of a point detector is 10.5 metres where as BS5839 recommends that beam detectors can be mounted up to a height of 25 metres.  Another advantage of a beam detector is that one beam detector can provide automatic fire detection for an area which would require several standard point detectors, which means that using beam detectors can be much more cost effective than using standard point detectors.

The Siting of Beam Detectors

• Beam Detectors can be used at heights of up to 25 metres
• For apex ceilings extended coverage can be achieved by 1% for each degree of angle
• Beam Detectors should be positioned within 600mm of the highest point of the ceiling
• Ceiling beams close to walls (500mm) or temporary obstructions should be avoided
• Transmitters & Receivers should be mounted on solid surfaces which are not effected by wind or natural temperature changes
• Additional units may be included in atria to detect at lower levels, to counter statification effect.

Bonding, Grounding and Earthing

Bonding, Grounding and Earthing

In Fire detection system or CCTV / Access Control System this 3 terms are very important during doing the errection, testing & commissioning activity.

• Bonding is more clear word compare to Grounding and Earthing but there is a micro difference between Grounding and Earhing.
• Earthing and Grounding are actually different terms for expressing the same concept. Ground or earth in a mains electrical wiring system is a conductor that provides a low impedance path to the earth to prevent hazardous voltages from appearing on equipment. Earthing is more commonly used in Britain, European and most of the commonwealth countries standards (IEC, IS), while Grounding is the word used in North American standards (NEC, IEEE, ANSI, UL).
•  We understand that Earthing and Grounding are necessary. We need to understand that there are really two separate things we are doing for same purpose that we call Grounding or Earthing.
• The Earthing is to reference our electrical source to earth (usually via connection to some kind of rod driven into the earth or some other metal that has direct contact with the earth).
• The grounded circuits of machines need to have an effective return path from the machines to the power source in order to function properly (Here by Neutral Circuit).
•  In addition, non-current-carrying metallic components in a System, such as equipment cabinets, enclosures, and structural steel, need to be electrically interconnected and earthed properly so voltage potential cannot exist between them. However, troubles can arise when terms like “bonding,” “grounding,” and “earthing” are interchanged or confused in certain situations.
• In TN Type Power Distribution System, in US NEC (and possibly other) usage: Equipment is earthed to pass fault Current and to trip the protective device without electrifying the device enclosure. Neutral is the current return path for phase. These Earthing conductor and Neutral conductor are connected together and earthed at the distribution panel and also at the street, but the intent is that no current flow on earthed ground, except during momentary fault conditions. Here we may say that Earthing and grounding are nearly same by practice.
• But In the TT Type Power Distribution System (In India) Neutral is only earthed (here it is actually called Grounding) at distribution source (at distribution transformer) and Four wires (Neutral and Three Phase) are distributed to consumer. While at consumer side all electrical equipments body are connected and earthed at consumer premises (here it is called Earthing). Consumer has no any permission to mix Neutral with earth at his premises here Earthing and grounding is the different by practice.

But in both above case Earthing and Grounding are used for the same Purpose. Let’s try to understand this terminology one by one.

Bonding:

§  Bonding is simply the act of joining two electrical conductors together. These may be two wires, a wire and a pipe, or these may be two Equipment’s.

§   Bonding has to be done by connecting of all the metal parts that are not supposed to be carrying current during normal operations to bringing them to the same electrical potential.

§  Bonding ensures that these two things which are bonded will be at the same electrical potential. That means we would not get electricity building up in one equipment or between two different equipment. No current flow can take place between two bonded bodies because they have the same potential.

§  Bonding, itself, does not protect anything. However, if one of those boxes is earthed there can be no electrical energy build-up. If the grounded box is bonded to the other box, the other box is also at zero electrical potential.

§  It protects equipment & Person by reducing current flow between pieces of equipment at different potentials.

§  The primary reason for bonding is personnel safety, so someone touching two pieces of equipment at the same time does not receive a shock by becoming the path of equalization if they happen to be at different potentials.

§  The Second reason has to do with what happens if Phase conductor may be touched an external metal part. The bonding helps to create a low impedance path back to the source. This will force a large current to flow, which in turn will cause the breaker to trip. In other words, bonding is there to allow a breaker to trip and thereby to terminate a fault.

§  Bonding to electrical earth is used extensively to ensure that all conductors (person, surface and product) are at the same electrical potential.  When all conductors are at the same potential no discharge can occur.  

Earthing:

§  Earthing means connecting the dead part (it means the part which does not carries current under normal condition) to the earth for example electrical equipment’s frames, enclosures, supports etc.

§  The purpose of earthing is to minimize risk of receiving an electric shock if touching metal parts when a fault is present. Generally green wire is used for this as a nomenclature.

§  Under fault conditions the non-current carrying metal parts of an electrical installation such as frames, enclosures, supports, fencing etc. may attain high potential with respect to ground so that any person or stray animal touching these or approaching these will be subjected to potential difference which may result in the flow of a current through the body of the person or the animal of such a value as may prove fatal.

§  To avoid this non-current carrying metal parts of the electrical system are connected to the general mass of earth by means of an earthing system comprising of earth conductors to conduct the fault currents safely to the ground.

§  Earthing has been accomplished through bonding of a metallic system to earth. It is normally achieved by inserting ground rods or other electrodes deep inside earth.

§  Earthing is to ensure safety or Protection of electrical equipment and Human by discharging the electrical energy to the earth.

Grounding:

§  Grounding means connecting the live part (it means the part which carries current under normal condition) to the earth for example neutral of power transformer.

§  Grounding is done for the protections of power system equipment and to provide an effective return path from the machine to the power source. For example grounding of neutral point of a star connected transformer.

§  Grounding refers the current carrying part of the system such as neutral (of the transformer or generator).

§  Because of lightening, line surges or unintentional contact with other high voltage lines, dangerously high voltages can develop in the electrical distribution system wires. Grounding provides a safe, alternate path around the electrical system of your house thus minimizing damage from such occurrences.

§  Generally Black wire is used for this as a nomenclature.

§  All electrical/electronic circuits (AC & DC) need a reference potential (zero volts) which is called ground in order to make possible the current flow from generator to load. Ground is May or May not be earthed. In Electrical Power distribution it is either earthed at distribution Point or at Consumer end but it is not earthed in Automobile( for instance all vehicles’ electrical circuits have ground connected to the chassis and metallic body that are insulated from earth through tires). There may exist a neutral to ground voltage due to voltage drop in the wiring, thus neutral does not necessarily have to be at ground potential.

§  In a properly balanced system, the phase currents balance each other, so that the total neutral current is also zero. For individual systems, this is not completely possible, but we strive to come close in aggregate. This balancing allows maximum efficiency of the distribution transformer’s secondary winding

Difference between Earthing & Grounding:

There is no major difference between earthing and Grounding, both means “Connecting an electrical circuit or device to the Earth”. This serves various purposes like to drain away unwanted currents, to provide a reference voltage for circuits needing one, to lead lightning away from delicate equipment. Even though there is a micro difference between grounding & earthing.

(1) Difference in Terminology:

§  In USA term Grounding is used but in UK term Earthing is used.

(2) Balancing the Load Vs Safety:

§  Ground is a source for unwanted currents and also as a return path for main current some times. While earthing is done not for return path but only for protection of delicate equipment’s. It is an alternate low resistance path for current.

§  When we take out the neutral for a three phase unbalanced connection and send it to ground, it is called grounding. Grounding is done to balance unbalanced load. While earthing is used between the equipment and earth pit so as to avoid electrical shock and equipment damage.

(3) Equipment Protection Vs Human Safety:

§  Earthing is to protect the circuit elements whenever high voltage is passed by thunders or by any other sources while Grounding is the common point in the circuit to maintain the voltage levels.

§  Earth is used for the safety of the human body in fault conditions while Grounding (As neutral earth) is used for the protection of equipment’s.

§  Earthing is a preventive measure while Grounding is just a return path

§  The ground conductor provides a return path for fault current when a phase conductor accidentally comes in contact with a grounded object. This is a safety feature of the wiring system and we would never expect to see grounding conductor current flow during normal operation.

§  Do not Ground the Neutral Second time When It is grounded either at Distribution Transformer or at Main service Panel of Consumer end.

§  Grounding act as neutral. But neutral cannot act as ground.

(4) System Zero Potential Vs Circuit Zero Potential:

§  Earthing and Grounding both is refer to zero potential  but the system connected to zero potential is differ than Equipment connected to zero potential .If a neutral point of a generator or transformer is connected to zero potential then it is known as grounding. At the same time if the body of the transformer or generator is connected to zero potential then it is known as earthing.

§  The term “Earthing means that the circuit is physically connected to the ground and it is Zero Volt Potential to the Ground (Earth) but in case of “Grounding” the circuit is not physically connected to ground, but its potential is zero(where the currents are algebraically zero) with respect to other point, which is also known as “Virtual Grounding.”

§  Earth having zero potential whereas neutral may have some potential. That means neutral does not always have zero potential with respect to ground. In earthing we have Zero Volt potential references to the earth while in grounding we have local Zero Volt potential reference to circuit. When we connect two different Power circuits in power distribution system, we want to have the same Zero Volt reference so we connect them and grounds together. This common reference might be different from the earth potential.

Illegal Practice of interchange Purpose of Grounding & Earthing wire

§  Neutral wire in grid connections is mandatory for safety. Imagine a person from 4th floor in a building uses Earth wire (which is earthed in the basement at Basement) as neutral to power his lights. Another Person from 2nd floor has a normal setup and uses neutral for the same purpose. Neutral wire is also earthed at the ground level (as per USA practice Neutral is Grounded (earthed) at Building and as per Indian Practice it is Grounded (earthed) at Distribution Transformer). However, ground wire (Neutral wire) has a much lower electrical resistance than Earth Wire (Earthing) which results in a difference of electrical potential (i.e. voltage) between them. This voltage is quite a hazard for anyone touching a Earth wire (Metal Body of Equipment) as it may have several tens of volts.

§  The second issue is legality. Using ground wire instead of neutral makes you an energy thief as the meter uses only the Phase and neutral for recording your energy consumption. Many Consumers make energy theft by using Earthing wire as a Neutral wire in an Energy meter.

How to Apply to Fire Safety Systems and Installations

The NFPA 70, or NEC, sets the foundation for electrical safety in residential, commercial, and industrial occupancies in the United States. Updated every three years, it's a living document that evolves alongside the ever-advancing field of electrical technology. Let's go through a fast summary of NFPA 70 (also expressed as NFPA70 and NFPA-70). This will allow you, as a fire safety professional, to navigate and use its provisions more effectively.

Application of NEC to Electrical installation Directly

The NEC is divided into 9 chapters, with each chapter further divided into parts, articles, and sections. The chapters cover specific aspects of electrical installations, ranging from wiring methods, equipment for general use, special occupancies, special equipment, to special conditions.

·        Article 210: This covers branch circuits, the smallest units of an electrical system that directly supply power to electrical equipment. It includes standards for circuit ratings, protection, and load calculations.

• Article 215: It focuses on feeder circuits, which transport electricity from service equipment to branch circuits. Understanding this section is essential for ensuring proper feeder size and load.
• Article 230: This provides guidelines for service installations, the point where a building or system receives power from the utility source. It covers service conductors, equipment, and overcurrent protection.
• Article 240: Overcurrent protection is a crucial safety element of any electrical system. This section provides guidance for the selection and installation of overcurrent protective devices.
• Article 250: This focuses on grounding and bonding, two critical aspects of electrical safety. It defines how to properly ground electrical systems and equipment, ensuring safety and reducing the risk of electrical shock and fires. Digitize equipment is thoroughly grounded to protect itself from surge damage, and this obviously depends on the earth ground available in the building.
• Article 760: This section is of particular interest to you, as it pertains specifically to fire alarm systems. NEC Article 760 covers fire alarm system power sources, circuit identification, installation, and more.

Application of NFPA 70 to Fire Alarm Installations Directly

One of the most direct overlaps between the NEC and fire safety is found in NFPA 70 Article 760, "Fire Alarm Systems," within the NEC. This article deals specifically with the installation of wiring for fire alarm systems.

NFPA 70 Article 760 outlines the specific wiring methods for both non-power-limited and power-limited fire alarm circuits. This distinction is critical as power-limited circuits are designed to limit electrical energy to a level that does not ignite a fire in normal or fault conditions. Non-power-limited circuits, on the other hand, may have enough energy to ignite a fire. That's why they require more robust safety measures.

Power-limited fire alarm circuits are generally easier to install due to fewer restrictions, as they don't pose as much of a fire hazard. They can often be installed using similar techniques to those used for regular low-voltage wiring.

Non-power-limited circuits, however, must adhere to stricter standards. They need to be installed in such a way that reduces the potential for faults and ensures a higher degree of safety. Specific types of cable or conduit may be required, and separation from other electrical cables may be mandated.

Additionally, the NEC dictates how fire alarm circuits should be protected from accidental damage or disconnection. The use of dedicated circuit breakers or disconnecting means, identifiable and accessible control equipment, and appropriately labeled circuits are all examples of NEC provisions related to fire alarm installations.

Other Overlaps between Fire Safety and NFPA 70

Beyond fire alarm systems, several other aspects of the NEC directly influence fire safety. For example:

• Grounding (Article 250): Proper grounding of electrical systems is crucial for preventing electrical fires. By providing a path for electric current to follow in the event of a fault, grounding reduces the risk of electrical shock and fire.
• Overcurrent Protection (Article 240): Overcurrent protection devices, such as circuit breakers and fuses, are designed to cut off electrical power when the current exceeds a certain level. This prevents overheating of wires and potential electrical fires.
• Wiring Methods and Materials (Articles 300-399): These articles provide guidelines for the proper installation of wiring and related equipment to ensure safety and prevent electrical fires. This includes specifications for conductors, cables, raceways, and boxes, among other components.

Understanding and applying these codes is crucial in minimizing fire risks and ensuring optimal safety in all types of buildings and facilities.

How to Stay Up to Date with NFPA 70 / NEC

Given the rapid evolution of electrical technology, it's important to stay updated with the latest NEC revisions. The NFPA offers numerous resources for ongoing education, including online training, webinars, and certifications.

Regularly reviewing and understanding changes in the NEC can enhance your ability to install, maintain, and use fire safety systems in compliance with the highest safety standards.

Conclusion:

Ground is a source for unwanted currents and also as a return path for main current. While earthing is done not for return path but only for protection of delicate equipment’s. It is an alternate low resistance path for current. Earth is used for the safety of the human body in fault conditions while Grounding (As neutral earth) is used for the protection of equipment’s.

BASIC FIRE SCIENCE

BASIC FIRE SCIENCE

A.   FIRE TRIANGLE

Four things must be present at the same time in order to produce fire:

 
1.    Enough oxygen (O2) to sustain combustion,
·         Oxygen (O2) is an element from the atmospheric air that supports the combustion process.  It makes up 21% of the total air mixture.
2.    Enough heat to raise the material to its ignition temperature,
·         Heat is a form of energy which may be produced by mechanical or chemical reaction and transmitted via radiation, conduction and/or convection.
3.    Some sort of fuel or combustible material,  
·         Fuels are combustibles that may exist in solid, liquid, molten and/or gaseous states. Examples of fuels are paper products, wood products, molten chemicals, flammable fluids and gases.
4.    The chemical, exothermic reaction known as fire.

Fire occurs when fuel and oxygen come into contact with substantial amount of heat energy. This will cause a chain reaction to take place which will in turn produce sufficient heat energy to propagate the burning in the presence of subsequent fuel and oxygen.

Oxygen(O2), heat, and fuel are frequently referred to as the "fire triangle." Add in the fourth element, the chemical reaction, and we actually have a fire "tetrahedron."

Combustion will continue as long as all these factors are present. Removal of one of the factors will lead to the collapse of the "triangle/ tetrahedron" and combustion stops, and we will not have a fire or the fire will be extinguished.

B.   METHODS OF EXTINGUISHING FIRE

Fire Safety, at its most basic, is based upon the principle of keeping fuel sources and ignition sources separate.

Fire can be extinguished with one or a combination of the following methods:

 
1.    Starvation: Removal of the combustible substances.
 
2.    Smothering: Limiting the amount of oxygen (O2) that supports combustion.
 
3.    Cooling: Reduce the temperature to the point of fire extinction.
 
4.    Breaking (blocking) or interfering with the chemical reaction occurring in the flame.
  

C.   BASIC STAGES OF FIRE DEVELOPMENT

Fir
Fires can be commonly seen to develop in the following stages:
1.    Initial
The fire develops from its point of origin. If the combustion reaction is capable of emitting further heat in the presence of subsequent fuel and oxygen, the reaction will propagate and temperature will continue to rise.
 
2.    Flashover
This is the transition stage between the Initial and the full development stage. The fire spreads rapidly and large merged flames are produced. Flashover will especially occur in a confined space fire.
 
3.    Full
Fire will be fully developed burning at/near its development maximum intensity.
 
4.    Decay
Fire is slowing down due to depletion of Oxygen and/or fuel. Introduction of subsequent Fuel and/or Oxygen will however propagate a second initial development.

D.   CLASSIFICATION OF FIRE

The different types of fire are classified into the following "Classes":
 
1.    Class A : For fires involving the burning of ordinary combustible materials like wood, paper, cloth, furnishing, plastics and rubber.
 
2.    Class B : For fires involving flammable liquids, solvents, oils, paints, thinner and flammable gases.
 
3.    Class C : For fires involving live electrical equipment like electrical mains, transformers and electrical appliances, etc and where extinguishing medium used must be electrically non-conductive. (If the electrical equipment is de-energised, extinguishers for Classes A and B can be used.)
 
4.    Class D : For fires involving combustible metals, eg. Potassium (K), magnesium(Mg), titanium(Ti), sodium(Na), lithium(Li) and zirconium.

 E.   IGNITION AND COMBUSTION

1.    IGNITION
 
i.      Ignition is the process of starting a combustion process through the input of energy.  
 
ii.     If the substance is a liquid, ignition can only occur when
·         There exists a gas/air mixture whose composition lies within the flammable range for that substance. A small ignition source, e.g. a spark or small flame, will then start ignition.
·         Enough vapour is given off the vapour surface so that a vapour/air mixture is formed whose properties lie within the flammable range for that substance. The flash point is the temperature at which the liquid gives off just enough vapour to form this flammable vapour/air mixture. For temperatures above the flash point a small igniting source will start ignition.
 
iii.    If the substance is a solid, ignition will only start when either:
·         The solid is heated so that destructive decomposition starts and flammable vapours are given off. When these vapours form a flammable vapour/air mixture combustion can be started if a small flame is applied. Most organic materials will come under third category. The size of ignition source needed will depend on the physical form and shape of the material. If it is in the form which has a high surface area to mass ratio only a small igniting source is required.
·         The solid is heated in air so that the surface oxidation is initiated of sufficient vigour to be self-sustaining reaction. Carbon and most metals are examples f this type of combustion. A high surface area to mass ratio is needed for ignition and this means that the metal has to be in he form of finely divided powder. An intense source of heat is required in most cases to ignite solids which burn this way.
 
 
2.    COMBUSTION
 
i.      Combustion is a reaction between a substance and oxygen in which heat is given out, i.e. it is an exothermic reaction, and enough heat must be given out either to maintain the temperature of the reactants or to continuously increase their temperatures so setting up a chain reaction.
 
ii.     In some cases the reaction between a substance and oxygen requires heat to be applied to maintain the reaction and the reaction stops when the applied heat is removed, this is not combustion.
 
iii.    All combustible materials burn as vapour phase reactions, i.e. they are reactions between two gases, one of which is usually oxygen. The combustible materials which are usually solid will, because of high temperatures involved, either melt and then vapourise and burn, or decompose by pyrolysis and give off flammable vapours which will then burn. There are exceptions and there can be reactions between oxygen and solid surfaces which are vigorous and exothermic and so can be termed combustion, e.g. carbon and certain metals. Liquids also vapourise before combustion takes place.
 
iv.   The following are the products of a combustion process:
 
Thermal           -           Flame & Heat
 
Non-Thermal   -           Smoke & Gases
 

·         Damage to property is due mainly to thermal effects although smoke and gases can also cause considerable damage. Some of the gases produced during combustion are corrosive, or become corrosive in solution with water.
 
·         Most fire deaths or injuries, however, may be attributed to smoke or gases arising from smoke inhalation and suffocation. Smoke can also cause panic among occupants which is likely to cause further injuries and death. Heat and flamers are primary causes of fewer deaths or injuries than smoke or gases.
 
·         The heat generated in the process of combustion can completely destroy a building. Steel will have lost two-thirds of its strength by the time it has been heated to 600 ° C. This is by no means uncommon temperature in a domestic fire. Concrete is more resistant material; but as reinforced concrete depends on steel for its tensile strength, there needs to be sufficient insulation of the steel to prevent it reaching its critical temperature. Timber, of course, burns but is a very good structural material as burning occurs at a fairly constant rate and so structural timbers can be oversized to provide a known measure of fire resistance. Bricks provide one of the best fire-resistant materials as they have already been kiln-fired at high temperature during manufacture.