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A firewall is a fireproof barrier used to prevent the spread of fire between or through buildings, structures, electrical substation transformers, or within an aircraft or vehicle.
- Firewalls can be used to subdivide a building into separate fire areas and are located in accordance with the locally applicable building code. Firewalls are a portion of a building's passive fire protection systems.
- Firewalls can be used to separate high value transformers at an electrical substation in the event of a mineral oil tank rupture and ignition. The firewall serves as a fire containment wall between one oil-filled transformer and other neighbouring transformers, building structures, and site equipment.
There are three main classifications of fire wall: firewalls, fire barrier walls, and high challenge firewalls. To the layperson, the common use of language typically includes all three when referring to a firewall unless distinguishing between them is necessary.
- A firewall is a wall separating transformers, structures, or buildings or a wall subdividing a building to prevent the spread of fire and having a fire resistance rating and independent structural stability.
- A fire barrier wall, also referred to as a fire partition, is a fire rated wall assembly which is not a fire wall. Typically, the main differences is that a fire barrier wall is not structurally self-sufficient, and does not extend through the roof, or necessarily to the underside of the floor above.
Fire barrier walls are continuous from an exterior wall to an exterior wall, or from a floor below to a floor or roof above, or from one fire barrier wall to another fire barrier wall, fire wall, or high challenge fire wall having a fire resistance rating of at least equal rating as required for the fire barrier wall. They are continuous through all concealed spaces (e.g., above a ceiling), but are not required to extend through concealed spaces if the construction assembly forming the bottom of the space has a fire resistance rating at least equal of the fire barrier wall.
- A high challenge fire wall is a wall used to separate transformers, structures, or buildings or a wall subdividing a building with high fire challenge occupancies, having enhanced fire resistance ratings and enhanced appurtenance protection to prevent the spread of fire, and having structural stability.
Portions of structures that are subdivided by fire walls are permitted to be considered separate buildings, in that fire walls have sufficient structural stability to maintain the integrity of the wall in the event of the collapse of the building construction on either side of the wall.
- Fire rating - Fire walls are constructed in such a way as to achieve a code-determined fire-resistance rating, thus forming part of a fire compartment's passive fire protection.
- Design loads – Fire wall must withstand a minimum 5 lb./sq.ft., and additional seismic loads.
- Substation Transformer Firewalls are typically free standing modular walls custom designed and engineered to meet application needs.
- Building Firewalls typically extend through the roof and terminate at a code-determined height above it. They are usually finished off on the top with flashing (sheet metal cap) for protection against the elements.
- Building and structural firewalls in North America are usually made of concrete, concrete blocks or reinforced concrete. Old walls, often built prior to World War II, will consist of brick.
- Fire barrier walls are typically constructed of drywall/gypsum board partitions.
- Penetrations – Penetrations through fire walls, such as for pipes and cables, must be protected with a firestop assembly to prevent the spread of fire through the wall at this point. Penetrations (holes) must not imperil the structural integrity of the wall, such that the wall must be able to withstand a fire without threat of collapse.
- Openings – Openings in Fire walls, such as doors and windows, must be fire rated as fire door assemblies and fire window assemblies.
Performance based design 
Firewalls being used in different application may require different design and performance specifications. Knowing the potential conditions that may exist during a fire are critical to selecting and installing a firewall. For example, a firewall designed to meet National Fire Protection Agency, (NFPA), 221-09 section A.5.7 which indicates an average temperature of 800°F, is not designed to withstand higher temperatures such as would be present in higher challenge fires, and as a result would fail in a time less than the wall rating.
Performance based design takes into account the potential conditions during a fire. Understanding thermal limitations of materials is essential to using the correct material for the application.
Firewalls outside of building construction 
Firewalls are also regularly found in aircraft and in specially prepared cars for compartmentalisation and competition use. For example, a typical conversion of a production car for rallying will include a metal firewall which seals the fuel tank off from the interior of the vehicle. In the event of an accident, resulting in fuel spillage, the firewall can prevent burning fuel from entering the passenger compartment, where it could cause serious injury or death. In regular stock cars, the firewall separates the engine compartment from the cabin and can, at times, contain fibreglass insulation. Automotive firewalls have to be fitted so that they form a complete seal. Usually this is done by bonding the sheet metal to the bodywork using fibreglass resin.
The term firewall is also commonly used by automotive mechanics to refer to the barrier between the passenger and engine compartments of a vehicle. The inner and outer surfaces of the firewall are often coated with NVH-absorber to prevent most engine noise from reaching the passenger compartment.
- NFPA 221, Standard for High Challenge Fire Walls, Fire Walls, and Fire Barrier Walls, 2009 Edition, section 18.104.22.168, NFPA 850-10 Fire Protection fotric Generating Plants and High Voltage DC Converter Stations 2010 Edition section 22.214.171.124-4
- NFPA 221, Standard for High Challenge Fire Walls, Fire Walls, and Fire Barrier Walls, 2009 Edition, section 126.96.36.199
- NFPA 221, Standard for High Challenge Fire Walls, Fire Walls, and Fire Barrier Walls, 2006 Edition, section 188.8.131.52
- NFPA 221, Standard for High Challenge Fire Walls, Fire Walls, and Fire Barrier Walls, 2009 Edition, section 184.108.40.206, NFPA 850-10 Fire Protection for Electric Generating Plants and High Voltage DC Converter Stations 2010 Edition section 220.127.116.11-4
- NFPA 221, Standard for High Challenge Fire Walls, Fire Walls, and Fire Barrier Walls, 2006 Edition, section A18.104.22.168
- NFPA 221, Standard for High Challenge Fire Walls, Fire Walls, and Fire Barrier Walls, 2006 Edition, section 4.6
- NFPA 221, Standard for High Challenge Fire Walls, Fire Walls, and Fire Barrier Walls, 2006 Edition, section 4.2
- NFPA 221, Standard for High Challenge Fire Walls, Fire Walls, and Fire Barrier Walls, 2006 Edition, section 4.9
- NFPA 221, Standard for High Challenge Fire Walls, Fire Walls, and Fire Barrier Walls, 2006 Edition, section 4.8.3