By Shane Clary · January 15, 2012
For years fire detection has been performed through the use of spot-type detection devices such as heat and smoke detectors. For the majority of applications these devices provide a level of coverage that meets the goals of life safety, property protection and mission continuity for a particular protected premise. But there are spaces that require protection that simply do not permit the use of a typical spot-type detector. Video image detection (VID) is a newer technology that has evolved the past 10 years to address the problems associated with these difficult areas.
While linear smoke detection (projected beams) and radiant heat detectors (flame detectors) have been around for a number of years, they may not be ideal choices for all applications. To function properly, all smoke detection devices, including spot-type, smoke beams and aspirating detectors, require that the products of combustion reach the detectors. This can be an issue when dealing with factors such as height, air velocity, transport delay and stratification. Even putting these factors aside, the cost of installing traditional spot detectors in these locations would be difficult at best, if not impossible.
With VID, the products of combustion when detecting for smoke are not required to travel to the sensor. VID can detect anywhere within a given field of view, which means it is not limited by the factors of distance and area as are standard detectors. However, VID is bound to the restraints of sharpness, contrast and illumination within the field of view. Let’s investigate further.
How NFPA Looks at Video Detection
VID was first described in the 2007 edition of NFPA 72, The National Fire Alarm Code, in Section 7.5.6, Video Image Smoke Detection. The requirements for VID with the 2007 edition are based on a performance design.
The Technical Committee on Initiating Devices was uneasy with the allowance of VID without further requirements on the siting of this technology. A project was conducted by the NFPA Fire Protection Research Foundation by Hughes Engineering on the uses of VID, and a report called Video Image Detection Systems Installation Performance Criteria Research Project was issued (www.nfpa.org/assets/files//PDF/Research/RPT_FPRF_VID_Performance_Criteria.pdf).
In the 2010 edition of NFPA 72, the National Fire Alarm and Signaling Code, additional requirements were added to Section 17.7.7, the former Section 7.5.6.
The use of VID in the current standard is through performance-based design. Does this mean VID meets the prescription requirements of 72 for the space intended to be protected, and can it meet the requirements of the fire or building code for the occupancy classification and/or the overall system design requirements?
Monitoring Smoke and Flames
To answer this question, an understanding of the basic principles of VID is necessary.
Originally developed in Europe, the first VID technology application in the United States was conducted by the U.S. Navy for volume detection within ships. Volume detection is the protection of a whole space. Fires onboard a ship can be devastating with early detection being paramount for a vessel’s survival.
Video detection is achieved through analog or digital video cameras and, depending on the system architecture selected, may involve devices also being used for security within a building. There are two configurations of VID. The first is having multiple cameras feed into a central processor, which is then connected to the fire alarm system. In this case, the “smarts” of the system is not in the cameras, but in the processor. The second method is by having the processor within each camera, which in turn is connected to the building’s fire alarm system.
The first generation of VID used the former configuration, while newer generations have the latter available as well. The multiple camera systems typically have either four or eight cameras feeding into the processor. One point to consider with this architecture is power consumption. Unless the VID is to be supplemental detection, the power requirements of NFPA 72 must be followed, which means a minimum of 24 hours of standby power. Single camera products have lower power requirements. These systems function with either 12 or 24VDC and with a relatively low power draw.
Both of these methods use analytical algorithms within their processors for the detection of either smoke or flame. Different methods are used for flame and smoke, but some common methods between the two are the use of fuzzy logic, neural networks and looking for pixel changes within the field of view or image.