Right on Target

Industry coming closer to perfect fire alarm system

• By David George
• May 02, 2007

INTELLIGENT, spot laser smoke detectors perform two distinct functions in a fire-protection system.

Capable of sensitivities 100 times greater than standard smoke detectors, spot laser units respond to incipient fire conditions as low as .02 percent-per-foot obscuration. Also, laser smoke detectors, functioning like standard smoke detectors, go into full alarm mode, activating a building’s fire-alarm or fire-suppression system if higher thresholds—2 percent-per-foot obscuration—of smoke are detected.

With this dual function, laser smoke detectors offer greater protection often required in areas that house high-value assets such as data centers, switch rooms, vital record storage areas and areas preserving historical artifacts.

Protecting High-Value Assets
Laser smoke detectors are designed to protect large areas where traces of smoke can cause significant damage and disruption of operations. Using an extremely bright, controlled laser diode, a laser beam is transmitted through a specially designed chamber that eliminates any reflection. If a single particle of smoke enters the laser’s path, intense light will scatter in the photoelectric chamber, triggering a response.

Laser smoke detectors also include a microprocessor-controlled device that pinpoints the exact location of a fire by identifying the responding detector’s address. This not only reduces response time, as smoke at extremely low levels is often not visible to humans, but also provides more time to identify what caused the detection to react.

Because of the high sensitivity, laser smoke detectors are programmed to screen out false alarms via multi-stage drift compensation, internal self diagnostics and transient-signal rejection algorithms. Signal confirmation is another way laser detectors minimize false alarms. For example, if a piece of dust enters a laser smoke detector’s sensing chamber, a signal from the detector tells the panel to check with other devices in the nearby area to see if they are registering the same stimulus. If no other detector is sensing the same increase in stimulus level, the system panel will generate a trouble condition signal to alert that the first detector requires service.

If in a real fire scenario, two laser smoke detectors confirm an increase in stimulus level, and a pre-alarm, an early-warning output signal is sent to the control panel. Typically, the signal does not notify the fire department or actuate an extinguishing system to operate. Instead, local personnel are notified, prompting an investigation of the area. Many times, the solution is as simple as shutting off a piece of equipment.

But if a fire develops and a laser smoke detector senses smoke at 2 percent-per-foot obscuration, the panel will again perform a cross-zone check. If two detectors confirm the higher amount of smoke, the control panel will issue a number of programmed output control instructions.

One instruction may be to release an extinguishing agent. However, after the command is issued, a 30-second delay lapses before the extinguishing agent is released. This is to protect valuable or irreplaceable property from unnecessary damage. As a last failsafe, most extinguishing systems have built-in abort stations to stop or delay discharge. This gives responders one last chance to stop the release if the situation warrants. In systems with preaction sprinklers, the pipes must be loaded with the extinguishing agent, which is released only when a ceiling sprinkler head melts at 165 degrees Fahrenheit.

Because most electrical fires that start small—with origins such as overheating cables or malfunctioning printed-circuit boards—do not create a lot of heat initially, without high-sensitivity detection, the lag time between the incipient stages and the release of the extinguishing agent can be significant.

Protecting Data
The IT industry standard is to keep data centers at 70 degrees Fahrenheit with 50-percent relative humidity, allowing for small variances. Excess humidity may cause circuits to short, while dry air may cause a static charge, both of which wreak havoc on delicate electronics.

But creating these conditions presents challenges for smoke detection because of the high level of air turbulence needed to achieve strict temperature and humidity levels. With a raised, grated floor stacked with heat-producing computers and cabling underneath, a robust HVAC system blows a high volume of air from the ceiling, to the floor, to circulate air under equipment. In a static-air environment, like an office, where fire generates smoke that rises to the ceiling in a concentrated form. And in a data center, the massive airflow causes smoke dilution. That makes detection by means other than high-sensitivity devices slow and less responsive.

Like other inhabited space, data centers are required by codes to have standard smoke detection. However, the dual-function capabilities of laser detectors add value. Beyond the high-sensitivity detection, laser detectors are cost-effective. Electrical-installation labor represents about 80 percent of cost, and laser smoke detectors add only about 10 percent to the overall price. So the devices essentially perform the function of two detector systems for the price of one.

Cutting Response Time
The new edition of the fire-alarm controls standard may prove to be one of the most significant changes the fire- and life-safety industry has seen in the past 20 years.

The ninth edition of UL-864 is bordering more on revolutionary than evolutionary because of the time lapse since the publication of the UL eighth edition. During this interim, three cycles of the National Fire Alarm Code NFPA 72 were completed—in 1999, 2002 and this year. It has brought about a tremendous amount of redesign in annunciating devices because manufacturers are now allowed to specify operating parameters for the devices as a mechanism to establish compatibility.

Engineered systems designers will now be able to determine which devices are compatible with which control unit by comparing specifications for regulated Notification Appliance Circuits and notification appliances.

Under the ninth edition, the determination of compatibility will require a review of ratings rather than a lengthy and expensive test program by UL. Some manufacturers have designated some or all of its NAC circuits as special application. The new compatibility scheme would now not be applicable to these circuits.

Creating a Standard
The 9th edition also includes a major change in alarm-signal processing time that is driven by newer editions of the National Fire Alarm Code. The 90-second signal processing time for annunciation of an alarm from the time an initiating device is tripped has been reduced to 10 seconds. The change is a reflection of the capability of new technology incorporated in the new generation of alarm equipment.

Most importantly, the new edition standardizes the interface between fire alarm notification appliance circuits and annunciation devices. Under this standardized scheme called regulated circuits, considerable new latitude is afforded to installers in choosing compatible notification appliances. This is a significant enhancement from the old process, where if the notification appliance was not identified in the control unit installation wiring diagram, it was not compatible.

Under the new scheme, building owners will benefit from having a choice of multiple brands of A/V products and the ability to determine compatibility, especially in retrofit situations. With a standardized interface between fire alarm control and the annunciation devices, the owner gets relief. This was a primary concern when this section of UL-864 was rewritten. But to have flexibility, the voltage and current ratings of NAC circuits have to be clearly defined. By comparing ratings, a person can establish what is and isn’t compatible with a certain control panel.

Expandable Systems
During the life of a building, it may be expanded or remodeled to the extent that additional horns and strobes may be needed for continued code compliance. If the original devices are no longer available, it puts the building owner at a disadvantage because it often means the entire system needs to be replaced. The changes have strengthened requirements for compatibility among smoke detectors, annunciation devices and control panels, as well as status display and signal processing time.

The flexibility is the result of the standard’s complexity that requires lengthy and complicated testing to address in-rush current, steady-state current and maintaining voltage to operate the strobe and synchronization. To accommodate the complexity, UL has developed a testing protocol that permits a device to be rated for all of the parameters.

Ultimately, this puts the responsibility directly on the device manufacturers to correctly report product ratings. It also puts pressure on designers and installers to make critical product choices, as their reputations are linked to the installed system being correct.

The ninth edition puts the industry one step closer to the “perfect” fire alarm system. As the industry converges on this mythical system, there are opportunities for continuous learning. This allows an upgrade of the standards and performance requirements because all engineering, to some degree, is an approximation. In a perfect system, the earliest stage of fire would provide an instant annunciation of the alert signal. But then it becomes a matter of practicality because response in 10 seconds, as required by the new standard, is a short period of time. NFPA added a year to its normal, three-year cycle, and because the standard was not published until late 2006, it was designated the 2007 edition.