In September’s “Fire Side Chat” we talked about false alarms and how they can overwhelm public agencies, such as firefighters and police personnel, impact municipal budgets, and how end users are often fined to offset the negative monetary effects. A sidebar provided ideas on how to prevent or avoid them and statistical data was offered to substantiate and quantify the problem.
In a continuation of that theme, this month we’re going to look at some of the ways municipalities, fire departments, and fire alarm companies are working to reduce false and unwanted fire alarms. Examples include central station verification, cross-zoning and video verification. The goal is to minimize the negative impact that false alarms present to public agencies.
Cross-Zoning for Smoke Detection
There are two ways to avoid false alarms using the power of today’s fire alarm panels: cross-zoning (see diagram) and smoke verification.
Cross-zoning is the application of two detectors/sensors where one would usually suffice - in other words, the detection area of each smoke detector is degraded by 50 percent. In this application both detectors must discern a legitimate fire/smoke signature in order to set the system into alarm.
Table 10.4.2.2(17), NFPA 72, 2007 Edition, defines a cross-zone detection circuit as: “One sensor or detector on each zone shall be operated. Occurrence of correct sequence with operation of first zone and then with operation of second zone shall be verified.”
At the center of this application is the use of AND logic instead of OR. Where OR logic says, “An alarm will occur if detector 1 or detector 2 trips,” AND logic dictates “both detector 1 and 2 must trip before an alarm can occur.”
Most of the time fire alarm technicians use their automatic smoke detectors in conjunction with the OR logic mode. Where the AND configuration is most often used is special hazard systems where the risk of unwanted or false fire alarms can mean the loss of thousands of dollars of fire suppressant, such as DuPont’s Fe-25 (HFC-125) clean agent.
“I’m not a big fan of cross-zoning although it can be done. I would only do it where I’ve tried to solve a chronic problem and nothing works,” says Nick Markowitz owner of Markowitz Electric Protection of Verona, Pa. “Also, I would have the fire marshal’s permission to do it before I’d even think about it.”
The second method, smoke verification, involves the resetting of a smoke detector(s) after detection with a 60-second window during which the fire alarm panel waits to see if a second alarm occurs. If a second trip does occur, the alarm panel goes into alarm.
Of both methods, cross-zoning is the most expensive to implement because of the reduced coverage. Most modern fire alarm panels contain both features in programming.
Central Station Alarm Verification
Another method used to reduce false dispatches is central station alarm verification. In a word, an alarm must be verified either by a human eye, nose, feel or some other means. In most cases this means that a central station operator must call the premises before the local fire department operations center.
The issue of central station verification has been a hot button for fire alarm technicians and third-party central stations for a long time. In the past, a large percentage of centrals refused to verify before dispatching firefighters. This was especially true in commercial applications, in particular high-rise buildings. In most cases the practice of immediate dispatch continues today — that is unless the authority having jurisdiction (AHJ) decides it’s not in the community’s best interest to approve its use. Residential settings are another matter.
NFPA 72 specifically allows central station verification in residential applications and many central stations now use it as a false alarm reduction tool. Section 18.104.22.168, NFPA 72, 2007, states that, “Remote monitoring locations shall be permitted to verify alarm signals prior to reporting them to the fire service, provided that the verification process does not delay the reporting by more than 90 seconds.”
This has given the green light to central stations, allowing them to call the homeowner before they dispatch. The catch is they must be able to do it within the allotted time of 90 seconds.
Although residential settings are approved for alarm verification under NFPA 72, 2007 Edition, commercial applications are not. And yet there are municipalities that require verification before they will dispatch firefighters.
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Fire/Life Safety · Systems Integration · Fire/Life Safety 2 ·
Central Stations ·
False Fire Alarms ·
Fire Side Chat ·
Fire Side Chat with Al Colombo ·
NFPA 72 ·
Reducing False Alarms ·
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