The latest technologies for eliminating nuisence alarms in fire detection applications

March/April 2000 Fire & Safety

Today more than ever, there's no place for nuisance alarms in fire detection and alarm applications. No matter the building or environment being protected , nuisance alarms should by and large be considered unnecessary and unacceptable.

In most cases, they are much more than a nuisance. Indeed, nuisance alarms can be downright devastating, and here are just a few reasons why:

* Nuisance alarms can be very costly to electrical contractors, due to the expenses associated with call backs during the initial warranty period.

* Nuisance alarms may reflect negatively on the quality of the installation.

* Nuisance alarms are costly to the fire service and unnecessarily absorb fire department resources.

* Depending on the setting, nuisance alarms can result in thousands if not millions of dollars in lost business.

* Nuisance alarms are demoralising and potentially dangerous to fire-fighters.

* Nuisance alarms are disruptive to building occupants.

* Over time, repeated nuisance alarms can result in building occupants ignoring all alarms – a development that can have tragic consequences.

Considering the danger, havoc and loss that nuisance alarms can cause, it's easy to see why building owners, electrical contractors, architects and engineers are so intent these days on reducing and/or eliminating them. The good news on that front is that with continuing advances in detectors, control panels and fire alarm systems, the tools and technologies now available to combat nuisance alarms are extremely sophisticated and effective.

The evolution of detector technology

In examining the latest techniques for eliminating nuisance alarms, it's informative and instructive to review the evolution of detection technology in order to gain a better understanding of how today's advanced capabilities developed over the past four decades.

In the early 1960s, ionisation detection technology had a strong foothold in the fire detection industry. Back then, ionisation detection was considered leading-edge technology and it was used primarily in high-risk applications such as telephone switching rooms.

In the1970s, photoelectric detection technology arrived on the scene and began to be used in any number of applications. Photoelectric smoke detectors provided a wider range of sensitivity and stronger capabilities for avoiding nuisance alarms. Even then, however, ionisation technology – because of its lower cost , remained a driving force in the industry. But while ionisation-type smoke detectors were considerably less expensive, there was a down side to their use , they contributed to a great many false alarm problems.

The 1980s brought the development of the multisensing detector , a device designed to combine the nuisance alarm immunity of photoelectric smoke detectors with improved flaming fire detection. The idea was that a multisensing detector , in bringing together the strongest features of two distinct technologies - would deliver enhanced detection capabilities.

However, because they relied heavily on ionisation detection technology, multisensing detectors were found to be prone to false alarms. Multisensing detection does continue to play a role in the in the fire detection industry today, and is considered an appropriate technology where certain types of fuels are present.

Photoelectric detection technology rises to prominence

Detection technology continued to evolve through the late 1980s and into the early 1990s. And throughout the industry, the number of nuisance alarms was significantly reduced by the introduction of photoelectric detection technology that enabled systems to provide drift compensation to offset the accumulation of dirt and dust.

Today, photoelectric detectors are used in an overwhelming majority of fire detection applications. That's because photoelectric detector characteristics have been found superior for the typical conditions that exist in most facilities being protected. Wood, paper, cardboard and cotton are the potential fuels most often present in commercial, educational and health care facilities. In a smouldering situation, photoelectric has proven to be the best detection technology for those potential fuels.

Ionisation technology is better suited to flaming fires, which produce visible smoke in relatively small amounts and quickly generate substantial amounts of heat. Consequently, ionisation-type detectors are now typically applied in areas where highly flammable material such as heptane or toluene is stored.

The latest technologies

Here is an overview of some of the latest technologies that can be employed today to combat nuisance alarms.

Drift compensation

The build-up of dirt and dust in smoke detectors , often the result of improper maintenance , is a major cause of nuisance alarms. As dirt accumulates over time in the chamber, the detector begins to 'drift' away from its selected sensitivity. In effect, the accumulation of dirt makes a detector more sensitive. As dirt continues to accumulate, a detector drifts toward the alarm level , the threshold that must be reached to cause an alarm.

Because of increased sensitivity caused by dirt build-up, trace particles of ambient smoke , which wouldn't activate a clean detector , can set off a device which is dirty. Transient conditions - such as radio frequency interference from cellular telephones - can similarly activate a dirty detector.

Recent advances have brought building owners powerful new technological tools that enable analogue addressable systems , by evaluating environmental data and compensating for contamination , to maintain detector sensitivity and nuisance alarm immunity. Many analog systems can now provide drift compensation, which enables detectors to maintain their original sensitivity setting over time despite the accumulation of dirt in the chamber.

In those systems, the control panel is able to continuously measure, analyse and average ambient conditions in the detector chamber , and to automatically compensate for dirt accumulation. Each sensor's average value is constantly monitored as part of a software filtering process that quantifies the build up of contamination. The environmental data is used to make adjustments that maintain the desired sensitivity level by compensating for accumulated dirt and dust. The result is a significant reduction in the probability of nuisance alarms caused by shifts , either up or down , in sensor sensitivity.

In the most advanced systems, drift compensation is accomplished by moving both the zero reference and the alarm threshold proportionately , by an amount equal to the change in sensitivity resulting from the accumulation of dirt. As an example, let's take a detector with a sensitivity setting that would cause an alarm at 2,5% per foot of smoke obscuration. And let's say the build-up of dirt over time has resulted in a change in sensitivity that equals 1% per foot of smoke obscuration. With the drift compensation feature, the system automatically moves the zero reference to 1% and the alarm level to 3,5%. As a result , just as with the original setting -,it still takes 2,5% smoke obscuration to initiate an alarm.

Automatic smoke detector cleaning indication

Many analog addressable systems today employ so-called 'smart detector' technology where the control panel can identify ,well in advance of the point at which they might cause a nuisance alarm , individual sensors that need to be cleaned. These systems provide intelligent data evaluation and a running average which exceeds a predetermined level indicates that a detector needs to be cleaned. The control panel will warn system operators that a sensor is dirty , before its compensation limit has been reached. The dirty sensor alert occurs when the detector is approaching the end of the compensation range. At that point, there is still room for additional compensation - and the sensor is still functioning at its prescribed sensitivity setting.

Some advanced fire detection systems also have 'almost dirty' reporting capabilities. With that capability, the control panel can alert system operators about sensors that are approaching a dirty indicator level. When a dirty detector report is received at the panel, facilities personnel - using the almost dirty reporting capabilities - can also identify any sensor that might report a dirty condition in the near future. These modern fire alarm systems are also capable - over a dial-up telephone network - of transmitting alarm and trouble conditions as well as maintenance signals (such as dirty and almost dirty indications) to a central monitoring station.

The National Fire Alarm Code (US) does require that the sensitivity of smoke detectors be periodically tested. Smoke detectors found to be outside of their listed sensitivity rating should be cleaned - and programs should be established to clean on a regularly scheduled basis detectors that have been installed in dirty locations. But while the National Fire Alarm Code does specify periodic testing, the dirty and almost dirty reporting capabilities available in some systems can be invaluable because they alert building owners or maintenance companies of impending service requirements. That enables cleaning to be scheduled in a more efficient manner - and also ensures that only those devices in need of cleaning are actually serviced.

Conventional systems aren't able to tell building owners when detectors need cleaning. Therefore, detectors are more apt to get excessively dirty, which typically causes them to go into alarm - a nuisance alarm. The cleaning of those systems can also be expensive and inefficient. That's because all detectors must be removed, inspected and cleaned in order to make sure the dirty ones are actually cleaned.

While drift compensation can offset the accumulation of dust or dirt that often causes nuisance alarms, proper maintenance is still critical to the effective operation of any fire detection and alarm system. And that maintenance is best performed by a company regularly engaged in servicing fire detection and alarm systems - one with the necessary equipment, personnel and resources to handle the job properly.

Multiple sensitivity levels

Some advanced systems offer building owners a choice of sensitivity levels with the UL-listed range. In a system with this capability, each sensor can be individually set at the optimum sensitivity for the environment it protects. For example, in an elevator lobby or lounge area where smokers might gather - or in other areas where small amounts of smoke might normally be present - the sensor can be set at the least sensitive end of the UL window. In high-risk installations - such as computer rooms and telephone switching centres where very early warning is important - the sensors can be set at a more sensitive level of 0,5% or even 0,2%.

In both instances, the building owner can take advantage of the full range of UL-accepted sensitivity settings, closely match the sensitivity of the detector to the area it is protecting, and guard against nuisance alarms.

In systems that cannot provide multiple sensitivity levels, each detector's sensitivity setting would typically be based only on general guidelines about protection levels for different occupancies. A computer room, for example, would most likely be equipped with a "relatively more sensitive" detector than a conference room. But it would be difficult - without causing a nuisance alarm - to determine if a more sensitive detector could potentially be used in that computer room setting.

Peak value reporting

Some newer fire detection systems have the ability to provide an historical accounting that specifies how close a detector has come to its alarm point. That "peak value" analytical data is useful in customising a system to meet the precise fire detection application in a particular area or building.

It is important to note that detectors are set at a factory default sensitivity that is appropriate for most commercial, educational and institutional environments. Peak value logging can be valuable in applications where a more precise, experience-based sensitivity setting is desirable. In those settings, peak value logging can be used to help maximise protection and minimise nuisance alarms.

Here's how it works. All sensors can be set to a sensitivity of 2,5% at installation. After a period of time, perhaps 90 days, the sensitivity can be adjusted up or down based on an analysis of how close they actually came to being in alarm during that interval. A sensor in a conference room, for example, that might have had a peak value of 1,0% smoke could have its sensitivity lowered to 3%. A sensor in a computer room, with a peak value of 1% smoke, could be have its sensitivity increased to 0,5%. Peak value logging can be an important consideration when selecting a control panel because it enables a customer – by evaluating historical data about actual environmental conditions - to set sensors at the optimum sensitivity. The resulting sensitivity settings coincide with the fire risk in the protected environment and help prevent nuisance alarms.

Multistage alarm selection

This feature takes full advantage of systems that provide multiple sensor sensitivity levels. Through control panel programming, some systems can provide multi-stage operations for each sensor. For example, the control panel may be programmed so that in one individual sensor a 2,0% level will cause a warning that prompts further investigation - while a 2,5% or 3,0% level will automatically initiate a general evacuation alarm. The multistage alarm allows time for investigation before proceeding to evacuation.

When allowed by the Authority Having Jurisdiction (AHJ), this feature can reduce unnecessary evacuations and is particularly valuable in hospitals, hotels and dormitories or in jurisdictions where there is a charge for responding to false alarms.


Recent trends clearly indicate that analogue addressable systems may ultimately replace hardwired, zoned systems in small- to medium-sized facilities. The information contained in this article provides an overview of some of the advanced capabilities that can be used - in those environments and in higher-end applications - to enhance detection and reduce nuisance alarms.

In order to maximise the application of these and other technologies - and thus ensure customer satisfaction - electrical contractors are well advised to become members of the National Fire Protection Association and make themselves available for fire protection instruction and training.


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