Selecting and maintaining smoke detectors
Smoke detectors can have a lifecycle of up to 20 years if certain design and installation recommendations are followed and if annual maintenance is performed for each detector. Given the tight budget of many companies in today’s economy, a reliable fire system can be one less budget issue and one less concern for the owner, operator, or lessee.
Generally, when fire systems fail, it is due to lack of planning and maintenance. The following describes common problems associated with detectors that can be avoided at different phases of fire system design, installation, and maintenance.
Consider the application
When smoke detection devices are selected for a project, the type of detection must be suitable for the application. Ionization and photoelectric smoke detectors commonly are used in many applications.
Both detectors have detection limitations, however. The ionization smoke detector is not as effective at detecting flaming fires, while the photoelectric smoke detector is not as efficient with smoldering fires. Therefore, the fuel loads and fire scenarios must be considered before a detector type is chosen. If the wrong detector is selected for an application, the lifecycle of the detector can be shorter or negligible if the detector is rendered useless and needs to be replaced with the right detector.
Environmental issues such as altitude, temperature range, humidity, and air contamination need to be considered when selecting detector types. The ionization detectors are limited to installation at altitudes as high as 3,000 ft above sea level. If they are installed at higher than 3,000 ft above sea level, the detectors often create nuisance alarms and may affect detector responsiveness. The photoelectric smoke detector does not have an altitude limitation.
Both detector types can be affected by temperature range and humidity. When detectors are installed where temperatures are below 32 F, ice crystals or condensation can appear inside the sensing chambers and cause the detector to become overly sensitive or create nuisance alarms. At temperatures above 100 F, the detectors’ internal electronic components may not function properly. In humid areas, water droplets can accumulate inside the sensing chamber and affect detector sensitivity, which could create nuisance alarms.
Both smoke detectors can be affected by dirty environments, combustion particles, caustic vapors, gas, and fumes that may lead to maintenance issues and nuisance alarms. If air contamination is inevitable, then heat detection may be a better solution for the area in question. It is important to understand the environment a detector is protecting in order to select the correct detector type and increase the lifespan of a smoke detector.
Placement and location
The efficiency of ionization or photoelectric smoke detectors to detect smoke conditions can be adversely affected if obstructions, ceiling heights, and types of ceiling are not considered when determining their locations. Both detectors are capable of providing 900 sq. ft (30×30 ft)of coverage.
Today’s architectural design has challenged alarm designers with assignment of proper smoke detector coverage. When determining placement of detectors, NFPA 72 defines proper location and spacing and provides installation parameters that cover ceiling heights, types of ceilings, and obstructions, and provides minimum requirements for proper smoke detector placement. This national code is adopted by most, if not all, contemporary model building codes and should be referenced by all alarm designers.
Often overlooked, manufacturers’ installation requirements—as referenced by the device’s listing organization—are vital to correct operation. The manufacturers have had the smoke detectors tested and listed with Underwriter’s Laboratory (UL) for most installation scenarios. This should be one of the first references for design consideration; otherwise, additional costs may be incurred if detectors need to be relocated or removed. Manipulation of a previously installed detector could subject the detector to contamination and could shorten
Installation and interferences
The installation contractor should be aware of issues the smoke detector or smoke detector wiring can create if installed in an incorrect location. Smoke detectors that are installed within 1 ft of a fluorescent light can cause nuisance alarms. The fluorescent light generates an electrical interference that affects the smoke detector’s sensing chamber electronics.
Additionally, the fire alarm wiring to the smoke detector must be properly separated from lighting circuits as defined in NFPA 70 (National Electric Code). The lighting circuit wiring generates an electrical interference that can create noise on the fire alarm wiring. This also can be picked up by the smoke detector’s sensing chamber and cause nuisance alarms. In most cases when this problem occurs, the first solution is to replace the smoke detector instead of properly troubleshooting the problem and relocating smoke detector and/or the fire alarm wiring. If basic separationrequirements were followed at installation, the lifespan of the original smoke detector would not have been affected.
Installation too close to air diffusers can shorten the life of a smoke detector. Whether the diffuser is a supply or return, the air movement near the diffuser is strong relative to other places within a building, carrying more dirt and dust (contaminants) than normal. The dirt and dust will tend to build up faster within smoke detectors when they are too close to an air diffuser. This can result in nuisance alarms or trouble signals, and if the detector is not properly maintained, it will have a shorter life.
Lastly, the installation of smoke detectors should be done when all construction trades have properly cleaned the construction debris. However, most projects undertaken these days will not allow the installation contractor the appropriate time to install the devices after construction clean-up. As a result, protective coverings supplied with the detectors should be left installed until construction clean-up is completed. This will maintain the integrity of the detector while high contamination potential is present, thus prolonging the life of the detector. It is important to note that these protective covers are not UL listed to protect a smoke detector from contaminants, and some authorities having jurisdiction (AHJ) will not accept this type of contamination protection; all jurisdictions will require impairment procedures to be followed.
Set-up and commissioning
The next step in the process of installing smoke detectors is to program the fire alarm system for the smoke detectors. This includes setting up the inputs (smoke detectors) to control the sequence of the outputs (programmable modules). This process is known as establishing the sequence of operations.
Part of the sequence of operations can be to activate alarm verification or adjust the obscuration levels of the smoke detectors. Alarm verification is a process that is programmed to the smoke detector in which an alarm condition must exist for a predetermined period of time prior to notifying the fire alarm panel that the smoke detector has an alarm condition. Adjustment of the obscuration level is setting the sensitivity level of the smoke detector. Most fire alarm panels have a low-, medium-, and high-sensitivity adjustment.
The alarm verification and the sensitivity adjustment can reduce nuisance alarms and relieve the requirement for smoke detector replacement, thus prolonging the life of the detectors. These options are available on most fire alarm panels, but it is the AHJ who approves the use of alarm verification or adjustments in the smoke detector sensitivity levels. These programming changes should be applied cautiously, in consideration of the detector’s expected safety function, and are not a replacement for good judgment and consideration of environmental considerations. If misapplied, these programming changes can mask issues that can shorten the lifespan of a spot-type smoke detector.
Once programming is completed and pre-testing accomplished, the AHJ will commission the system for acceptance. This commissioning is helpful in several respects, but especially when identifying installation discrepancies and validation of the sequence of operations. (The term “a fresh pair of eyes helps a lot” applies here once installers and programmers look at the system for a substantial period of time; this is a chance to identify and resolve something that could get overlooked.)
Once the building is commissioned and occupied, the fire alarm system should be retested every 12 months. Part of this test will include recording the contamination levels of each smoke detector installed on the system. Most intelligent fire alarm panels provide this information through the password-protected maintenance screen and also will display the contamination levels of each smoke detector on the fire alarm system by device address. The contamination levels should be compared to the manufacturer’s operation and maintenance standards to determine acceptable contamination levels prior to nuisance alarms. The retesting contractor should then clean the deficient detectors.
Most fire alarm systems normally take 24 to 48 hours to reset sensitivity levels After that time, the detectors should be checked again through the fire alarm panel’s maintenance screen to validate that the cleaning of the detectors was successful in reducing the contamination levels. After this, deficient detectors will be removed from service, and cleaned detectors will have an increased lifespan.
The lifecycle of ionization and photoelectric smoke detectors include several factors that should be considered for the best safety function performance and lifespan. Fuel loads, fire scenarios, and related egress paths that should be the first consideration. Subsequently, environmental issues such as altitude, temperature range, humidity, and air contamination should be taken into account. Proper installation techniques should then be followed in reference to proper electrical separation, distance from air diffusers, and proper timing for smoke detector installation. The setup and commissioning of the fire alarm system and the periodic testing and maintenance of the ionization and photoelectric smoke detectors complete the process of keeping detectors functioning properly, active when needed, and silent when not.
|Authored by Rolf Jensen & Assocs. Inc., a fire protection and life safety subsidiary of the RJA Group.|