Spot and linear are two subclassifications of detection devices. Subclassifications group detectors based on their function and the types of scenarios they will be used in. Spot type devices are detectors that centralize their detection around a single point in the room. Around that point, the detector has a certain radius where it effectively detects fires. Since the detection centers around a single spot, you may need to plan your detector placement carefully to ensure full coverage in your building.
In contrast, linear type detectors have detection areas that concentrate along a path, and typically do not have a large radius of detection outside of that path. To clarify, the NFPA 72 defines line-type detectors as “a device in which detection is continuous along a path.” This means that they are typically more suitable for large areas, such as atriums or entertainment halls. Linear type detectors are usually more expensive than spot type detectors, however they have the ability to cover a much larger area than most spot detectors. Due to this difference in area coverage, economically using linear detectors can save you money and get your facility fully protected.
Some detectors that are associated with spot type detection are certain gas detectors, bimetallic detectors, fusible alloy detectors, pneumatic rate-of-rise detectors, carbon monoxide detectors, smoke detectors, and some thermoelectric detectors. Below, I will go into more detail of these types.
The most prevalent type of detection devices are photoelectric and ionization smoke detectors. These detectors contain sensors that can detect the presence of smoke, and thus, can activate an alarm to warn occupants about a fire. In photoelectric detection, smoke reflects light onto a sensor, activating the alarm state; however, in ionization detectors, a small amount of radioactive material creates a current, which smoke must obstruct to activate the alarm. To learn more about how these detectors work, you can check out our photoelectric vs ionization article here.
Sometimes, more simple applications use fusible alloys to detect heat. In the context of fire safety, fusible alloys are metals that are designed to melt at certain predefined temperatures. Often, this will physically cause a chain reaction of events that will cause a desirable response to the raise in temperature. Manufacturers commonly bond these alloys to fire sprinklers, where they refer to them as fusible links. At specific temperatures, the fusible links melt, allowing the sprinkler to release water. In other applications, fusible alloys are utilized as a pressure relief device in the event of overheating.
Keep Reading: The Difference Between Standard and Quick Response Fire Sprinklers >>
Thermoelectric detectors, exemplified by the Thermotech 302 series, sense heat through metal struts with opposing open contact points. Typically available as spot or linear types, these detectors spring into action when a specific temperature threshold is reached, causing the contact points to actuate and activate the alarm. Often, we hermetically seal these devices to protect sensitive electrical components from heat, dust, and dirt. Notably, the 302 series models feature reusability, and require no resetting or cleaning post-use, making maintenance extremely hassle-free.
Some examples of linear detectors include heat-sensitive cables (sometimes called linear heat detectors), projected beam smoke detectors, aspirating smoke detectors, and flame detectors
One example of a heat sensitive cable is the Protectowire Linear Heat Detector. It is a cable with two steel conductors covered by a heat-sensitive polymer cable. When a fire or high temperature is nearby, the polymer cable shielding will yield to pressure and may rupture. Afterwards, the resulting short circuit will trigger an alarm signal in the fire alarm control panel. The wire spans over a large area, and may detect fires over any part of the cable. In summary, the portion of the cable that is closest to the fire will likely short and activate the alarm state.
Protectowire heat sensitive cables have become popular for fire detection in cold, dry storage. These detectors, the CTI/CTM series in particular, are FM and UL listed to -60°F, and are designed for use in refrigerated storage applications. In contrast, most point smoke detectors have an operating temperature range above 32°F. To add, point smoke detectors are not guaranteed to function with condensation or icy conditions. Thus, for food or refrigeration protection, linear heat detection is an ideal solution.
Beam detectors emit light across the protected area. After that, a reflector will send that light back to the transceiver unit. Under normal circumstances, the light consistently reflects back to the transceiver. However, if smoke is present, it blocks the light, which activates an alarm signal within the device. It’s a straightforward yet effective method for detecting smoke over larger areas, such as in warehouses or large open spaces.
The primary types of flame detectors are infrared (IR), ultraviolet (UV), video, Multi-Spectrum Infrared (MSIR), and IR/UV combination detectors. Each of these flame detectors have a sensor that detects particles emitted by flames. However, these flame sensors may not perform as effectively in harsh conditions, such as high fog or humidity.
Video and image detection (VID) has also become more prevalent as flame and fire recognition technology improves. As a testament to this, VID detectors that are inspected, tested, and maintained properly are officially recognized by the NFPA. Although VID can provide convenience, there is no occupancy that outright requires the use of video image technology.
In an aspirating smoke detector (ASD) system, a network of polymer tubing draws in air and carries it to a detector in a separate location. Afterwards, the detector analyzes the air for the presence of smoke. Since air is drawn in by fans and subsequently filtered, ASD systems are extremely accurate and provide very early fire detection even in highly contaminated or high airflow spaces. This tubing is easily hidden, so ASDs are used for either aesthetic or efficiency purposes. Common places to use ASD systems are office spaces, historic buildings, data centers, and water treatment plants.
Knowing the differences between various detectors helps you make better purchase decisions for your home or business. While you may still keep up to code by simply installing as many point smoke detectors you can buy, it is often not as economical to do so for large businesses. Incorporating line detectors offers numerous benefits, including cost-savings, faster fire response, and improved detection accuracy for covering larger areas. Finally, a full fire protection system will likely incorporate both line and point detectors to increase coverage and reduce risk, especially in high-hazard environments.
