Heat Detectors

Heat detectors are probably the most widely used initiating device for general-purpose automatic fire alarm systems. Some common types of heat detectors are discussed below.

SPOT TYPE OF FIXED-TEMPERATURE DETECTORS. - Fixed-temperature heat detectors that are categorized as spot type have a detecting element or elements that respond to temperature conditions at a single point or in a small area.

These detectors are shown in figures 7-7 and 7-8. Other fixed-temperature detectors are manufactured in the style shown in figure 7-9. The

Figure 7-8.\Replaceable-element fixed-temperature heat detector.

Figure 7-9.\Combination fixed-temperature/rate-of-rise heat detector.

spot type of fixed-temperature detectors is used mainly in unattended spaces to detect smoldering fires that increase the temperature of a detector above its design value, usually 135F to 145F or 185 to 200F. The higher temperature devices are used in spaces that may reach higher temperatures under ordinary conditions, such as boiler rooms, attics, or cooking areas.

The device usually is actuated by the melting or fusing of an element made of a fusible metal alloy. Actuated devices usually can be detected by visual examination.

In the devices shown in figures 7-7 and 7-8, the smaller diameter part in the center drops away. In figure 7-9, the dimple becomes a hole when the detector operates.

Fixed-temperature devices are often designed for one-time operation, and the whole device (figs. 7-7 and 7-9) or the element (fig. 7-8) needs to be replaced.

RATE-COMPENSATED DETECTORS.\ This type of detector is shown in figure 7-10. For low rates of temperature change (up to 5F per minute), rate-compensated detectors operate like fixed-temperature detectors. For higher rates of temperature change, the detector anticipates the rise in temperature to its set point and operates faster than the usual fixed-temperature detector. It automatically resets and is reusable when the

Figure 7-10.\Rate-compensated heat detector.

temperature drops below its design value. There is no difference in external appearance between an actuated device and an unactuated device; therefore, its status must be checked electrically.

RATE-OF-RISE DETECTORS.\These detectors are found in the styles shown in figures 7-7 and 7-9. Rate-of-rise detectors cause an alarm whenever the rate of temperature rise exceeds about 15F per minute. Heating causes an increase in air pressure inside the detector. A slow increase in pressure bleeds off through a breather valve, while a fast increase operates a bellows type of diaphragm, which operates the alarm contact, causing a signal. The detectors automatically reset after actuation and are reusable. Actuation is not visually indicated.

COMBINATION DETECTORS.\These detectors are found in the styles of figures 7-7 and 7-9. The combination detectors contain both fixed-temperature and rate-of-rise elements. If either element actuates, an alarm results. The fixed-temperature element is visible and actuates only once. If the fixed-temperature element actuates, the whole device must be replaced. The rate-of-rise element automatically resets and is reusable.

TESTING HEAT DETECTORS.\Test heat detectors semiannually on a rotation schedule to ensure that all devices will be tested over a 5-year period. During the semiannual tests, select at least one detector from each initiating circuit (zone) for

testing. Nonreusable detectors with replaceable elements can be tested by removing and reinstalling the element. Test and replace all nonreusable detectors in a 5-year period. The testing provides training opportunities and improves the alarm system reliability.

Keep accurate records of devices tested, their locations, and the rotation scheme so no devices are overlooked and so that other personnel can do the testing.

The spot type of heat-actuated detectors can be tested using various sources of heat. If the detector is located in a hazardous area that may contain explosive fumes or other highly flammable materials, use an explosionproof lamp. For nonhazardous areas, the heat source may be an infrared lamp, a hair dryer, or a hot-air gun. Be careful to avoid heat or smoke damage to reusable detectors and to the surroundings.

To test combination detectors that have a nonreuseable fixed-temperature element, test both the rate-of-rise and fixed-temperature features. First, use a higher heat level for a short period and direct it away from the fusible fixed-temperature element, if possible, to actuate only the rate-of-rise element. When an alarm occurs, allow cooling; reset; and then apply more gradual heat to actuate the fixed-temperature element.