Smoke detectors are employed where the type of fire anticipated will generate invisible and visible products of combustion before temperature changes are sufficient to actuate heat detectors.

Photoelectric detectors
Photoelectric detectors are of the spot type or light-scattering type. In each, visible products of combustion partially obscure or reflect a beam between a light source and a photoelectric receiving element. The disruption of the light source is detected by the receiving unit and an alarm actuated.

Combustion products detectors
Ionization detectors and condensation nuclei detectors are spot detectors that alarm at the presence of invisible combustion products. Ionization detectors have proven to be reliable and are the most common type of early warning detectors.

They will alarm in the presence of both visible and invisible combustion products. Condensation nuclei detectors operate on the cloud-chamber principle, which allows invisible particles to be detected by optical techniques.

Flame detectors
Flame detectors are spot detectors that are usually used in flammable liquids operations and have limited use in substations. These detectors alarm at the presence of light from flames, usually in the ultraviolet or infra-red range.

Detectors are set to detect the typical flicker of a flame. Detectors may be provided with a time delay to eliminate false alarms from transient flickering light sources.

Air sampling detectors
Air sampling detector systems continuously draw air samples through sampling heads or ports to detect submicron particles generated during the incipient stages of a fire. To detect the presence of these particles, the systems use either the light-scattering or cloud chamber method.

Both systems are capable of several levels of preprogrammed alarm thresholds. It is feasible to provide a staged, early-warning regime that responds to increasing levels of concern.

Linear beam smoke detectors
Linear beam smoke detectors consist of a light transmitter and a light receiver that electronically evaluates the received light. If smoke passes through the beam and the received signal falls below a preset value, an alarm is activated.

Slow changes to the received signal caused by dust accumulation or other environmental influences are offset by means of a compensating circuit. If the limits of compensation are reached, if the beam is obstructed, or if the housing cover is removed, the receiver initiates a fault signal.

These detectors are capable of monitoring over long distances and are ideal for large or narrow rooms, e.g., corridors, store-rooms, and machine halls.

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