INDICATING CIRCUIT (also known as an INITIATING CIRCUIT):
Describes the field wiring and terminations in the FIRE ALARM or SECURITY SYSTEM's common control to which the building's INPUT DEVICES are connected.
INPUT DEVICE: Any of a family of devices designed to detect unauthorized access, fire, smoke, flood, motion or any condition requiring notification or response by a building's occupants or a CENTRAL STATION. INPUT DEVICES can be MAGNETIC DOOR CONTACTS, GLASS BREAKAGE DETECTORS, MOTION SENSORS, PHOTOELECTRIC BEAMS, SMOKE DETECTORS, WATER DETECTORS, LOW TEMPERATURE DETECTORS, etc.
INTELLIGIBILITY: Is the means of measuring of the clarity of verbal announcements over a public address system. Two measurement scales have been established: CIS (Common Intelligibility Scale) and STI (Speech Transmission Index).
ISOLATOR: Any of a family of field devices designed to ensure survivability of the fire alarm system if one segment thereof becomes compromised by an open circuit or short circuit condition. There are three types of isolators which may be deployed:
Data Communication Loop (DCL) Isolators (sometimes referred to as SLC Isolators) ensure communications with active field devices such as heat, and smoke detectors, manual initiating stations, and signalling modules remains unaffected (or is minimized) by a circuit trouble condition. While it is recommended that DCL isolators be located in an electrical service room, they can also be deployed in the field;
Signal Circuit Isolators are normally installed to serve suites in a residential occupancy. One such isolator will handle two suites. If the in-suite signalling appliance (usually a buzzer) is compromised for any reason, the signalling devices in the other suites serving the floor will remain unaffected. A “trouble” condition will be displayed on the common control however;
Power Buss Isolators are designed to protect the integrity of a power distribution riser usually employed by a distributed (addressable) system (some conventionally wired systems may also employ these units).
We review the testing procedures of these three types of isolators in this FAQ.