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Siemens OP121 Photoelectric Smoke Detector 

OP121 SIEMENS Detector placement for a clear air, 0 to 4000 ft/min velocity application, use 30 foot center spacing (900 sq ft) from NFPA Standard 72 initiating devices chapter and CAN/ULC-S524 as a starting point for a detector installation layout. This spacing, however, is based on ideal conditions – smooth ceiling, no air movement, and no physical obstructions. In some applications, considerably less area is protected adequately by each smoke detector. In all installations, place the detector on the ceiling, a minimum of 6 inches from a side wall, or on a wall, 6 inches from the ceiling.

TO AVOID NUISANCE ALARMS: DO NOT locate the OP121/OH121 detector where excessive smoke concentrations exist under normal conditions, or in areas of prolonged high relative humidity where condensation occurs.

DO NOT locate the OP121/OH121 detector next to an oil burner, kitchen, or garage where exhaust fumes can trigger an alarm. Other causes of false alarms are dust accumulation, heavy concentrations of steam, heavy pipe or cigar smoke, and certain aerosol sprays.


Before a detector can sense a fire, the products of combustion or smoke must travel from the fire to the OP121 Siemens. Since their travel is especially influenced by air currents, consider the movement of air in the design of the system.

While combustion products tend to rise, drafts from hallways, air diffusers, fans, etc., may help or hinder the travel of combustion products to the detector. When positioning a detector at a particular location, give consideration to windows and doors, both open and closed, and to influencing air movement. Never install a detector in the air stream of a room air supply diffuser. It is better to position a detector closer to an air return.


To assure proper operation of the detector and control panel, it is required that both the Sensitivity test and the Functional test be conducted using the procedures outlined below.


The OP121/OH121 monitors its smoke sensitivity automatically and requires no test equipment. A green flash of the detector LED about every 10 seconds indicates that the smoke sensitivity is within its listed limits.


To test the detector and control panel for functional operation, use SIEMENS Test Gas, P/N 500-649750. Follow the instructions on the gas canister label.

Smoke entry test mode: To test the detector for smoke entry, place the detector into a test mode. The test mode condition is automatically started after the initial power up and after every reset for a period of 3 minutes. It is recommended that a control panel reset be performed after every smoke entry test is conducted to restart the 3 minute test period. The test mode is indicated when the green LED is double blinking every 10 seconds. A normal operational mode is indicated

NOTE: this part replaces Siemens PE-11.

Datasheet OP121.

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Additional information

Weight 0.2 lbs
Dimensions 5 × 5 × 5 in
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