The System Sensor 2251E was an analogue addressable optical type smoke detector for use on System Sensor protocol fire alarm system loop circuits. This detector has now been made obsolete by System Sensor but has now been replaced by the System Sensor 22051-26-IV which can be used as a like for like replacement.
The System Sensor 22051E-26-IV uses the same B501 type detector base as the old 2251E unit and can be used as a like for like replacement. This means that the detector can be simply installed into the existing base if it is a replacement for a faulty detector. The System Sensor 22051E-26-IV Smoke Detector also has the same Ivory (Off-White) colour as the original 2251E detector, which means it will match the colour of the existing base.