The model number suffix  “-2CO”, indicates that the detectoir is a double circuit with one set of Normally Open (N/O) contacts, and one set of Normally Closed (N/C) contacts. The second set of N/C contacts is often used to activate a local ancillary function that includes elevator recall, release of magnetically-held doors, local signal operation, local annunciation, etc.  In many cases this set of contacts will be connected to a controlling relay that is used to switch heavier voltages.

 Combination Rate-of-Rise & Fixed Temperature

The prefix “CR” in the model number indicates that the detector is a combination Rate-of-Rise and Fixed Temperature,(often referred to as “Dual-action”), unit. The Rate-of-Rise function allows the detector to close one set and open the other set of its contacts when the temperature at the ceiling increases at a rate of 8.4 Celsius degrees (15 Fahrenheit degrees) per minute.  In most cases, the closing of one set of contacts initiates the Fire Alarm sequence.  The second set of contacts is commonly used to initiate an ancillary function. The Fixed Temperature portion consists of a spring-loaded plunger held in place by a eutectic solder that will fuse at the specific temperature (in Fahrenheit degrees) as indicated by the Model Number i.e. 135,165, 200 and 285 degrees.
Fixed Temperature Only
The prefix “CF” in the model number indicates that the detector is Fixed Temperature Only, and will not respond to a rate of temperature increase but will operate when the detector fuses at the prescribed (Fahrenheit) temperature as indicated by the model number, i.e. 135,165, 200 and 285 degrees.  This detector is referred to as “Fixed Temperature Only, non-restorable”.