In any fire alarm system, the smoke detector detects smoke, and, because the smoke detector detected smoke, the fire alarm control panel activates into alarm.
Conventional Fire Alarm Systems
On a conventional fire alarm system, the smoke detector, when it detects enough smoke, latches into alarm. It also sets the Initiating Device Circuit (IDC or Zone Input Circuit) into alarm. The IDC activates the control panel's zone into alarm, and the control panel latches into alarm.
The power to operate the detector comes in on the IDC. The only way a conventional smoke detector will un-latch from the alarm condition is for power on the IDC to be dropped and restored.
Pressing the reset button on the control panel will cut power on the IDC for a few seconds, and unplugging the smoke detector from the IDC cuts power. Either way, the smoke detector is reset.
If there's still an alarm on the IDC, the control panel can't reset.
With the conventional fire alarm system, if a device is removed from the IDC for any reason, the IDC no longer receives an alarm from that device.
Addressable Fire Alarm Systems
On an addressable system, which uses a Signaling Line Circuit (SLC) to power the addressable detectors and modules, things work a little differently. Using the SLC, each detector and each module sends digital information to the control panel, and using the SLC, the control panel sends digital information to each individual detector and module.
Included in this information on the SLC is the specific address of the detector or module. That way, the panel can specify which device should take action, and the panel can determine which device sent information.
Sensing Smoke
For some addressable fire alarm systems, when a smoke detector senses smoke, the detector latches into alarm and also uses the SLC to send the alarm to the panel.
For other addressable fire alarm systems, when a smoke detector senses smoke, the detector uses the SLC to send the information to the panel, and the panel decides if there is an alarm. With these other systems, the panel then uses the SLC to tell the detector that the detector is now in alarm.
The Panel and the Detector in Alarm
Any time a detector or input module detects a fire, two devices latch and stay in alarm: the control panel and also the detector or module. A key issue in the problem you are experiencing is the stopping of data between an alarmed smoke detector and the panel.
When there is an alarm, the sequence of going into alarm is, first, a fire is sensed by a detector or input module, and then second, the panel goes into alarm.
When the system is reset, the sequence of resetting is the panel's reset button is pressed, the panel uses the SLC to send a global reset command. After receiving the global reset command, if the specific detector that sensed the fire in the first place uses the SLC to say that it's reset, the panel can reset.
If the panel cannot get back the signal saying the formerly-alarmed detector is now reset, the panel will stay in alarm.
Not-Normal System
Physically removing a smoke detector that is showing an alarm isn't normal-to-the-system. Whenever something is done to the system that is not-normal, not-normal results are expected.
In this case, the detector was already in alarm, and the panel was in alarm when the detector was taken down. Depending on the exact programming of the computer inside the fire alarm panel, several outcomes can happen when the alarmed-detector was removed.
Apparently, on this system, even though the system was silenced, when the panel lost communication with an already-in-alarm smoke detector (that's when the detector was pulled down), the panel defaulted to re-activating the alarms and strobes. This could be a life-safety feature, or it could be a just-the-way-it-is feature.
Technical Support
Contact the fire alarm system manufacturer's technical support team to get further information.
Douglas Krantz
