Alarms indicate faults in the HVAC plant and building automation and control system, and let you initiate corrective action, where appropriate. The management of alarms (generation, signaling, acknowledgement) is in compliance with the BACnet standard.
There are two alarm types:
- OFFNORMAL
- FAULT
OFFNORMAL
OFFNORMAL alarms (process alarms) occur when a process variable assumes an inadmissible value. What is inadmissible is determined during engineering. The relevant parameters are stored in all alarm-generating objects. An OFFNORMAL alarm always indicates a fault in a plant, while the automation system itself works properly.
Examples of OFFNORMAL alarms:
- Temperature in HTHW circuit is too high or too low
- Alarm generated by fire detection system
- A damper-motor feedback signal has not been received
- A time schedule cannot carry out a command
FAULT
FAULT alarms are faults in the automation system itself (internal alarms). You cannot define the cause of a FAULT alarm during engineering. Nor is it possible for the user to suppress or otherwise influence the monitoring of FAULT alarms. FAULT alarms are intrinsically linked to the system. A FAULT alarm always takes precedence over an OFFNORMAL alarm from the same alarm source, because in the case of a FAULT alarm, there is some uncertainty about the reliability of the alarm source.
Examples of FAULT alarms:
- Faulty sensor (open circuit, short circuit, etc.)
- Buffer for storage of non-volatile data full
- Access to an I/O module failed
- Bus open circuit (RX integration)
Alarm detection procedure
Every alarm (OFFNORMAL or FAULT) can be uniquely allocated to a source. The alarm monitoring system is based on the principle of Intrinsic Reporting or Algorithmic Reporting as defined in the BACnet standard.
Intrinsic reporting
Intrinsic reporting means that alarm monitoring (target-actual comparison) takes place within the alarm-generating object itself (the alarm source). For this purpose, the function block contains the entire alarm state machine. Alarm detection does not require any function blocks with external functions. The alarm behavior of the object is defined by setting variables in the alarm-generating object (function block).
Algorithmic reporting
Algorithmic Reporting means that alarm suppression (target-actual comparison) occurs outside the alarm source. The alarm state machine is not located in the function block of the alarm source. For alarm detection, function blocks with external functions are required. The object's alarm response is not parameterized using variables of the monitored object (function block).