While the aforementioned monitors can determine line integrity during a supervisory state and a short circuit in an alarm device in an alarm state, the monitors do not indicate where in the system a break has occurred during a supervisory mode or whether a break has occurred in the alarm mode. The monitors also fail to indicate which alarms are inoperative due to a break in the wiring of the system or due to a failure of an alarm device. Information regarding the location of the break and the operability of the alarms can be useful to emergency personnel. Without alarm notification, occupants may remain in a building during an alarm state, for example. Knowledge of where a break in line integrity occurs can provide emergency personnel with information regarding which occupants should be personally warned of an alarm state in a building.
During a fire emergency in the aforementioned alarm systems, the electrical conductors and alarm devices themselves are subject to damage caused by a fire or the resulting heat. Certain types of Circuit Integrity wiring can withstand direct flame for up to two hours. The characteristics of the wire, however, will change with this exposure. For example, the resistance of the wire will increase when exposed to direct flame. With such a change in the wire, the alarms used to warn of the fire may become inoperative. The change in resistance of the wiring, leading to alarm failure, cannot be detected with the current alarm systems.
The present alarm system detects the failure of an alarm device connected to the system. The alarm system will also detect not only a break in the line integrity of the system, but the location of the break. Furthermore, the alarm system can detect the change in resistance of the wiring in the system caused by exposure to heat which, in turn, can predict the potential failure of an alarm system.
The alarm system can include an electrical conductor, a plurality of alarm devices powered from the electrical conductor and a load sensor which senses the electrical load on the electrical conductor to indicate failure of one or more devices. The electrical load measured by the load sensor is proportional to the number of alarm devices powered from the electrical conductor. A decrease in the electrical load of the system indicates failure of at least one alarm device. The alarm system can also include at least one wire integrity sensor to monitor for breaks in the electrical conductor during supervisory mode.
The plurality of alarm devices in the system can be notification appliances, such as audible devices or light strobes. The alarm devices can also be sensors, such as smoke or temperature sensors. The load sensor can measure either current in the electrical conductor, such as by sensing voltage across a resistor connected in series with the electrical conductor, during an alarm state and compare this measurement against a baseline or initial electrical load value. Any deviation between the initial load and measured load indicates failure of an alarm device. The initial electrical load in the alarm system can be measured during the initialization of the system. When the load sensor is active, during an alarm state, the sensor indicates the number of alarm devices active in the alarm system.