In conventional systems, detectors are hard wired into a circuit or zone. They signal a fire condition to a control panel by changing from a high to low impedance. The panel detects this impedance change by current monitoring and identifies the zone. Individual detectors cannot be identified. In two-wire conventional systems, fire detection and alarm products are connected to the same pair of supply wires, making installation quicker, less expensive and more flexible than traditional four wire systems. Two-wire systems are particularly well suited for small to medium sized buildings.