The Purpose of the Form C Relay in Smoke Detectors

Relay Types

• Relay forms are categorized by the number of poles and throws in addition to the default position of the relay. Form A is a single-pole single-throw (SPST) relay with an open default position. Form B is an SPST relay with a closed default position. Form C relays are single-pole double-throw relays and break the connection with one throw before making a connection to the other pole.

Control Panel

• Smoke detectors that are equipped with an auxiliary form C relay can be interconnected and optionally controlled from a central panel. The control panel can test all of the alarms in the series from one location. The operator of the control panel is able to choose whether all alarms sound together or individually.

Interconnection

• A series of interconnected smoke detectors that are not connected to a control panel will all sound when one alarm is activated. Each detector in the series has an exterior test button and you will need to test each unit individually.