The purpose of the Thermal Time Delay Relay is to limit the self energizing time period of the Ignition and Safety Relays and to stop the Siren Engine upon completion of the "Warning" or "All Clear" Signal, after a suitable Engine Idling time period.

This is accomplished by controlling the operations of the Thermal Contact Relay, which it does by opening and closing the energizing circuit to the Thermal Contact Relay, by its single pole switch.

The Thermal Time Delay Relay has a normally open single pole switch and a Thermal Coil. The Thermal Coil, when energized, heats up in Ten to Twelve seconds and closes the switch, when the circuit to the Thermal Coil is opened, it will cool in approximately One to Two minutes and open the switch.

THERMAL CONTACT RELAY

The purpose of the Thermal Contact Relay is to complete a Ground Circuit to the Ignition and Lock-in Relays, at a definite and timed sequence. It is energized and controlled by the Thermal Time Delay Relay.

The Thermal Contact Relay has a double set of contacts, one set normally open, the other set normally closed. When the Relay is energized, the normally open and closed position of the contacts is reversed.

The first act of the Thermal Contact Relay is to furnish a Ground circuit through its normally closed contacts, #3 to #5, to the Ignition Relays SOL terminal, to keep the Ignition Relay energized, after the initial Four second Ground Circuit from the Remote Control Relay energized it to start the Siren Engine. After Ten to Twelve seconds, the Thermal Time Delay Relay will heat sufficiently to close its contacts to energize the Thermal Contact Relay, causing its contacts, #3 to #5, to open the Ground circuit to the Ignition Relay and close its contacts, #3 to #4, to complete the Ground circuit to the Lock-in Relay TH terminal, energizing the Lock-in Relay. When the Siren Signaling operations are completed and a suitable Engine Idling time period has elapsed, the Thermal Time Delay Relay will cool and open its contacts, this will open the circuit energizing the Thermal Contact Relay which in turn will open its contacts, #3 to #4, breaking the circuit to the Lock-in Relay, and the Siren Engine will stop.

The operations of these two Relays are performed in conjunction with the Control Circuits when the Siren is operated from the Remote Control Station. The Thermal Time Delay Relay and the Thermal Contact Relay does hot have any control during the Local Control Station Operation.