What role does the relay play when main power is available in an emergency circuit?

In emergency circuits, the role of the relay when main power is available is to connect the lamp to the power source. This function is crucial because it ensures that the lamp receives the necessary power when the main supply is active, allowing it to operate as intended. The relay serves as an electrically operated switch that can close the circuit, enabling the lamp to illuminate when mains power is present, thereby providing necessary illumination during an emergency situation when other power sources are not required or in use.

In contrast, the relay does not disconnect the storage battery, as that would hinder the circuit's ability to function during a power outage. Turning off the sensor is not relevant in this context, as emergency circuits typically require all components to be operational to provide the intended service. Stopping the current flow would also disrupt the functioning of the lamp, which is counterproductive in an emergency setting. Thus, the function of connecting the lamp to the power source highlights the relay's critical role in maintaining illumination when the primary power is available.