Most fire alarm control panels can provide a limited amount of power for the horns and strobes, but extra power for the horns and strobes can be provided by a Booster Power Supply (BPS). The term "Trigger Voltage" for the BPS is a shorthand term for "Whatever voltage is necessary to turn on the Booster Power Supply". Depending on how the manufacturer wants to call the box, the BPS could be called NAC Expander, Signal Power Expander (SPX), Expansion Power Supply, Power Expander, Expander Panel, Etc.
Of course, when there is an alarm, "Trigger Voltage" is required to turn on the BPS; the "Trigger Voltage" is used to turn on the BPS. When the "Trigger Voltage" is applied to the control input terminals of the BPS, the BPS turns on, and the BPS stays on as long as the "Trigger Voltage" is present. Once the trigger voltage is removed, the BPS turns off.
Most BPSs will turn on (activate into alarm) when a positive
8 to 30 volts is applied to the control input to the BPS. Usually, even a 9-volt battery can be used to provide this voltage. When sounding the alarm, most Fire Alarm Control Panel NAC circuits also provide a positive
24 volts to a Notification Appliance Circuit. This positive
24 volts falls within the positive
8 to 30 volt turn-on range for the BPS; the BPS turns on into alarm.
Depending on the manufacturer, when they are not in alarm, most NAC circuits provide a negative
1.8 to 19 volts. This negative
voltage does not fall into the positive
voltage range required to turn on the BPS; the BPS stays off.
It all boils down to how the BPS is turned on. If the BPS is connected to the NAC output on the Fire Alarm Control Panel, the BPS will turn on whenever the control panel is sounding the alarm.