We have to look at what horns and strobes are in the first place. Horns and strobes are On-Off devices:
- When they are on, they are making lots of noise and flashing lights, they are sounding the fire alarm. Think of a light in the room being turned on - the light is using lots of current; the horns and strobes are using lots of current.
- When they are off, they are silent and not flashing, they are not sounding the fire alarm. Think of a light in the room being turned off - the light isn't using any current; the horns and strobes aren't using any current.
Constant Checking of the Wires
Wires are used to carry the electricity to the horns and strobes. However, except on rare occasions, the horns and strobes aren't being used, so the wires aren't being used. Then means that almost all of the time, the wires just sit there.
A very long time ago, people concerned with fire alarm systems figured out that the wires sometimes come loose from connections, or even sometimes break. If the wires aren't being used, no one will know that there's a problem with the wires. If a real fire happens, and a wire has come loose from a connection, no one will be warned about the fire.
To solve that problem, the came up with a way of always checking the wires. They ran a very small electrical current through the wires to "Supervise" the wires. That way, if the current stopped, the panel could sound the trouble buzzer, and the loose connection or the broken wire could then be fixed.
Forward and Backward Current
- When there's a fire alarm, the panel provides lots of forward current. The panel has to provide lots of current to run the horns and strobes. The horns and strobes are made so they only use forward current.
- When there isn't an alarm, the panel provides very little reverse current. The panel only provides a very small current because the only thing it is doing is supervising the wires. The horns and strobes are made so they won't use reverse current.
When the panel is supervising the wires, the current is too small to power any horns or strobes. That's why, for practical purposes, the panel doesn't provide any current for the horns and strobes when the panel isn't in alarm.