Fire Alarm -- Description
Early fire alarm systems used a second source of power just for the trouble sounder. If the primary source of power was lost, the trouble sounder could at least alert someone of the trouble.
What Is Fire Alarm Trouble Power?
By Douglas Krantz
In the "Old School" type of fire alarm system, before the smoke detector was invented, even though the fire alarm systems used AC, all voltages were in a steady state: either on or off. Until someone threw a switch (manual station), the power to the fire bell stayed off.
At this point in time, because the fire alarm systems used power line AC voltages, batteries couldn't be used as a standby power source.
IDC as a Turn-On Circuit
On the Initiating Device Circuit (IDC), all input devices were just on/off switches; when there was a fire, the glass was broken on the manual station and a push button switch popped out, turning on the power to the fire bell.
Supervising the Wiring
Normally, the switch (manual station) and the bell were wired in series and connected to building power. Like turning on a light, turning on the switch inside the manual station would turn on the bell.
But, what told anyone when a wire broke or a connection came loose? A broken wire would effectively disable the fire alarm system --- this would be bad.
In case a wire broke or a connection came loose, the wiring to the manual station and the bell had to be monitored for integrity; it had to be supervised. This was accomplished using a trouble relay inside a centrally located fire alarm control box.
Adding a Trouble Relay
So it could supervise all the wiring, the trouble relay coil was wired in series with the switch and the bell. Across its contacts, the manual station switch had a resistor allowing enough supervision current to bypass the open switch and keep the relay energized.
The resistor, though, was high enough in resistance to keep the supervision current down so the bell wouldn't sound.
The trouble relay would switch between the "normal" indicating light on the panel and the "trouble" indicating light. A buzzer would also come on when the trouble light came on.
If the supervision current stopped, like when the wire broke or a connection came loose, the trouble relay would de-energize, turning off power to a "normal indicating light" on the control box, and switching power to the trouble indicating light and the trouble buzzer.
Still there was a big issue with the AC power for the fire alarm system. If the fuse providing power to the fire alarm system blew, the fire alarm system wouldn't work, and the trouble buzzer also would not sound. Unless the box was in a prominent place, the normal light failing to be lit up on the control box wouldn't be noticed for quite a while; a fire in the meantime would be a serious problem.
Second Source for Power
It was decided that the power for the indicating lights and trouble buzzer would come from another source. Usually this other source of power was an emergency power panel on a separate power entry from the building's main power panel. Because this was a separate power entry into the building, even the main fuse for the building would be bypassed to run the fire alarm trouble light and buzzer.
A few of the old AC only fire alarm systems are still in use, but newer fire alarm panels (half a century old is considered new by some standards) have a standby battery system instead of a separate trouble power source.
In case a wire breaks or power to the main panel is lost, this standby battery system is not only used to keep the trouble light and buzzer (sounder) operating, but when there is a power loss to the building, this battery (always kept charged by the fire alarm control panel) keeps the whole fire alarm system operating.