When a fire alarm system is installed correctly and there is no damage to the system anywhere in the building, the fire alarm system isn't connected to ground anywhere outside the panel. The first part of the following explanation will be only about the pull station and how the
detector circuit (Initiating Device Circuit or IDC) works to detect fire and to detect an open wire or connection. The second part is about ground faults.
Normal
When the fire alarm system is green light normal, there are no alarms and all wires from the panel to the devices are complete.
At this time, the panel is
checking continuity of the wires to make sure all devices are connected. It's supervising the wires. The panel is causing a small amount of current to flow out of the panel on one of the screw terminals of the circuit, pushing this current all the way to the
end of line resistor
through one of the wires, through the end of line resistor, and back through the other wire and into the other screw terminal.
Trouble
Using this technique, the panel is checking to make sure all the devices are connected. If a device is disconnected, if a wire breaks, if a wire comes off of a connector, the current stops and the panel turns on its yellow light saying "
Trouble".
The end of line resistor limits the current from the panel. It is not a short on the circuit, so the panel doesn't consider that to be an alarm.
Alarm
A pull station closes a switch to "Short" the two wires of the circuit together. It is a wire-to-wire short. The panel considers this to be an alarm because lots more current flows.
A smoke detector in alarm is not a short, but it does draw a lot more current from the circuit. The panel also considers this extra current to be an alarm.
Ground Fault
The panel has a ground fault detection circuit inside it. This should be the only connection to building ground in the entire fire alarm system, anywhere in the building. There is a current limiting resistor in the ground fault detection circuit to prevent very much current at all from going from building ground to the panel. If any current does go through this connection, the panel not only turn on its trouble light, but because
a second ground fault is so dangerous, the panel turns on the ground fault light to say "Danger".
To get this ground fault light to turn on, another wire, somewhere in the building's fire alarm system has to make
contact with building ground. That connection makes a complete electrical circuit between the power supply in the panel, through one of the wires in the detector circuit, through the ground fault, through the building ground, through the ground fault detection circuitry, and back into the power supply in the panel.
Because the ground fault circuitry is in that path, the ground fault light turns on as a warning.
Single Ground Fault
That was a single ground fault. The panel turns on its trouble light because
a ground fault is a trouble, but everything else that detects fire and warns people of fire still works.
Second Ground Fault
That was a single ground fault. A second ground fault is different.
A second ground fault is a wire-to-wire short. If both wires of a circuit are making contact with building ground, the two wires may as well be shorted together like what a pull station does - Alarm!
The second ground fault doesn't have to be on the same circuit as the single ground fault. The fire alarm system is really just one big circuit, with many sub-circuits. If a ground fault is on one circuit in one part of the building, and a second ground fault is in another part of the same building, there could be a false alarm, or the horns and strobes could be shorted, or some other bad thing happens.
Pull Station vs Ground Fault
To send its alarm to the panel, a
pull station performs a wire-to-wire short; a
ground fault is a short between only one of the wires of a circuit and building ground. You don't want a second ground fault to ever occur, which is why there's a separate ground fault light to warn you to fix any single ground fault.
Douglas Krantz
