Since AC current flows rapidly in both directions, if and how was supervision achieved without sounding the horns?
They were wired in series, not parallel.
I know the old ones were, but was it the same for newer AC systems like the AC version of the 2001?
And even in the case of a series circuit, doesn’t supervision still involve passing current through the circuit?
Early AC fire horns were wired in series, not parallel, and there was no EOL in the field.
The first fire alarm control panels just provided power for the horns and supervised the horn wiring. Sometimes they had an auxiliary relay (for a Master Box or something).
A typical NAC would output between 24-240 volts AC with 1-3 amps available in alarm. As such the voltage at the individual horn would be equal to the output / the number of devices.
Let’s set up an AC control panel with some devices. Our panel contains a 240VAC 1-amp NAC and we want to put 24 horns on it: 240/24 = 10 volts, so each device would have to be rated to draw 10 volts and 1-amp.
Suppose we only wanted to use 12 horns: 240/12 = 20 volts, so if we still wanted to use 10 volt horns we would have to reduce the output of the NAC using a variable transformer (kind of like a potentiometer) to reduce output voltage to 120VAC.
Now, how would we supervise our NAC? Simple: during supervision, the panel engages a set of relays that put maybe 10-12VAC across the NAC. Recall, our series NAC with 24 signals divides the voltage across all the devices. 12V / 24 = .5VAC at each device. This would not be sufficient voltage to sound the horns. However, at the other NAC terminal at the panel is another relay. If the 12 Volts flows out across the series loop and the loop is complete, the 12 volts will keep the trouble relay energized. If a wire broke somewhere or a horn failed causing an open circuit, 12 Volts would not flow back into the panel and this supervision relay would switch states, turning on a light and trouble bell. Thus the NAC is supervised. But, because our horns are series wired, if any of them fail, all of them fail.
Once a pull station shorts an IDC, it turns on a relay that switches the NAC power from the 12-Volt supervision supply to the 240 volt horn supply, and the horns sound (assuming there are no broken connections in the circuit).
theboginator has it correct. In the older systems like 4247 or 4208 in supervisory mode a resistor and a relay coil were in series with the horns. Therefore only a few milliamps passed through the horn circuit. That is only a tiny fraction of the current it takes to operate the horns. The 2001-2077 series signal module used a very low current DC current to supervise the horn circuit. The operating voltage was supervised by another circuit on the module.
The 4247 age panels used a 100 ohm 160 watt adjustable resistor to set the current in the circuit. The actual name on the schematic is Gong Circuit Compensating Resistor. The 4208 and 2001 systems used a variac autotransformer to set the voltage to match the operating current of the series horn circuit.