Douglas Krantz - Technical Writer - Describing How It Works
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Quadruple Use of the Initiating Device Circuit (IDC)

An IDC or Initiating Device Circuit does more than carry an alarm signal from a detection device to the control panel, it also watches for shorts or breaks in the wire, provides power for some devices, and reset some devices.

Four uses of the IDC - carrying the alarm, self-supervision of the wiring, powering the smoke detectors, resetting the smoke detectors.
The Initiating Device Circuit is a pair of wires, but what it does is carry alarms to the control panel, check its own wiring, provide power, and reset the detectors.

By Douglas Krantz

When troubleshooting, knowing how a circuit works makes it easier to understand the problems. Take, for instance, an Initiating Device Circuit. Just how does an Initiating Device Circuit or IDC work? Another way of asking the same question is, what does the IDC do?

On the surface, the IDC looks like a pair of wires that is installed in a precise method (at least according to NFPA Code). The trouble is that you don't come on site to perform service on a fire alarm system that is working (except for performance testing to confirm that the fire alarm system does indeed work); you come on site to perform service to a system that is not working. When looking for problems, until you know what the problem is, everything has to be suspect.

Having come on site, your job is to find out what is wrong so it can be fixed. As with any other circuit, if you understand how an Initiating Device circuit is supposed to work, the problems you came to fix can be found easier. The book "Make It Work - Conventional Fire Alarms" explains the following in greater detail.

Initiating Device Circuit

In a fire alarm system, the Initiating Device Circuit (IDC) is an input circuit or detection circuit: it carries the alarms detected by devices like flow switches, pull stations, heat detectors, tamper switches, etc.

A second use of the Initiating Device Circuit is to monitor itself, it supervises its own wires throughout the building. If the wire loses continuity, the panel will show that there's trouble on the circuit. To do this, in essence, the panel pushes current through the wires, and if the current stops, the panel detects a broken wire.

Most conventional fire alarm systems, however, also use the IDC for other purposes: to operate smoke detectors, the Initiating Device Circuit provides power; during the reset process to reset the smoke detectors, the IDC interrupts the power.

Dual Use --- Original Fire Alarm System IDC

Originally, input circuits for fire alarm systems sent signals one way: alarms from the input device to the fire alarm panel. A pull station, for instance, was used mainly as an on-off switch for the fire horns. For all practical purposes, a device in alarm shorts out the IDC.

However, it was understood that the wires carrying the alarm was extremely important. If a wire broke, the fire alarm system was useless (no shorted circuit detected at the control panel). In order to supervise the wires, the control panel ran a small amount of current through the wires and the end-of-line (EOL) resistor to check the continuity of the wires.

Right away, if a wire broke or a connection came loose, the panel could detect the interruption of current, sound the trouble buzzer, and the wire would be fixed.

For many years, these were the only purposes of the Initiating Device Circuit.

Third Use --- Power for the Smoke Detectors

Around the middle of the last century (1930's to 1960's is the middle of the last century) smoke detectors were developed. The problem is that smoke detectors are active devices; they take power to operate.

The first smoke detectors were powered using utility power or an external auxiliary power source. (Conventional Duct Smoke Detectors still use utility power or auxiliary power.) Eventually, though, power to operate most common area smoke detectors came from another source - - the Initiating Device Circuit or IDC.

This was a common-sense source of power. Those designing fire alarm systems saw that supervision power was already provided on the circuit. If the power used to supervise the wires was already there, all that had to be done was to boost this supervision power a little so the power-hungry smoke detectors had enough power.

Another issue with the extra power is that if the smoke detector just shorted out the IDC, the voltage on the IDC would be shorted out, and without the voltage to latch it in alarm, the smoke detector would quit shorting the wires. That would be kind of self-defeating, wouldn't it? To solve this problem, smoke detectors just draw more current (so they wouldn't short out the latching voltage) and the control panel was designed to go into alarm when the IDC just drew more current. (This becomes a compatibility issue if the fire alarm control panel and the smoke detectors aren't properly matched.)

Fourth Use --- Resetting the Smoke Detectors

Of course, resetting the smoke detectors to a normal state after an alarm was an issue. Pull stations are easy; resetting a manual pull station can be done by simply returning the pull station's internal alarm switch back to a normal position. Resetting a smoke detector is a little harder.

When first powered up, by code, all fire alarm devices have to be in a normal condition: not in alarm. This idea is used to reset the smoke detectors.

By turning off the power to a smoke detector (turning off the supervision voltage), any alarm condition on a detector is cleared. Once a detector has been turned off, re-applying the power to the detector means that the smoke detector is now normal - - ready to detect smoke again.

In essence, by cycling the IDC's supervision power off and on during the panel's reset process, the panel resets the detectors.

Four Uses of the Initiating Device Circuit

The Initiating Device Circuit (IDC) not only carries alarm signals from switches like manual pull stations, waterflow switches, heat detectors, etc., the IDC supervises its own wiring, provides power to operate the smoke detectors on the circuit, and it resets the smoke detectors.
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