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Fire Alarm -- Description

The IDC is daisy chained so the wires in the circuit are carrying current at all times. The curret goes through the input terminals on each device and through the end of line resistor.
The Initiating Device Circuit uses Voltage produced by the fire alarm panel to drive Current through the wires of the circuit.

The end of line resistor has resistance to allow normal current through the circuit while keeping the voltage at a normal level, showing a normal circuit to the panel.

If the total resistance of the circuit goes up because a wire breaks or a connection comes loose, the current stops, the voltage goes up, and the panel sees trouble.

If the total resistance of the circuit goes down because a device on the circuit lowers its resistance, the current increases, the voltage goes down, and the panel sees an alarm.
Douglas Krantz -- Fire Alarm Engineering Technician, Electronic Designer, Electronic Technician, Writer

How Does Resistance Affect the Initiating Device Circuit (IDC)?

By Douglas Krantz

IDC circuits (Initiating Device Circuits) are input circuits to the fire alarm control panel, and the wiring is either a Class A or a Class B configuration.

Devices connected to the IDC cause the panel to do something: sound the alarms, operate relays, and call the monitoring company, among other things.

The IDC circuits look for alarm and supervisory signals from pull stations, smoke and heat detectors, waterflow switches, tamper switches, etc. These are all input (initiating) devices.

Two Wire DC Circuit

IDC circuits are two wire circuits, and are supervised for normal condition, alarm condition, and trouble condition. The voltages, currents, and resistances on the IDC are steady - - no AC or variations in DC levels, except when the circuit changes from normal to alarm or trouble.

Two-Way Supervision of the IDC

The supervision voltage is produced by the panel on its IDC screw terminals at all times; this voltage pushes the electrical current through the circuit. In return, the IDC, changing its resistance to affect the amount of current flow, changes the voltage seen by the panel on the screw terminals. This two-way realationship of voltage pushing the current and the resistance affecting the current can be seen in the book "Make It Work - Conventional Fire Alarms".

Three Resistance Conditions of the IDC - - Read by the Panel

The supervision voltage, seen by the panel using an internal voltmeter, tells the panel the circuit is Normal, the circuit has an Alarm or Supervisory, or the circuit is in Trouble.

Normal - - End-of-Line Resistor Resistance

To confirm that there are no breaks in the wire or the connections, the wires are supervised. Under normal conditions, the current through the IDC is restricted by the End-of-Line Resistor.

The resistance of the wires themselves is too small to be of any consequence. However, for the end-of-line resistor to be detected by the panel, the supervisory current has to pass through all the wires. To confirm that the devices are connected, this supervision current also has to pass through the input terminals of the devices.

By wiring the devices in a Class A or Class B configuration, the panel may not be able to tell if a particular device is working. At least, however, if the device becomes disconnected, the supervision current stops and the panel then shows a trouble condition to the owner.

Alarm - - Shorted or Partially Shorted Wires - - Low Resistance

This is a condition where one of the initiating devices (detectors, switches, etc.) is shorting out the Initiating Device Circuit, or partially shorting it.

The short causes the supervisory current on the circuit to rise. The rise in supervisory current causes the voltage at the IDC terminals of the panel to drop. The panel interprets this drop in supervision voltage as an alarm or a supervisory signal.

Of course, any other wire-to-wire short on the IDC will also be interpreted as an alarm, even if it's only water on the wires.

Trouble - - Open Wires or Bad Connection - - High Resistance

If the wire breaks or a connection comes loose, the supervision current drops. The drop in current causes the supervision voltage at the IDC terminals of the panel to increase.

The panel interprets this increase in voltage as a trouble condition.

T-Taps on the IDC Aren't Acceptable

Yes, if there's an alarm on a device on a T-tap, the panel will see this as an alarm and do what it's programmed to do. However, when there's a T-tap, the supervision Current doesn't pass through the wires or the ins and outs of devices on the T-tap. If a wire breaks on the T-tap, or a connection to one of the T-tapped devices comes loose, there's no change in the supervisory current so the panel doesn't detect a trouble condition.

No one finds out about the trouble condition until there's a fire, at which time it's too late to start troubleshooting.

To be properly supervised, the devices have to be daisy-chained in a Class A or Class B configuration, and there can be no T-taps.

Note: IDC (Initiating Device Circuit) power, used by the detectors, will be addressed in a later issue of Short Circuit.

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Quadruple Use of the Initiating Device Circuit (IDC)-- 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... Read More

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What is the Difference Between an End of Line Resistor and a Terminating Resistor? -- In electronics, at the end of the communication line, there's often a Terminating Resistor. In fire alarm and security systems ... Read More

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