Burglar Alarm System Components: Circuit Theory and Supervisory Loops

A burglar alarm system is a network of supervised electrical circuits where sensors alter the loop's resistance or state to trigger a control panel's logical response. In a real installation, this architecture changes a simple open/closed switch circuit into a fault-tolerant supervisory loop capable of detecting not just an open door, but a cut wire or a shorted tamper switch. Beginners commonly confuse a supervised security zone with a basic doorbell circuit, or mix up the Normally Closed (NC) wiring standard for burglar zones with the Normally Open (NO) standard used in fire zones.

The Core Theory: Supervisory Loops vs. Simple Switches

If you wire a magnetic door contact directly to a microcontroller or a basic relay, you have created a simple switch. When the door opens, the circuit breaks, and the alarm sounds. The fatal flaw in this basic setup is that an intruder can simply cut the wires before opening the door. The panel sees an open circuit and triggers the alarm immediately, which is annoying, but worse: if the intruder strips the wires and twists them together (shorting the circuit) before opening the door, the panel sees a closed circuit and assumes the door is secure.

To solve this, professional burglar alarm system components utilize a Normally Closed (NC) supervisory loop paired with an End-of-Line (EOL) resistor. The EOL resistor is wired in series with the sensor's dry contact at the furthest point of the circuit. The control panel constantly monitors the total resistance of the loop.

By monitoring resistance rather than just continuity, the panel can distinguish between three distinct states: Secure (normal resistance), Alarm (infinite resistance/open circuit), and Trouble/Sabotage (zero or near-zero resistance/short circuit).

Numeric Breakdown: Calculating Loop Resistance and Voltage Drop

Let's look at the actual numbers on the bench. A standard Honeywell/Resideo Vista series panel expects a 2,000 ohm (2kΩ) EOL resistor for standard zones, while DSC panels typically use a 5,600 ohm (5.6kΩ) resistor. The panel's internal analog-to-digital converter measures the voltage drop across the loop to determine the resistance, usually allowing a tolerance of ±10% to ±15% to account for wire resistance.

Suppose you are wiring a hardwired PIR motion sensor in a detached garage using 18 AWG solid copper wire. The run from the panel to the garage is 200 feet. Because the loop requires an out-and-back path, the total wire length in the circuit is 400 feet.

1. Wire Resistance Calculation: 18 AWG copper has a resistance of approximately 6.385 ohms per 1,000 feet at 20°C. For 400 feet, the wire resistance is 400 × (6.385 / 1000) = 2.55 ohms.
2. Total Loop Resistance: The 2kΩ EOL resistor plus the 2.55 ohms of wire equals 2,002.55 ohms.
3. Panel Tolerance Check: A 10% tolerance on a 2,000 ohm target gives an acceptable window of 1,800 to 2,200 ohms. Our measured 2,002.55 ohms falls perfectly in the 'Secure' window.

If the PIR detects motion, its internal relay opens. The panel now sees infinite resistance (an open circuit) and triggers the alarm. If an intruder cuts the wire in the garage, the panel also sees infinite resistance and triggers the alarm. If they short the wires, the panel sees roughly 2.55 ohms (well below the 1,800 ohm minimum threshold) and triggers a 'Trouble' or 'Sabotage' alert, refusing to arm the system.

Where You Meet This In Practice: Component Roles

Understanding how these components interact on the jobsite is critical for debugging false alarms and designing reliable zones. Below is a breakdown of the primary hardware you will terminate on the board.

Real-World Scenario Walkthrough: The 'Shorted Wire' Sabotage

Theory is clean; jobsites are messy. Here is a classic failure mode that occurs when installers misunderstand supervisory loop theory.

The Setup: An installer is wiring a hardwired PIR motion sensor in an unfinished basement. They run 22 AWG 4-conductor wire from the control panel to the sensor. To save time and avoid climbing the ladder to open the PIR housing, they twist the 2kΩ EOL resistor directly onto the zone terminals at the control panel board. They connect the PIR's NC relay contacts to the other end of the wire in the basement.

The Numbers: The panel expects 2,000 ohms. Because the resistor is at the panel, the panel measures 2,000 ohms in parallel with the wire loop. When the PIR relay is closed, the wire loop has near-zero resistance, but the panel's measurement circuit reads the 2kΩ resistor perfectly. The keypad shows 'Secure'.

The Outcome: The homeowner arms the system and goes to sleep. An intruder enters the unfinished basement and notices the exposed alarm wires running along the joists. They strip the two zone wires and twist them together, creating a dead short.

What Went Wrong: Because the EOL resistor was installed at the panel, the short circuit in the basement completely bypasses the resistor. The panel's zone terminals are now shorted directly together, reading 0 ohms. On many standard NC configurations, a dead short reads identically to a closed, secure relay. The intruder walks past the PIR. The PIR's internal relay opens, but the shorted wires keep the panel seeing 0 ohms. No alarm triggers. The system was defeated by a basic sabotage technique that a properly placed EOL resistor would have instantly caught. Always follow the manufacturer's wiring diagrams, which explicitly mandate EOL placement at the furthest device (Bosch Security Intrusion Guidelines).

FAQ: Common Installation and Theory Questions