Circuit Types & Wiring Classes
Class A and Class B circuits, EOL resistors, SLC and NAC fundamentals, circuit supervision, and Class N network pathways.
- Distinguish between Class A and Class B circuit configurations
- Explain the purpose of end-of-line resistors in supervised circuits
- Describe Signaling Line Circuits and Notification Appliance Circuits
- Identify the survivability advantages of Class A over Class B wiring
- Explain Class N network pathway concepts
Lesson 1
Class A & Class B Circuit Fundamentals
Circuit Classification
Fire alarm circuits are classified by their survivability - how the circuit continues to function when a single open or ground fault occurs. The two primary classifications are Class A and Class B.
Class A
Wiring: Loop (outgoing and return path)
Open fault: All devices still function
Conductors: 4 wires (2 out, 2 return)
Cost: Higher - more wire required
Survivability: Superior
Class B
Wiring: Tee-tap (single path with EOL)
Open fault: Devices past the break are lost
Conductors: 2 wires with EOL resistor
Cost: Lower - less wire required
Survivability: Basic
Class B Circuits
Class B is the most common wiring method in fire alarm systems. A single pair of wires runs from the panel to each device, with an end-of-line (EOL) resistor at the last device. The EOL resistor allows the panel to supervise the circuit - if the wire breaks (open fault), the panel detects the loss of the supervision current and generates a trouble signal.
The limitation of Class B is that any devices connected beyond the break point lose communication with the panel. They cannot report alarms, and notification appliances beyond the break cannot be activated.
Class A Circuits
Class A circuits provide superior survivability by wiring devices in a loop. The outgoing pair and the return pair both connect back to the panel. If a single open fault occurs, the panel can still communicate with all devices by sending signals from both ends of the loop.
Exam Tip - Class A Key Advantage
The key advantage of Class A is single open fault survivability. All devices continue to operate because the panel communicates from both directions. The exam frequently asks about this advantage.
Class B uses two wires with an EOL resistor - devices past a break are lost. Class A uses a return loop to the panel - all devices survive a single open fault. Class A costs more but provides superior survivability.