How the drill works
Each set shows all 15 scenarios in a shuffled order. Every diagram draws the pair as two lines, with the panel on the left, devices numbered from the panel out, and any isolators, end-of-line resistor or return path drawn where they really sit. A fault is marked in color: an open is a break in one conductor, a short bridges the two conductors, and a ground runs from a conductor to earth. Pick one of four answers. The right one is there with three mix-ups that techs really make, and the explanation tells you why each one fails.
The scenarios fall into five concepts: circuit type, pathway class, isolators and shorts, the end-of-line resistor, and supervision and faults. Your score is one point per scenario, out of 15. At the end you get your score by concept and the guide to study for your weakest one. Hit New shuffle for a fresh order, and use the set number if you report an error.
Worked examples
Example 1: count the conductor on a Class B NAC
Step 1: trace the route. The pair runs 150 ft from the panel to the last appliance and stops at the EOL, so it is Class B. Step 2: count conductors. Current goes out on one conductor and back on the other, and a pair is two wires, so 150 ft of route is 300 ft of conductor. Step 3: check the trap. Counting only the 150 ft route halves the drop and makes a long run look fine. For a return-wired Class A NAC, use the length the panel manual tells you to, then check the result with the fire alarm voltage drop calculator.
Example 2: a short on a Class B loop with isolators
Step 1: find the isolators on each side of the short. One sits between devices 1 and 2, the other between devices 3 and 4. Step 2: open them. The shorted section, holding devices 2 and 3, is cut off, and device 1 keeps talking to the panel. Step 3: ask whether anything past the section has another way home. On Class B there is no return, so device 4 is lost too. The answer is devices 2, 3 and 4. Draw a return on the same loop and device 4 comes back, which is why the class and the isolators have to be read together. SLC loop wiring, isolators and shorts walks through finding the fault section by section.
Class A vs Class B vs Class X at a glance
A plain summary of how the wired classes behave. NFPA 72 (2022) §12.3 is where the classes are set out, §12.3.6 covers Class N network pathways, and §12.4 covers survivability, which is a separate check from the class.
| Class | Route | One open | One short |
|---|---|---|---|
| B | Out from the panel and stops, at an EOL on a conventional circuit | Trouble; every device past the break is lost | Trouble; with no isolators the whole circuit can go down, and with them everything past the first open isolator is lost |
| A | Out and back to a second set of panel terminals | Trouble; every device is still reached from one end or the other | Trouble; the return does not help, so with no isolators the whole loop can go down, and with them only the shorted section is lost |
| X | Out and back, with isolators placed so a short is contained, as the manufacturer's design sets out | Trouble; every device is still reached | Trouble; the isolators cut out the shorted span and the other devices keep working |
Three traps to carry into the test. NAC, SLC and IDC name what a circuit does, and A, B and X name how it is wired, so a NAC can be Class A and an SLC can be Class B. Older drawings and manuals use style numbers instead of classes, so trace the wiring and check the manual before you translate one. And on a conventional IDC, many panels read a short as an alarm, because a device operating does the same thing to the pair. NAC vs SLC circuits goes deeper on supervision and the pathway classes.
Fire alarm circuit FAQ
What is a NAC circuit in a fire alarm system?
NAC stands for notification appliance circuit: the circuit that powers the horns, strobes and other appliances that tell occupants to act. On a conventional system the panel supervises it in standby with a small current through an end-of-line resistor, then reverses polarity in alarm so every appliance operates. NAC names the job, not the wiring class.
What is the difference between Class A and Class B wiring?
Class B runs out from the panel and stops, so one open loses every device past the break. Class A runs out and comes back to a second set of panel terminals, so one open still leaves every device reachable. Both report the open as trouble. Neither protects against a short by itself; that is the job of isolators and Class X. NFPA 72 (2022) §12.3 sets out the classes.
Do I need to know circuit classes for the NICET Level 1 test?
Yes. Landing a return, placing an EOL and predicting what an open or a short does sit in the Installation and Maintenance domains of the Level I content outline. Knowing the definitions is the start; this drill practices what each fault does on each class, which is where the mix-ups happen.