By Fire Alarm BT, Founder, Fire Alarm Blueprint Trainer · Creator of the FireAlarmBT YouTube channel · reviewed against NFPA 72 & NFPA 70 principles
NAC Open Circuit Troubleshooting: Silent Horn/Strobes on a Class B Notification Circuit
On a Class B notification appliance circuit, everything downstream of an open loses power — which is exactly why the appliances that still work tell you where to look. This case study walks through an intermittent NAC 2 OPEN found during an elementary school's annual fire alarm test, with a 90-minute window to find and fix it.
The setting
Annual test at a two-story elementary school, addressable FACP with four Class B NACs. The building is occupied; your test window is 3:30–5:00 PM.
TROUBLE: NAC 2 OPEN
NOTIFICATION CIRCUIT 2 · EAST WING
What the technician found on arrival
During the first alarm test the west-wing horn/strobes operate, but the four horn/strobes in the east-wing corridor stay silent and dark. When you reset, the panel posts a NAC 2 OPEN trouble that the custodian says 'comes and goes.'
The question
You have 90 minutes with the building. How do you find why NAC 2 is open — and why it's intermittent?
Step-by-step: what the technician did and why
- 1
Reset, silence, and map which appliances failed
On a Class B NAC everything downstream of an open loses power, so the last working appliance brackets the fault. Compare against the as-built.
FindingNAC 2 runs west→east. The last working appliance is HS-2-1 at the wing entrance; HS-2-2 through HS-2-5 are dead.
- 2
Measure NAC 2 at the panel in standby
A supervised NAC sits at a reverse-polarity supervision voltage and reads the EOL resistance. An open reads high/infinite instead of the EOL value.
FindingMeter flickers between ~1.2 kΩ and open when you tap the conduit near the wing door — an intermittent open near HS-2-1/HS-2-2.
- 3
Open the last working appliance (HS-2-1) and inspect the terminals
An intermittent open is almost always a loose landing, a broken strand, or a poor splice at a device — not a broken wire mid-run.
FindingThe OUT+ terminal screw on HS-2-1 is barely snug; two strands are broken and the connection heat-cycles loose.
- 4
Check for hidden T-taps or splices
NAC appliances must be wired in-and-out so the EOL supervises every conductor. A T-tap passes power but defeats supervision.
FindingWiring is in-and-out with no T-taps. Good.
- 5
Repair, torque, and re-test the whole circuit
Re-strip and land the conductor, torque per the manufacturer, re-measure the EOL at the panel, then run a full alarm test and verify each appliance visually and audibly.
FindingPanel reads 4.7 kΩ steady (correct EOL). Alarm test: all five east-wing appliances operate; 22.1 VDC at the last device — above the appliance minimum.
- 6
Document and flag the pattern
Report the finding and proactively check other appliances installed by the same crew for the same loose-terminal issue.
FindingTwo more appliances in the wing had marginal torque and were corrected preventively.
Key takeaways
- •On a Class B NAC, the last appliance that works points to the fault.
- •Intermittent opens live at terminals and splices, not mid-cable.
- •No T-taps on a NAC — supervision only sees what the EOL sees.
- •Prove voltage at the last appliance after any NAC repair.
Code references
NFPA 72 · 12.3 — Pathway Class Designations
A Class B pathway loses operation beyond a single open; the panel must annunciate the trouble.
NFPA 72 · 12.6 — Monitoring Integrity
Notification circuits are supervised; wiring that defeats EOL supervision (T-taps) is not permitted on a Class B NAC.
NFPA 72 · 14.4 / Table 14.4.3.2 — Notification Appliances
Test all appliances for operation annually; record failures and corrective action.
Section numbers vary between editions. Always verify against the edition adopted by your Authority Having Jurisdiction (AHJ).
Sample service report
Found
Annual test 15:35. East-wing horn/strobes HS-2-2 through HS-2-5 failed to operate; FACP posted NAC 2 OPEN trouble (intermittent per custodian).
Cause
Loose OUT+ terminal with broken strands at HS-2-1 (last operating appliance); thermal cycling produced an intermittent open downstream.
Action taken
Re-stripped and re-landed conductor, torqued per manufacturer spec; inspected and torqued HS-2-2 and HS-2-3 preventively.
Verification
NAC 2 EOL reads 4.7 kΩ at FACP. Alarm test: all NAC 2 appliances operated; 22.1 VDC at HS-2-5. Panel NORMAL 16:50. Remaining annual test items completed; full report attached.
Practice this call in the Troubleshooting Lab
Work through the same scenario step by step, with wrong-move feedback and a downloadable completion certificate. Free, no account required.
Frequently asked questions
Why do some horn/strobes work while others stay silent on the same NAC?
On a Class B NAC power flows in one direction from the panel to the end-of-line resistor. An open anywhere on the circuit removes power from every appliance beyond it, so the last appliance that still operates brackets the fault location.
Why are T-taps not allowed on a notification appliance circuit?
Supervision only sees the conductors between the panel and the end-of-line device. A T-tapped branch still receives power but is invisible to supervision, so an open on that branch would never generate a trouble signal.
Related guide: Commercial Fire Alarm Troubleshooting: Opens, Shorts & Ground Faults
This case study is educational. It does not replace the manufacturer's instructions, the currently adopted codes and standards, a licensed designer or engineer, or review by the Authority Having Jurisdiction.