Troubleshooting guide

Why Does the Fire Alarm Panel Show an Open-Circuit or NAC Trouble?

A supervised NAC reports trouble for a break, missing end-of-line device, incompatible device or configuration error. What the panel measures.

Fire Protection · Last reviewed October 17, 2026

The panel watches its notification appliance circuit (NAC) by sending a small reverse-polarity current through it. A correctly installed device blocks this current in the normal direction, and an end-of-line device completes the supervision path. If the panel does not see the expected current, it reports an open circuit or other trouble.

A NAC trouble means some or all of the horns and strobes on that circuit may not sound. It is an impairment of a life-safety system and must be handled by qualified fire alarm technicians following the manufacturer instructions, the applicable standards, and the site and authority having jurisdiction requirements for impairments. Do not open or remove devices on a live building system without that approval.

What you observe	Likely cause	What to check
Open-circuit trouble on a new or modified circuit	Break in the wire, a loose terminal, or an end-of-line device missing	Continuity of each leg, the terminations and the end-of-line device
Trouble came after a device was added or removed	The wiring or the end-of-line device was disturbed	The last device on the line, and polarity at every device
Short-circuit trouble	Conductors touching, a pinched wire, or a device wired backwards	Insulation and pinch points. Polarity at devices
Trouble on a circuit with many devices	Voltage drop or current above the circuit rating, or the panel limit	Circuit load against the rating and the voltage drop
Trouble appears with some brands of device	Incompatible devices, which draw the supervision current or do not block it	Device listing against the panel compatibility list
Trouble after programming changes	Circuit configured as the wrong type (Class A or Class B), or wrong style	Panel programming against the installed wiring
Trouble comes and goes	Intermittent connection or a device that is failing	Terminal torque, corroded splices and temperature effects

What the panel is measuring

Most panels apply a small supervisory voltage, in the opposite polarity to the alarm voltage. Polarized devices contain a diode that blocks the current in that direction. The current that flows through the end-of-line device (such as a resistor of a value given by the panel maker, for example 4.7 kΩ) is the signal the panel expects to see:

Condition	What the panel sees
Normal	The small, expected supervisory current
Open circuit (broken wire or missing end-of-line device)	Almost no current
Short circuit	Much more current than expected
Device wired backwards	The diode no longer blocks, so the current is wrong

A broken wire in the middle of a Class B circuit isolates every device beyond the break. They do not sound in an alarm, even though only one connection failed. That is why the open is reported as a trouble.

Write a useful problem statement

Panel make and model, the trouble message, which circuit and when it started.
Circuit class (A or B), the number and types of devices, wire gauge and length.
What changed before the trouble: new devices, wiring work, programming or a modification of the building.
Results of any measurements made by the technician: end-of-line value, voltage, current.

Related tools and labs

Calculators, labs and simulators are part of the All-Access Pass.

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Questions

Can I disable the circuit to clear the trouble?

Disabling a NAC removes the audible and visible alarm from that area. This is an impairment that needs the approvals and notifications that the standards and the site require.

What is the end-of-line device for?

It completes the supervision path, so the panel can tell that the wiring is continuous and that nothing was removed from the end of the circuit.

Why does polarity matter?

Notification devices are polarized so that the supervisory current is blocked in the normal state. A device wired backwards defeats the supervision and may not sound.

Educational material. Examples use simplified, hypothetical inputs and are not a design, a construction document or professional engineering advice. Confirm requirements with the codes and standards adopted where the work is built and with a licensed engineer.

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