What voltage level indicates that a fire alarm circuit must not be AFCI protected?

A fire alarm circuit must not be Arc-Fault Circuit Interrupter (AFCI) protected if it operates at voltages under 300 volts. This requirement is based on the understanding that the nature of fire alarm systems is different from standard branch circuits. Fire alarm circuits are designed to detect fires and require reliability and functionality that could be compromised by AFCI devices.

AFCIs are designed to prevent fires caused by electrical arcing faults, which can occur in typical residential circuits. However, the devices used in fire alarm systems are more sensitive and need to maintain a certain level of operation at all times, especially during emergencies. If an AFCI were to trip due to electrical noise or minor faults in the system, it could lead to the failure of the fire alarm system, potentially putting lives at risk.

This is why circuits operating at voltages under 300 volts are excluded from AFCI protection. Higher voltage levels, although not directly relevant to the operation of typical fire alarm systems, also indicate different safety considerations and conditions under which AFCI protections might not apply.

Understanding these voltage requirements ensures that fire alarm systems can operate reliably without unnecessary interruptions, which is critical for safety in buildings.