Circuit Classifications
Chapter 12 of NFPA 72 provides detailed requirements for the circuits and pathways that connect fire alarm system components. The chapter defines circuit classifications that specify how a circuit must perform under fault conditions. Class B circuits operate on a single pathway; a single open fault can disable devices beyond the break. Class A circuits use a redundant return path, ensuring that all devices remain operational even with a single open fault. Class X circuits provide even greater survivability through redundant pathways that are physically separated.
Pathway Survivability
Pathway survivability is categorized into three levels. Level 0 provides no additional protection beyond standard wiring methods. Level 1 requires pathways to be protected using one of several approved methods, such as 2-hour fire-rated cable, 2-hour fire-rated enclosures, or approved alternative means. Level 2 requires full survivability through the use of approved performance alternatives, such as redundant pathways routed through different parts of the building. Higher-risk occupancies and certain system types mandate higher survivability levels.
Shared Pathways and Network Communications
Modern fire alarm systems often use networked communications and shared pathways. Chapter 12 addresses these configurations, requiring that shared pathways maintain the integrity and performance required by the code. When fire alarm signals share pathways with non-fire-alarm communications, the fire alarm signals must take priority and the pathway must meet all applicable survivability requirements.
Wiring Methods
Wiring for fire alarm systems must comply with both NFPA 72 and NFPA 70 (National Electrical Code). Fire alarm circuits must be installed in a manner that protects them from physical damage and electromagnetic interference. Circuit conductors must be properly identified, separated from power-limited and non-power-limited circuits as required, and installed using approved cable types and methods.
