2.3 Cable Jackets & Fire Ratings

2.3 Cable Jackets & Fire Ratings

When installing fiber optic cables inside residential, commercial, or industrial buildings, fire safety is a paramount concern. Standard cable jackets made of polyethylene (PE) or general-purpose plastics are highly flammable; when exposed to flame, they ignite easily, propagate fire along ceilings and walls, and release thick, black smoke and toxic gases. To prevent communication cables from acting as fuel or conduits for fire, building codes—most notably the National Electrical Code (NEC) in the United States—mandate strict fire safety and material flammability standards.

National Electrical Code (NEC) Article 770

In the United States, NEC Article 770 governs the installation of optical fiber cables and raceways. A fundamental distinction made by the NEC is the division of fiber cables into two primary categories based on the presence of conductive elements:

1. Non-conductive Cables: These cables contain only optical fibers and dielectric materials. They have no metal components whatsoever—no metallic strength members, no metallic armor, and no copper conductors. The NEC prefix for non-conductive cables begins with OFN (Optical Fiber Nonconductive).
2. Conductive Cables: These cables contain optical fibers along with metallic components, such as a steel strength member, a metallic armor sheath, or copper wires used for power delivery. The NEC prefix for conductive cables begins with OFC (Optical Fiber Conductive).

This distinction is critical for safety: conductive cables can accidentally carry electrical currents from contact with power lines or lightning, necessitating strict rules on grounding and separation from electrical wiring. Non-conductive cables, being fully dielectric, are immune to electrical induction and have fewer routing restrictions.

Plenum Space Ratings (OFNP / OFCP)

A plenum space is any building compartment used to facilitate environmental air handling, such as heating, ventilation, and air conditioning (HVAC) systems. Common examples include the space above suspended drop ceilings and below raised computer floors.

The Plenum Hazard

Because plenum spaces are designed to circulate air throughout a building, a fire originating in or entering a plenum space can spread rapidly. Furthermore, the air circulation system will distribute toxic smoke and fumes throughout all floors of the building. This poses a major hazard to building occupants.

Cable Requirements

To mitigate this risk, any cable installed in a plenum space must have a plenum rating. Under the NEC, these are designated as:

• OFNP: Optical Fiber Nonconductive Plenum
• OFCP: Optical Fiber Conductive Plenum

Plenum-rated cables must pass the rigorous NFPA 262 (National Fire Protection Association) standard, which tests flame travel distance and smoke optical density. To meet these standards, plenum jackets are made from highly specialized fluoropolymer materials, such as Fluorinated Ethylene Propylene (FEP) or Polyvinylidene Fluoride (PVDF). These materials do not support combustion, resist flame spread, and produce minimal, low-optical-density smoke when exposed to extreme heat.

Riser Space Ratings (OFNR / OFCR)

Riser spaces are vertical shafts, conduits, or utility chases that run floor-to-floor within a building.

The Riser Hazard

Vertical shafts act as chimneys during a building fire. The 'stack effect'—where warm air rises and draws cold air in from below—can pull fire and smoke upward through riser shafts at high speed, spreading the fire to upper floors.

Cable Requirements

Cables installed vertically from floor to floor must carry a riser rating, designated as:

• OFNR: Optical Fiber Nonconductive Riser
• OFCR: Optical Fiber Conductive Riser

Riser-rated cables must pass the UL 1666 (Underwriters Laboratories) vertical tray flame test. While riser cables are flame-retardant to prevent fire from traveling up the shaft, their testing standards are less stringent than plenum cables regarding smoke production. Riser jackets are typically made from flame-retardant PVC (polyvinyl chloride) compounds. Riser-rated cables are prohibited in plenum spaces unless they are enclosed within a plenum-rated conduit (raceway).

General Purpose Ratings (OFN / OFC / OFNG / OFCG)

For horizontal runs in areas that are neither plenum air-handling spaces nor vertical riser shafts—such as standard office walls or horizontal cable trays—general-purpose cables are used.

Designations

• OFN / OFNG: Optical Fiber Nonconductive General Purpose (or General Purpose Nonconductive)
• OFC / OFCG: Optical Fiber Conductive General Purpose (or General Purpose Conductive)

These cables are designed to resist flame propagation over short distances (typically tested under the UL 1581 standard). They are less expensive than plenum or riser cables but have no ratings for vertical flame spread or low-smoke emission.

Low Smoke Zero Halogen (LSZH) Cables

In many international markets—particularly in Europe under the Construction Products Regulation (CPR) and in specific maritime or transit sectors—Low Smoke Zero Halogen (LSZH) cables are mandated.

Standard PVC cable jackets contain halogens (such as chlorine). When PVC burns, it undergoes thermal decomposition and releases hydrogen chloride gas. When this gas contacts moisture, such as water from fire sprinklers or sweat in a human airway, it turns into highly corrosive hydrochloric acid. This acid is lethal to humans who inhale it and chemically destroys electronic circuitry in servers and switches.

LSZH cables utilize jackets made of thermoplastic or thermoset compounds (such as polyolefin filled with metal hydrates like aluminum trihydrate) that contain zero halogens. When burned, LSZH jackets release only a tiny amount of non-toxic white smoke and water vapor, with no acid generation. LSZH is standard in subways, tunnels, ships, aircraft, and high-density computing facilities.

Cable Substitution Hierarchy

The NEC establishes a strict hierarchy for cable substitutions based on the principle that a cable with a higher fire-safety rating can always replace a cable with a lower rating, but never the reverse.

The Hierarchy Rules

1. Plenum (OFNP / OFCP) is the highest rating. An OFNP cable can be used in plenum spaces, riser shafts, and general-purpose areas. It can substitute for OFNR and OFN.
2. Riser (OFNR / OFCR) is the middle rating. An OFNR cable can be used in vertical shafts and general-purpose areas. It can substitute for OFN, but it cannot be used in plenum spaces.
3. General Purpose (OFN / OFC) is the lowest rating. It can only be used in general-purpose horizontal runs. It can never substitute for riser or plenum cables.
4. Conductive vs. Non-conductive: A non-conductive cable can substitute for a conductive cable of the same or lower tier (e.g., OFNP can replace OFCP, and OFNR can replace OFCR). However, a conductive cable cannot replace a non-conductive cable where electrical isolation is required.

Substitution Summary Table

(Note: In all cases, a non-conductive cable can substitute for a conductive cable of the same or lower tier.)