Quick answer: The glass fiber carries light, not current — but the jacket, buffer and strength members around it are plastic, and in a data center tray they add up to tons of fuel. That is why flame retardant fiber optic cable is a real category with its own ratings: in the US, OFNP (plenum, NFPA 262) and OFNR (riser, UL 1666); internationally, LSZH jackets tested to IEC 60332 with smoke and acidity limits, and CPR Euroclasses in the EU. The twist buyers miss: US plenum ratings are usually earned with halogenated materials, while LSZH bans halogens — the two big systems solve the same fire with opposite chemistry.
“Why would a glass cable need to be flame retardant?” is a fair question, and the answer decides real money in every data center build. A single fiber weighs nothing; two thousand cables stacked in a riser shaft or plenum space are a vertical fuel column connecting every floor of the building. The fire codes do not care what signal the cable carries — they care what the jacket does when the fire arrives.
The ratings map: same fire, four exams
| Rating | Test | Where it applies | Chemistry behind it |
|---|---|---|---|
| OFNP (plenum) | NFPA 262 flame + smoke | US air-handling spaces (above drop ceilings) | Typically halogenated (FEP, plenum PVC) |
| OFNR (riser) | UL 1666 vertical shaft | US floor-to-floor risers | FR PVC and similar compounds |
| LSZH | IEC 60332 flame + IEC 61034 smoke + IEC 60754 acidity | International; enclosed spaces, transit, ships, data halls | Halogen-free: EVA/PE + ATH/MDH mineral |
| CPR Euroclass (B2ca–Eca) | EN 50399 / EN 60332-1-2 | EU permanent building installs | Class-dependent; higher classes lean LSZH |
Two practical readings of that table. First, ratings are about the installation space, not the cable’s job — the same duplex patch cord is OFNP in a Chicago plenum and LSZH in a Frankfurt data hall. Second, a higher US rating substitutes downward (OFNP can go in riser spaces; OFNR cannot go in plenums), but US and international ratings do not substitute for each other at all.
The philosophy split: suppress the flame, or clean up the smoke
Here is the part worth understanding before you spec a global product. The US plenum approach says: the cable sits in the building’s airflow, so above all it must not spread flame or generate smoke into that air — and the materials that pass NFPA 262 most easily are fluoropolymers (FEP) and specially formulated plenum PVC. Both are halogenated; they win by aggressive flame suppression.
The LSZH philosophy says the opposite: in an enclosed space full of people and electronics, the killer is not the flame but the combustion products — dense smoke that blinds evacuation and corrosive halogen acids that destroy equipment far from the fire. So LSZH bans halogens entirely and builds the jacket from polyolefin loaded 55–65% with ATH/MDH minerals, which release water vapor as they decompose. It passes flame tests and the smoke/acidity tests halogenated compounds cannot.
Consequence: a cable cannot be meaningfully “OFNP and LSZH” through one chemistry — the two ratings pull the formulation in opposite directions. Exporters serve them with different jacket lines, which is why your RFQ should name the destination market before the compound is quoted. The full deep-dive on the halogen-free side lives on our flame retardant cable compounds page; the philosophical trade-off between the two chemistries is the subject of halogen-free vs halogenated flame retardants.
What is actually in a fiber cable jacket compound
- LSZH jackets: EVA or grafted PE host loaded with 55–65% aluminium or magnesium hydroxide, plus coupling agents to keep mechanicals alive at that filler level. The engineering challenge is staying extrudable and flexible at a mineral load that high — dispersion and surface treatment decide whether the line runs smooth or the jacket cracks on the bend test.
- Riser-grade PVC jackets: flexible PVC with the antimony/zinc-borate smoke-suppression packages we detailed in the flame retardant PVC cable guide — same chemistry family, tuned for UL 1666’s vertical shaft.
- Tight buffers and strength elements: the layers directly on the fiber follow the jacket’s halogen policy — an LSZH cable with a PVC buffer inside fails the “zero halogen” claim on IEC 60754 acidity, a detail that surfaces in third-party testing, not in the datasheet.
One more distinction worth keeping sharp: everything above is flame retardant performance — stopping spread and managing smoke. If the spec also demands the circuit survive the fire (alarm and emergency systems), that is fire resistance, a different construction entirely — the difference is unpacked in fire resistant vs flame retardant cable.
2026: the data center buildout is a jacket-compound story
The AI data center wave is, from a materials seat, a fiber volume explosion — hyperscale halls consume fiber by the hundreds of kilometers, in dense trays where fuel-load math gets serious. Two procurement trends follow: hyperscalers increasingly standardize on LSZH globally (one halogen-free spec across regions beats managing OFNP/LSZH splits), and CPR class requirements in the EU keep tightening under the new (EU) 2024/3110 framework. For compounders and cable makers, that translates to growing demand for high-filler halogen-free jacket compounds that still run fast on modern extrusion lines — exactly the ATH/MDH dispersion problem that separates a good halogen-free flame retardant package from a cheap one.
How Rectivas supports fiber cable jacket projects
We supply the flame retardant side of the jacket: halogen-free ATH/MDH packages with surface treatment and coupling systems for LSZH compounds, and the antimony/zinc-borate smoke-suppression packages for FR PVC riser grades — as powders or pre-dispersed concentrates. Straight boundary as always: we do not supply FEP or extrude cable; our lane is the compound chemistry that passes IEC 60332, 61034 and 60754 on your line. Loading ladders run on our twin-screw line, burn and LOI checks happen in-house, and every recommendation ships with a CTI (Hebei) third-party report. Tell us the jacket host (EVA/PE/PVC), the target rating and market (LSZH/CPR class or riser), and your line speed — request a quote and we will match the package.
FAQ
Why does a fiber optic cable need to be flame retardant if it carries no electricity?
Because the fire codes regulate the plastic, not the signal. Jackets, buffers and strength members are combustible, and bundled runs in risers and plenums form a continuous fuel path through a building. Ratings like OFNP, OFNR and LSZH exist to stop cable trays from becoming fire highways.
What is the difference between OFNP, OFNR and LSZH fiber cable?
OFNP (plenum, NFPA 262) and OFNR (riser, UL 1666) are US installation-space ratings, usually met with halogenated materials. LSZH is the international low-smoke zero-halogen approach, tested to IEC 60332 for flame plus IEC 61034 (smoke) and IEC 60754 (acidity). OFNP can substitute for OFNR; US and LSZH ratings do not substitute for each other.
Is LSZH fiber cable plenum rated?
Not automatically — and usually not. LSZH is defined by halogen-free chemistry and smoke/acidity limits, while US plenum listing requires passing NFPA 262, which is most easily met with halogenated materials. A few specialty products chase both, but as a rule you choose per the installation code that governs the building.
What material is an LSZH fiber cable jacket made of?
A polyolefin host — typically EVA or grafted PE — loaded with roughly 55–65% aluminium hydroxide (ATH) or magnesium hydroxide (MDH). The minerals release water vapor when heated, cooling the flame and diluting smoke, with coupling agents keeping the jacket flexible at that filler level.
Which rating do data centers use?
In the US, OFNP or OFNR by space, per NEC. Internationally — and increasingly as a global corporate standard — LSZH, because one halogen-free spec covers EU CPR classes, protects equipment from corrosive smoke, and simplifies multi-region procurement. Confirm the operator’s own cabling standard; hyperscalers often exceed local code.
—— Rectivas Materials 团队 技术工程师老陈