LSZH CPR vs OFNP vs OFNR Rated Cabling in Data Centers

LSZH CPR vs OFNP vs OFNR Rated Cabling in Data Centers

Key Takeaways
 - When selecting cable types, always follow local fire regulations
 - Cable in the US that installed in data center Plenum or Riser spaces must pass NFPA262 and UL1666 fire standard tests. OFNP rated cables are commonly used for trunks, and OFNR for in-cabinet patching
 - In Europe, cables permanently incorporated into buildings and construction works must be CPR rated with each country defining its own regulation
 - Cca and B2ca rated cable types are emerging as the preffered cabling within data centers
 - US OFNP/OFNR ratings put emphasis on reduced flammability whereas the European approach allows for moderate flammability with focus on gas/smoke safety
 - A cable that meets a high CPR rating and also passes Plenum requirements is hard to achieve

Data communication cables are manufactured to fulfil specific certain fire test standards depending on global locations. It is a pre-requisite to comply with existing local regulations or, in the absence of these, to follow building owner or insurer commercial requirements. In the event that there are no such requirements, one must abide by local standards, surveys, and guidance. Common fire regulations across the US for data center cabling follow NEC (National Electrical Code) referencing, NFPA (National Fire Protection Administration), and UL (Underwriter Laboratory) fire test standards for cables applied in Plenum or Riser spaces. In Europe, all cables that are permanently incorporated into building or construction works must be tested with respect to their reaction to fire as per EN 50575 and a specific CPR rating must be assigned to each cable with each European country determining which CPR rating to apply for specific building types or applications.

This article explains and provides insight into the differences between European LSZH CPR and US Plenum/Riser fire performance tests. 

The National Electrical Code
When faced with the choice between Riser and Plenum or determining which LSZH cable CPR rating to apply, it is pre-requisite to always comply with the legal requirements specified in existing regulations. In the US, the NEC (National Electrical Code) is commonly applied nationwide with no exceptions, although adoption and implementation of NEC codes depend on the state and can even vary between specific cities. The NEC rules that cable used for Optical Fiber Nonconductive Plenum (OFNP) spaces must pass test NFPA262 (“Standard Method of Test for Flame Travel and Smoke of Wires and Cables for Use in Air-Handling Spaces”) and cable used for Optical Fiber Nonconductive Riser (OFNR) spaces must pass the UL1666 (“Standard for Test for Flame Propagation Height of Electrical and OpticalFiber Cables Installed Vertically in Shafts”) test.

What are plenum and riser spaces?
The most common fiber optic cable used in US data centers today are Plenum and Riser; Plenum for trunks and Riser for in-cabinet patching. The NEC defines Plenum as any enclosed area that facilitates the handling of environmental air; spaces between walls, above a suspended ceiling or below a raised floor that do not have closed air ducts to allow the Heating, Ventilating, and Air Conditioning (HVAC) system to transport air. Riser spaces refer to vertical runs in a shaft that penetrate more than one floor. These pathways do not handle environmental air but they have the potential to conduct a fire between floors. 

Construction Product Regulation (CPR) – EU
The Construction Product Regulation (CPR), EU No.305/2011, defines standard EN 50575 in respect of the “reaction to fire” of Low Smoke Zero Halogen (LSZH) cables permanently incorporated into buildings and construction works. “Reaction to fire” is broken down in to the following categories:
 - Heat and Flames (*Aca, *B1ca, B2ca, Cca, Dca, Eca, **Fca)
 - Smoke (s1a, s1b, s1, s2, s3)
 - Flaming Droplets (d0, d1, d2)
 - Acid Gases (a1, a2, a3)

The regulation specifies that only certified laboratories can provide test approval for Dca and Eca with higher ratings requiring additional regular product and manufacturing facilities audits. The CPR ruling does not stipulate which CPR Euroclasses are to be used in specific applications, it is left up to EU member states to decide and implement regulations. Ultimately, it is the building owner who is obligated to specify the correct Euroclass assigned by the national regulation. As is stands, not all EU countries have a regulation in place meaning commercial contracts are drawn up, referencing existing local standards, surveys, guidance bodies (typically a group of manufacturers) or insurance provider requirements. In Europe, more and more data centers are opting for a Cca class rating for trunk cabling. B2ca s1 d1 a1 deployment does exist but is more sporadic and reserved for applications involving a high risk to people; escape routes, hospitals, areas of high flammability or places with high importance electronic devices.

How do EU LSZH CPR and US plenum/riser ratings compare?
Both test standards – EN 50575 in Europe and FNPA262/UL1666 in the US – test cables for their reaction to fire but nature of these tests is the result of different, historic, and independent, long-term approaches. It is difficult to conduct an exact comparison of both tests as they use a different set-up and distinctive test parameters. Nevertheless, it is worth noting that, American NFPA Plenum and Riser tests place emphasis on increased resistance to flammability (not allowing to burn in first place) with higher tolerances for toxic gases and smoke, whereas European standards allow for moderate flammability but focus on preventing smoke, toxic gases, flaming droplets etc. when the cable burns.

Could I specify cable that meets both the OFNP and CPR Cca/ B2ca ratings?
Different material structures are required to fulfil higher CPR or Plenum pass criteria, and these requirements typically exclude themselves; data cables do not typically meet both B2ca/Cca standards and Plenum ratings. For example, achieving a
high CPR LSZH rating and passing Plenum tests may not be possible. Reduced flammability is achieved through the addition of halogens to improve the flame retardant properties (chlorine, bromine, fluorine, iodine) of the cable materials. The
disadvantage of halogen additives is that, when burnt, more smoke and acid gas is emitted. A lack of halogen additives would assist in passing the CPR smoke and acid gas tests but at the cost of reducing flammability, meaning it may prevent
the cable from meeting the Plenum pass criteria. Naturally, custom designs are possible but the design of such a cable could deviate from typical physical properties of data center cabling and wouldn’t necessarily be cost effective.

Summary
 - USA and European fire regulations for data cabling follow two different approaches established over many years
 - US standards for data center cabling set more rigorous requirements regarding the flammability of a cable and tests are less focused on smoke generation
 - European standards have allowances for cable flammability but are more stringent with respect to smoke, gases, acidity/ corrosiveness, and flame droplets

(Source from AFL Hyperscale)