In structured cabling systems, cables are often laid in bundles or bundles. For these cables, we must pay attention not only to their transmission characteristics but also to their fire resistance, as fire is a serious threat! So, what standards are used to determine the fire resistance rating of cables, and what are the different ratings?
There are many regulations and standards related to fire protection around the world, but in the structured cabling industry, the provisions of ISO/IEC 60 332 and NEC 800 are the most frequently followed and referenced. They provide detailed and rigorous definitions of the fire resistance, flammability indicators, and testing methods for communication cables.
IEC 60 332 is the standard for testing electrical cables under fire conditions. This test is used to control the spread of fire and is applicable to both PVC (polyvinyl chloride) sheathed cables and LSZH (low smoke, halogen-free) sheathed cables. IEC 60 332 mainly consists of two series of standards: IEC 60 332-1 and IEC 60 332-3. 332-1 is the procedure for determining the vertical burning of a single cable; 332-3 is the procedure for determining the vertical burning of bundled cables. Obviously, the flame retardancy rating of the 332-3 standard is higher than that of 332-1. Furthermore, the 332-3 series is further subdivided into more detailed standards, such as 332-3-22, 332-3-23, and 332-3-24, corresponding to classes A, B, C, etc., respectively. Sometimes we see cables marked IEC 60 332-3C; their flame retardancy rating conforms to the IEC 60 332-3-24 fire test requirements. (332-2 is a test specification for vertical burning of a single small cable).
Of course, LSZH cables usually require two additional tests: IEC 60 754-2 (testing combustion gases to verify the content of toxic halogens) and IEC 61 034-2 (smoke density test). Only cables that meet the corresponding standards can be called LSZH cables. LSZH and flame retardant rating are two completely independent concepts and should not be confused.
NEC 800 requires all communication cables installed within buildings to meet Class 4 fire resistance standards, with appropriate markings on the cable label indicating compliance. Underwriters Laboratories (UL) has developed test procedures for verifying cable compliance with NEC specifications, ranging from highest to lowest: UL 910, UL 1666, and UL 1581. The corresponding cable markings and test procedures are shown in the table below.
To facilitate memorization and understanding, we will provide further explanations of the four levels: CMP, CMR, CMG/CM, and CMX.
In CMP, CM stands for Communication, indicating cables intended for communication purposes; P stands for Plenum, which refers to pressure ventilation ducts. This means that cables marked CMP can be installed in pressure ventilation ducts or other open spaces; even in wind-driven fire environments, CMP cables maintain their flame-retardant rating. Therefore, CMP represents the highest level of flame retardancy for cables.
The "R" in CMR stands for Riser, meaning vertical shaft. When CMR cables are laid vertically, such as in shafts, they have flame-retardant properties to prevent the spread of flames across floors. The fire-retardant rating of CMR is lower than that of CMP. If CMR cables are to be run in air supply ducts, they must be placed inside metal conduits.
In CMG/CM, "G" stands for General, indicating general purpose. CMG/CM cables can be used anywhere within a building, except for pressure ventilation ducts and shafts. As horizontal cabling, CM-level network cables are sufficient; within the same floor, they can be bundled and laid in open cable trays. CMG cables can also be placed in pressure ventilation ducts and shafts using metal conduits.
The "X" in CMX stands for Dwellings, meaning communication cables used in residential homes. In home wiring, these cables are typically run in conduits and often as single strands. It's easy to understand which is more likely to ignite and spread a fire: a single cable or a bundle of cables. Therefore, CMX cables have a lower fire rating than CM cables, and are the lowest fire-resistant cable of all types.
Similarly, the fire resistance and flame retardancy ratings of optical cables are OFNP, OFNR, and OFN; however, there is no OFNX, which is easy to understand.
As for the specific test methods and requirements for various flame retardant and fireproof levels in the standards, this article will not go into detail. Interested readers can consult the standards themselves.
