|Cabling jacket fire ratings|
|Question: (by Michael L. Clarey) |
What is the difference between "plenum-rated" cable and cable that is sheathed by "fire retardant" material. Are they saying the same thing?
Actually, any cable jacket that you may use indoors shell be made of fire retardant materials for the sole reason - you bring lots of polymer resins - highly combustible materials - into the place, which is not suppose to be a
fireplace. That's why there is a 15 meters limitation for OSP cable placement within a building. Polyethylene (usual outer sheath material for OSP cables) is not considered fire retardant at all, because it burns like an oil (emitting pretty the same amount of energy).
PVC composites are known as producing not as much heat as Polyethylene and maybe even more important - having slower flame spread, that gives people more time to escape a fire. In an office environment (the type of environment, where structured cabling is usually installed) PVC composites are most often used as outer jacket materials. Composites means that some ballast mineral materials added to slow down a flame spread. So they are "fire retardant" in comparison to Polyethylene, but still they burn and produce pretty much heat. That's why it is dangerous to use this PVC composites is such a spaces, where may be a presence of some air movement, because fresh oxygen will diminish all the "fire retardancy" efforts and fire will spread fast again. Such a spaces, whatever their primary purpose is, are called "plenum(s)". HVAC tunnels, above-the-ceiling spaces, any other cavity, that may create a return path for conditioned air, are good examples of plenum spaces.
In this case special jacket materials are used and cable is called "plenum rated". Teflon FEP is the most well known plenum rated cable jacket material. Its what DuPont called "Low fuel-load fluoropolymer".In few words its the material that produces 30 times less energy than Polyethylene while burning, and gives zero flame spread. For the identification purposes NEC in Article 800-51 sets listing requirements for communications wires and cables, where All cables are for communications use and
CMP is plenum cable
CM and CMG -general purpose
So look at a cable mark and you'll be able to say that some particular cable is suitable for some particular installation. And one very small remark. There is no big difference between CM and CMR cables, they are all PVC-based, but riser cables have more of those ballast materials to even more slow flame spread down. You may check it yourself. Try to burn CM cable while horizontal. It should burn only above a naked flame of your lighter. Rotate cable 90 degrees - and flame will move up (and quickly if the cable isn't that good) because of convection. Ideally, you shouldn't see this flame moving up with riser rated CMR cable.
There is DuPont site on the Internet located at http://www.dupont.com/teflon/cablingmaterials/generic/firetest.html It contains many interesting facts about cables and fires.
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