How Low Is Your Smoke?
Europeans are different than Americans. And it’s not just in the languages we speak or the food we eat. We have some basic differences in the philosophy of safety. And nowhere is this more apparent than in the differences between wire and cable made for each location.
You might see cable from Europe marked with unfamiliar letters such as LSOH (which can also be marked LSNH or LSZH, they mean the same thing). Those letters mean “low-smoke, zero-halogen.”
This is different from the “plenum” rating used in the United States, and you cannot substitute one for the other.
You might wonder what the letters LSOH mean when describing certain cables like this shipboard product from Belden.
Plenum cables were invented by the National Fire Protection Association when it started looking for ways to make buildings safer. (I’ve discussed some of the famous fires that started the process in past articles.) The NFPA concluded that cables need to resist burning and to not serve as fuel for the fire. And their key test, the Steiner Tunnel Test, does precisely that. It lights one end of a cable and sees how far the flame spreads.
Of course, you can get a fire that’s so hot it consumes everything, even “fire-resistant” cables. So it’s not that they won’t burn, it’s that they won’t be fuel. The Steiner Tunnel Test also looks at smoke generation, because smoke is what panics most people during a fire. If during a test the flame spreads too far or produces too much smoke, the cable fails.
While the definition of a “plenum” is really up to the fire marshal, building inspector or other Authority Having Jurisdiction, most people agree that a drop ceiling or a raised floor is a “plenum” area when it forms the “air return” for the air conditioning. Anything burning in there will not be visible and will be fed by the air conditioning to the rest of the building.
In Europe, they start with low smoke. They don’t care if the cable is on fire, or if it is fuel, as long as you can get to the door! The non-halogen portion of their requirement also minimizes the toxic effect of the gasses produced, so you can breathe, see the door and get out.
The LSOH style is becoming more popular in applications on our side of the world. The telephone companies have specified LSOH pairs and coaxes for a long time. Since there are essentially no humans inside a “central office” (the huge telco building with no windows), plenum ratings don’t apply. The advantage for them is that non-halogen cables, when burning, have much less effect on equipment than the chlorine gas produced by burning PVC or the fluorine gas produced by burning Teflon, both of which can destroy equipment.
These advantages have worked their way into the transportation industry, such as trucking and aircraft. Vehicles are not covered by the NFPA. Another major user is the marine industry, especially cruise ships. In fact, there are whole lines of cables, video cables, data cables, control cables and power cables that are LSOH just for these customers. Funny that these cables can then also be used in Europe!
But here in North America, we’re still working with “plenum” and plastic compounds that resist burning.
There are PVC compounds that resist burning, and we have FEP (fluorinated ethylene propylene), TFE (tetrafluoroethylene), PVDF (polyvinylidene fluoride) and many other compounds.
Many of these are generically called “Teflon,” but this is a DuPont trademark. Some of these compounds can go as high as 260 degrees Celsius before they even start to melt. But, as pointed out, all of these can be melted by a fire out of control around them.
Now, neither the LSOH nor the plenum system is perfect. Both have limitations and represent compromises between having no wire and having wire that can hurt you in a fire. In today’s technological world, “no wire” simply is not an option, so minimizing the effect is the only solution.
In fact, comparing our “plenum” to LSOH really isn’t fair. The flame tests for LSOH are really closer to our “CM” grade, a fact that is evident by the rating on many of those shipboard cables, LSOH and CMG.
CMG is a “harmonized” fire rating for both the United States and Canada. It’s equivalent to CM in the U.S. and FT4 in Canada. And both of those are below CMP, plenum ratings.
So all I’m really comparing here is the philosophy, not so much the cables.
That being said, there are expensive cables made for alarm systems that do more than just resist burning. They can survive two hours at 2,000 degrees Fahrenheit and still work! These are mostly speaker cables or similar designs to ring alarm bells or get an evacuation announcement out, even when the rest of the building is on fire. These cables will resist not just the fire but the high-pressure wash from of a fire hose! The secret to these high-temperature cables is the jacket material that essentially turns to glass under high heat.
So if you really need something to work as the building is burning down, you can buy the cable to make it happen, if you’re willing to pay for it.
Steve Lampen is multimedia technology manager for Belden. Past columns are archived at radioworld.com. His book “The Audio-Video Cable Installer’s Pocket Guide” is published by McGraw-Hill.
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