reason for plenum ratings -- Re: [TriLUG] OT: looking for "red" plenum zip ties

Jeremy Portzer jeremyp at pobox.com
Thu May 13 14:16:29 EDT 2004


On Thu, 2004-05-13 at 13:58, Aaron S. Joyner wrote:
> The purpose behind Plenum 
> ratings is to ensure that nothing in your open-air return ceilings is 
> going to cause the fire to spread from one section of the building to 
> another.  The situation in question is with a fire in one room of the 
> building, and the HVAC system circulating super-heated air from the fire 
> into the ceiling return area, will the wire ties burst into flame and 
> set something else on fire, thus causing the entire building to burn 
> down at a very rapid rate.  It's a certainly valid concern, and it's his 
> job to pro actively prevent it. 

This is not the main concern.  The main reason we have plenum-rated
cables and zip-ties is because standard cables are made of PVC
(poly-vinyl chloride) which when burned, releases deadly chlorine gas. 
If a fire were to get in a plenum area (return air ducts, or ceiling
area used as return air ducts), it could burn the PVC cable.  The HVAC
system would then blow this chlorine gas to other areas of the building,
instantly poisoning everyone.

Plenum-rated cables are made of Teflon or other plastics that don't
release  chlorine gas.  Teflon can burn, though -- but burning isn't the
main concern, it's the chlorine.  Teflon is a lot more expensive than
PVC, hence the higher cost of plenum-rated cables and zip ties.

Clearly the color is not a factor, except to the extent that this
inspector believes all plenum-rated zip ties should be red.  Inspectors
are used to this color coding (for example, bathroom-rated drywall is
green, certain types of electrical cable are yellow, etc.).  But if the
manufacturer of Jon's zip ties has  placed no other marking to indicate
plenum rating, I could see his concern.

--Jeremy

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