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

Jon Carnes jonc at nc.rr.com
Thu May 13 14:51:50 EDT 2004


On Thu, 2004-05-13 at 14:16, Jeremy Portzer wrote:
> 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

Jeremy's analysis is (as usual) correct. It's the deadly gasses given
off while burning that cause folks to use plenum rated materials in open
air ceilings.
  open air == no conduit

The new zip ties are made from nylon and are fully rated for use in open
air ceilings since they don't give off any spectacularly evil chemicals
when burnt.  Unfortunately the addition of nylon dyes would change
that... so they remain clear.

These new zip ties are taking over the market and the red ones are
almost impossible to find, so I'm sure that my errant inspector will not
make the same mistake later on, but for now he has too much face
invested in being right this time around.

Thanks to Jim though, I will have both a happy inspector and a happy
customer tomorrow!

BTW: I always use 2 zip ties at each anchor point:
 - one made into a loose loop that goes around the cables
 - the other to tightly secure that loop to a ceiling structure

The cables are held well above the tiles and the electrical wiring, and
yet slide effortlessly through the loop while I'm making my pull. I've
run a lot of wire in my days.

Jon




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