How is fire spread in server rooms and datacenters?

Every now and then I read that a severe fire has happened in some datacenter, lots of equipment has been damaged and customers have gone offline. Now I wonder what is there to support and spread fire?

I mean walls in a server room usually have little or no finish. Racks are made of metal. Almost all units have metal cases. Cables have (or at least should have) insulation of materials that don't spread fire.

What is spreading fire in a server room or datacenter?

From serverfault sharptooth

• Cables have (or at least should have) insulation of materials that don't spread fire.

  This is false. Most cable insulation is PVC, which is flammable.

  Pipe : It can be made of PVC, but some have retardants to avoid the fire spreed.

  Chris S : @Pipe, unless it has the "plenum" moniker the cable is almost always just plain PVC, which is very flammable once hot.
  From Ignacio Vazquez-Abrams

• What is spreading fire in a server room or datacenter?

  The air handling plant, mostly. Blowing air onto a fire is unlikely to put it out (Well, maybe if it was blowing hard enough)

  Combined with the amount of plastic cabling, ties, coatings on stuff, plastic plugs and sockets. Plenty of stuff to burn.

  From Tom O'Connor

• I think you could even test the air flow by using some kind of smoke machine, to visualize the directions of air in your data-center.

  Mitch Miller : Bad idea, and not enough points left to vote this down. Many data centres have VESDA (Very Early Smoke Detection Apparatus) - if you release smoke in the data centre, you may set off the alarms... at best you will get a bill from the fire department for a false alarm - at worst you may get to experience a halon dump!
  From Shaman

• One thing to remember is that what burns is not just the materials, but the gases they give off. A demonstration of this effect is to light a candle and blow it out. You can often relight the candle by touching a match (away from the wick) to the wisp of smoke.

  Here's a video on YouTube that demonstrates lighting candle smoke.

  From Dennis Williamson

• Here's a bit of information not generally published and at times even denied - the insulation used in most electronic components will burn and burn rather well once a suitable temperature has been reached. This includes the material circuit boards are made of, as well as the lacquer used to coat most components. Some types of insulation, once lit, will add to the problem by producing gases that themselves are somewhat flammable. Of course they also produce noxious gases that could incapacitate or kill anyone unfortunate enough to breath in too much of it.

  The insulation on normal (non fire resistant) cables burns extremely well. Even worse, prior to, as well as after, ignition the plastic flows very freely, which helps greatly in the spread of the fire. During a fire some components, such as capacitors, will explode, which further helps to spread the fire by throwing already burning bits around.

  Once the temperature is nice and high we start to factor in things like paint and other decorative coatings that can be hard to light but burn well once lit. Even powder coating will burn. Far from adding protection from an already lit fire the metal case will add to the fire by providing additional fuel (the paint or other coating) and will help to sustain the fire by retaining heat and feeding it back to the fire.

  Tom O'Connor : Get the temperature high enough, and you can burn the magnesium out of the alloy they make hard disk drives out of. Aluminium, and most other metals too, for that matter.. Talking epic high temperatures for this to happen though..

  Farseeker : Your wealth of knowledge never fails to amaze me

  Tom O'Connor : @Farseeker If that was directed at me, that's something that I get told a lot.

  Farseeker : @Tom, Hmm, it was directed at John, but hey, no harm in sharing ;)
  From John Gardeniers