Definitely personal preference. 

I've done both. I'd prefer to mount directly onto the unistrut framework, but then the trunking etc is also a little more difficult to mount, but not really. 

Just musing - what's Paxolin like for fire resistance?

I know it's been used for such things almost since the days of Faraday, but I'm wondering if current requirements for for resistant cable fixings (and sometimes fire resistant DBs) might be undermined somewhat by making them dependent on what I gather is basically paper and resin.

   - Andy.

fire rated fibreglass 'FR4' or similar - essentially thick pcb  (printed circuit board, not the oil) without the copper tracks may be preferred in those cases where you cannot go metal to metal for some reason..
Personally I'd only use paxolin where the fire rating of a hardwood would also be OK - which to be fair is most common situations.

Note that fibreglass, fire rated or not, makes a right mess of drills and cutter blades, so these need to be factored into the job cost as consumable items. Paxolin burns cutters if they are used too fast as it does not conduct heat well, for the same reasons it does not conduct electricity, but even when taken slow, the glass actually grinds away the steel cutting edge.

Mike

Sounds like making work for the sake of it

Humm information on the fire performance of paxolin (or FR-2 or FR-4) seems rather thin on the ground (or thin on Google at least) - "self-extinguishing" seems to be a specific as I can find, which as I recall is about the standard that plastic CU cases were supposed to meet, which might not be that reassuring.

   - Andy.

To be declared FR4, the material has to meet UL 94 V0 - when a flame is applied to a vertically held sample and then removed, it must not keep burning for more than 10 seconds, and if anything flakes off or drips off it must not be burning. Paxolin type material generally does not meet this though some brands do claim to meet the easier FH1. Similar classification ratings occur in IEC 60695-11-10.

We used to use a lot of sheets of Tufnol in varying thicknesses - if it got hot it just used to char and give off a awful smell, same when drilling and cutting the stuff. No idea if it is still available though.

I've also used Tufnol in the past for different applications. A quick look on RS shows they still sell it in various types. Whale is cotton weave and Kite is paper based. I seem to remember Carp being a variant as well.