I have had a groundless PVC ductwork on 7 machines in my garage dust collection for about 15 years. I use the IVac system that automatically leaves the gates open and cyclone running for an additional 50 secs after the machine is turned off. I have never encountered anything remotely resembling static on my pvc pipework. I do a pipework inspection once a year and have never encountered any saw dust build up in my duct system which could risk a fire.
Also, when I accidentally cut through a nail or screw and it gets sucked up the ductwork I quickly go to the sawdust bin and make sure it doesn't catch fire.
like John I never ever encountered a static build up in my dust collection system I can fill a 30 galleon trash container per week of saw dust.
Some time ago there was a big scare on the old Ships in Scale Yahoo forum about a static spark causing an explosion of saw dust. So I took my video camera out and tested it. If you have an open fire like a camp fire and toss in a handful of saw dust it will flare up and burn. If you have a pile of saw dust and spark igniter no way will you get the pile to burn, even if you had an open flame like a rolled up paper and put in into a pile of saw dust it will go out.
there is just not enough heat generated in a static spark to set saw dust on fire.
I heat my house with a wood burning stove and I produce a lot of saw dust, it crossed my mind to burn the saw dust. Lot easier said than done. You have to mix the saw dust with wax to get it burning. It just smolders and finally it will just die out.
as far as a fire in a dust collection system is not likely it is like a flame thrower you need an ignition flame and blow the saw dust into the flame to set it on fire. I tried it. I unconnected one end of my dust pipe and at the end used a spark and nothing happened I used an open flame and it burned.
I tried to start a fire by putting a flame in a pipe and blowing in saw dust. I burns with a WOOSH! and within seconds it is out. explode! not a chance.
there is not enough saw dust in a small home work shop not even enough in a wood working shop