The separator, assuming you empty it often enough, does
a great job of keeping big stuff from dinging the impeller.
Also keeps shavings/curlies from hand planing from
clogging up the DC's intake.
Another potential tip - if the unit you have has a Y inlet
to allow two 4 inch dust lines to be connected at the
DC, orient the one you won't be using down and the rubber
plug left on. If an impeller dinger gets through the
separator and hits the impellar it'll often drop into
the unused, downward pointing port - where you can
get it without having to reach in and find it. NEVER
reach in around the impeller unless you've personally
unplugged the DC or verified that the unit's unplugged
JUST prior to reaching in. One more thing to remember
it can hurt you.
Oh, and ditch the dust bags that come with the unit
unless you got the 5 micron option, 1 micron being
preferable. 1 micro oversized felt bags work pretty
well. The big stuff that the stock bags catch isn't
what will mess up your lungs - it's the very little
stuff that'll screw you up.
Here's a great site with plenty of useful info on