I have an application (*) where I need to two pole switch about 12 amps DC
at 24v into an inductive load, however at times the un-energised contacts
will have 415v AC between them. Now because of the 415v AC I will need to
use a contactor rated at that voltage, but will presumably have to derate
it's current carrying capacity when switching DC. What factor should I use?
Manufacturers give loads of ratings but only tend to specify for either AC
or DC but not both in the same contactor!

(*) the application is a DC injection brake. The three phase motor is
driven via a contactor from 415v AC. When the contactor is de-energised,
after a fraction of a second delay, a second contactor (the one in question)
will squirt about 12 amps from a 24v DC source into one winding for a
maximum of ten seconds then itself de-energise. (The delay between the first
contactor dropping out and the second one pulling in is obviously a self
preservation exercise to avoid a big bang and blown up 24v power supply -
they will also be interlinked to prevent dual energisation)

AWEM

(An alternative approach would be to use the braking inherent in a suitable
inverter drive or commercial DC braking units, but the three motors in
question are 7.5 HP 7.5 HP and 5.5 HP so unless someone has three inverters
or DC brakes of suitable beefiness available for a song they are ruled out
of my budget!)