Using a 30V Switch on a 28V load
Are these AC or DC?  It's probably not so very important at only 28 volts DC, but you should be in the habit of being very cautious about DC ratings.   

DC does not have zero crossings like AC does, and so once a DC arc is struck, nothing will stop it.  This makes things extra difficult for DC switches, particularly of higher voltages than this.  I've seen a 600VDC contactor goobed together out of a 3-pole AC contactor that is rated 960VAC (intended for 3-phase power).  All three poles were wired in series.  Even then, the device was only rated for 140 volts DC, so its spec was being exceeded even if we only ask each pole to interrupt 200VDC! 

Most switches will have a DC voltage and/or current rating quite below their AC rating.   You have to check it!