I came across some literature from Tyco on what they call "back EMF suppression", basically the reason everybody installs diodes on their contactor coils. After reading a bunch of literature, it appears that the diodes protect the switches on panel, but severely degrade the speed that the high amperage contacts separate. That's bad, since it increases the wear on the contacts significantly. I remembered that Eric at Perihelion Design sold Snapjacks that are supposed to be superior to diodes. They're Bi-Directional Zener Transient Voltage Suppressors, what a mouthfull. That is exactly what the Tyco literature explained.
Eric only lists a SnapJack kit for 12V airplanes. I need 24V and I only need a few SnapJacks. So I decided to track down what Eric is using in his SnapJacks. By tracking Eric's part #P6KE18CA, they also manufacture a P6KE36CA that increases the voltage to what I need for my customer. I figured Eric had a 50% headroom 12-18V for his Snapjacks, so I should go with 36V.
Bottom line, these seem to give you all the advantages of protecting your switch, and don't have the disadvantage of slowing down the contact separation time.
Here's a chart from Eric's literature on SnapJacks.
You could ask Eric about a 24V kit or track down the parts from the normal suppliers.
Mike Easley
Colorado Springs