I need to detect a break in 28 V DC voltage of the order of 100 micro seconds
Use a voltage divider and Arduino. Have it trigger an interrupt on level change (You can use that on a digital port). Then loop for 100 microseconds loooking for the level to change back up. If not, output your signal on some other pins.

Voltage divider should bring voltage down to around 4-5V, so 1:6 or so. 68kOhm and 10kOhm should be OK. Or 1kOhm / 6.8kOhm