I have 20 fluorescent light fixtures controlled by 4 motion detectors triggering a 20 amp relay. The detectors red trigger wires are all connected parallel to the relays coil. How come I keep frying my motion detectors. I've tried several different brand of detectors.
Sometimes the system will function for a few weeks.

The Motions need to be on the same circuit, If they were powered from different circuits, particularilly the opposite phase, I would see a problem.
I don't see the relay causing the problem as it surely draws less than 1 Motion can handle.
Can you verify they are on same circuit? Do they all go out?