class X and class Y capacitors are neat
class-X capacitors are guaranteed to fail short if they fail, and are to be placed between live and neutral, or between two live phases, so that if they fail, high current flows, which trips a breaker or blows a fuse and leaves no hazard
class-Y capacitors are guaranteed to fail open; they're put between live and ground and neutral and ground, so that a failing capacitor doesn't result in something meant to be grounded becoming live
-F