Suppression Capacitors

Suppression capacitors are meant for suppressing EMI.

class	duty	use	safety
X1	permantly installed/wired equipment	hot to hot or hot to neutral	capacitor must not be used where it can cause risk to human life if it fails
X2	pluggable equipment	hot to hot or hot to neutral	capacitor must not be used where it can cause risk to human life if it fails
Y1	line to ground in double insulated equipment	hot to ground or neutral to ground	capacitor must not fail in a way that causes risk to human life
Y2	line to ground in grounded equipment	hot to ground or neutral to ground	capacitor must not fail in a way that causes risk to human life

capacitor dielectric	capacitor conductor/plate	fail short	fail open	self healing	temprature tolerance	fire risk
paper	metallization	not directly	melt back 1	low residue self healing 2	high	can be self extingushing
paper	foil	common failure mode	uncommon	no	high	can be self extingushing
film	metallization	not directly	melt back 1	very low residue self healing 2	high	can be self extingushing
film	foil	common failure mode	uncommon	no	high	can be self extingushing
ceramic	foil	common failure mode low resistance dead short	uncommon	no	high	yes, not flammable by itself but may shoot molten copper and explode.
tantalum pentoxide	tantalum metal anode, manganese dioxide cathode	common failure mode	uncommon	none, small failures cause runaway	good-high	very high, small failure causes thermal runaway combustion is added by oxigen release from manganese dioxide and tantalum pentoxide
aluminum oxide on foil	aluminum foil, conductive liquid	uncommon	common	no	low	low, liquid electrolyte boils off during a thermal failure removing the short

note 1 self healing may leave a semiconductive residue behind. This buildup over many self healings may cause a thermal failure from fire
note 2 self healing causes the metallization to melt away from dielectric damage

see the following Application Notes for interesting information about when and where to use Suppression Capacitors.