alarm system vs warning device

alarm system and warning device both are nouns.

Word	Noun	Adjective	Verb	Adverb
alarm system	Yes	No	No	No
warning device	Yes	No	No	No

As nouns, alarm system and warning device are synonyms defined as:

alarm system and warning device: a device that signals the occurrence of some undesirable event

Other synonyms of alarm system include alarm.

alarm system (noun)	warning device (noun)
a device that signals the occurrence of some undesirable event	a device that signals the occurrence of some undesirable event