Protective Relay

According to IEEE    
 Relay :
 ‘‘an electric device that is designed to respond to input conditions in a prescribed manner
   and, after specified conditions are met, to cause contact operation or similar
   abrupt change in associated electric control circuits.’’

Relays are used in all aspects of activity: home, communication, transportation,
commerce, and industry.

Protective relay:
 ‘‘a relay whose function is to detect defective lines or apparatus or other power system conditions of an      abnormal or dangerous nature and to initiate appropriate control circuit action’

Fuse:
‘‘an over-current protective device with a circuit-opening fusible pat that is heated and severed
by the passage of the over-current through it’’