Relay works on the principle of electromagnetic induction. When no current is supplied to the core or supplied current is not sufficient to generate enough magnetic flux to attract the armature, relay remains initial position i.e. normally closed(NC) position.
When sufficient current is supplied to the core, it create sufficient magnetic field around it and then armature gets attracted to the magnetic field created by the core. Then, the armature moves towards the core as armature is made movable opening the relay, the relay changes its position to normally close (NO) and vice-versa. It is now connected in normally opened position and faulty section is disconnected from the supply.
Conclusion
Hope, you have clearly understand about the working of a relay. Make sure to leave your opinion and suggestion about this post in the comment section below.