LarryD:
One application for NC is in a power fail circuit.
If your relay is powered by AC mains the NC contact will be open.
You connect a battery to the contact then to a bell.
When your AC mains fails the relay drops and the NC contact closes.
Therefore the battery is connected to the bell and the power fail is annunciated.
WOW! perfect application. I never thought this way. but how can i power relay with ac 220v when it takes 5 v as input .