under what condition is dc applied safely to the primary of a transformer?

Answer 1

Under no condition unless you add sufficient resistance in series.
Transformer is an inductive load. the DC resistance is zero ( in theory) So it will short circuit. To avoid it you have to add a Resistance of sufficient value to limit the current.

What is the purpose of doing this. DC wont come on other side.

Answer 2

Why would you want to apply DC current to a transformer? A transformer uses AC current in the primary and induces an AC current into the secondary. From that point DC current is produced through a bridge rectifier.