Under what condition is D.C supply applied safely to the primary of a transformer?

 As i mentioned ,This is a simple electrical interview question. So if some one ask this question in interview or somewhere..then the right answer will be this....I think u got the main Idea 
Also
As we know that Transformer works only on A.C, in case of D.C Supply, the primary of transformer may start to smoke and burn....But this is the only one condition where we can operate a Transformer on D.C (although the Circuit is Useless)...For more information....in the above blog...U will see a post where we talked about the topic.....1 Transformer primary with A.C supply, 2. Transformer primary with D.C Supply.