hi,
we designed an electronic card which is supplied by a dc adapter(i.e. no safety ground connection). the card is in a box and the box is shielded. i wonder where should i connect the shield? because there is no earth connection.
The shield can function correctly even if it is not earthed. In your situation it might be best to connect the shield to the circuit reference on the card (ie. DC ground). However, it would be useful to know a few things first. Are there cables passing into and out of the box? Is it likely that a person would touch the sheild metal in normal use? Is the sheild there for a specific purpose, or just general protection?
The best bet would be to get a real earth (not the articial "earth" taken from neutral) shield connected to the box. The shielded cable should only be connected at the box to leave the other end unconnected.
The shielding in that particular case is not AF which requires shielding on both sides. Shielding AF cables on both sides is a cripple. A third neutral wire and an extra shielding is much better.