Electromagnetic field of a transformer

Hello, It was a long time that I was looking for the answer to this question that how one should stay away from transformer electromagnetic noise, now I think I understood the answer, I'll post it here to be disscused and possibly verified by Electromagnetic experts. If we want to shield a sensetive board from an external electric field, we will shield it by earth(physical earth) to provide a low impedance path for noise back to its source which is power generating company. There is no easy way to shield from magnetic field(especially low frequency one) other than keep distance from it.
Because in many sensetive boards signal ground and physical earth can't be connected directly it is better to put the transformer in a separate metal cage and connect it to physical earth. this will have little effect on magnetic field but good protection against electical field.
Thanks

i think that is right.