If I'm correct in what I think you're doing?, then the supply for the meter module needs to be completely floating from the main supply. You can either use a seperate transformer, or a seperate secondary winding on your main transformer.
Assuming you don't have a seperate unused winding?, then you need another mains transformer! - however, as it's only for powering the meter, it need only be a very small one.
I went through this years ago building a power supply at work, and ended bu adding another small mains transformer.