Double-check all connections to verify they are tight. Are the low-voltage ps passive or active devices (i.e., heavy transformers, or electronic devices)? Disconnect one fixture/transformer at a time and see if that helps isolate the problem. You could also have a nail or staple in a wire somewhere. Does it trip immediately, or intermittently after some arbitrary delay? Not all arc detecting cb are created equal...some have better filters in them to prevent nuisance tripping.