Isolated sensing should be used in high voltage and noisy measurement. The LEM LV25-P is a hall-effect voltage sensor. It has galvanic isolation in between the sensing voltage, and signal output. It has safety certification (EN 50178: 1997), thus make it a very expensive device.
This blog show the connection for LV25-P
http://electronictoofree.blogspot.my/2015/03/isolated-ac-voltage-sensing-using-lv25-p.html