A method and circuit for detecting a fault in a power transformer having an conductive shield layer sandwiched between electrical insulating layers separating the conductive shield layer from a first conductor and a second conductor, the second conductor opposite the conductive shield layer from the first conductor, and including, sensing a voltage energizing the shield layer, comparing the sensed voltage to a threshold voltage value corresponding to a fault, and upon satisfaction of the comparison, providing a fault indication when the comparison indicates the presence of a fault.