In one embodiment, a method for determining capacitive signature validity of a powered device (PD) attached to power sourcing equipment (PSE) having (i) an isolated side with a primary coil and (ii) a line side with a secondary coil connected to the PD. The method includes determining, on the isolated side, a first time T1 and a corresponding first voltage V1 across the PD. Then generating, on the isolated side, a switching signal used to generate an electrical current through the primary coil. Then determining, on the isolated side, a second time T2 and a corresponding second voltage V2 across the PD, wherein a difference between V2 and V1 is related to the electrical current provided to the primary coil. Then determining the capacitive-signature validity of the PD based on T1, T2, V1, V2, and a resistive signature of the PD.