A circuit state sensing device (100) includes a transformer (102) having a primary coil (104A) and a secondary coil (104B). In use, the secondary coil is connected to a target circuit (106) for which a state is to be sensed. The device further includes a measuring arrangement (112, 122) configured to take a measurement relating to impedance at the primary coil, and a processing arrangement (122) configured to process the impedance measurement to determine a state of the target circuit.