A ground fault detection device comprising: line (416, 716, 1016, 1116, 1216) and neutral (418, 718, 1018, 1118, 1218) conductors configured to connect an AC power source and a load; a differential current transformer (412, 712, 1012, 1112, 1212) comprising: a toroid (414, 714, 1014, 1114, 1214), through which the line (416, 716, 1016, 1116, 1216) and neutral (418, 718, 1018, 1118, 1218) conductors pass, a first secondary winding (420, 720, 1020, 1120, 1220) wound on the toroid (414, 714, 1014, 1114, 1214) configured to generate a current in response to an imbalance of magnetic flux in the toroid (414, 714, 1014, 1114, 1214), and a second secondary winding (421, 721, 1021, 1121, 1221) wound on the toroid (414, 714, 1014, 1114, 1214); a differential ground fault detector (404, 704, 1004, 1104, 1204) electrically connected to the first secondary winding (420, 720, 1020, 1120, 1220) of the differential current transformer (412, 712, 1012, 1112, 1212) and configured to compare the current generated in the first secondary winding (420, 720, 1020, 1120, 1220) to a predetermined threshold; a controller (406, 706, 1006, 1106, 1206) configured to control a test stimulus signal to be generated in the second secondary winding (421, 721, 1021, 1121, 1221); characterised in that the ground fault detection device comprises a DC power supply (410, 710, 1010, 1110, 1210) electrically connected to the second secondary winding (421, 721, 1021, 1121, 1221) wound on the toroid (414, 714, 1014, 1114, 1214) of the differential current transformer (412, 712, 1012, 1112, 1212), wherein the controller (406, 706, 1006, 1106, 1206) is configured to control the DC power supply (410, 710, 1010, 1110, 1210) to generate directly the test stimulus signal in the second secondary winding (421, 721, 1021, 1121, 1221) from the DC power supply (410, 710, 1010, 1110, 1210). line (416, 716, 1016, 1116, 1216) and neutral (418, 718, 1018, 1118, 1218) conductors configured to connect an AC power source and a load; a differential current transformer (412, 712, 1012, 1112, 1212) comprising: a toroid (414, 714, 1014, 1114, 1214), through which the line (416, 716, 1016, 1116, 1216) and neutral (418, 718, 1018, 1118, 1218) conductors pass, a first secondary winding (420, 720, 1020, 1120, 1220) wound on the toroid (414, 714, 1014, 1114, 1214) configured to generate a current in response to an imbalance of magnetic flux in the toroid (414, 714, 1014, 1114, 1214), and a second secondary winding (421, 721, 1021, 1121, 1221) wound on the toroid (414, 714, 1014, 1114, 1214); a toroid (414, 714, 1014, 1114, 1214), through which the line (416, 716, 1016, 1116, 1216) and neutral (418, 718, 1018, 1118, 1218) conductors pass, a first secondary winding (420, 720, 1020, 1120, 1220) wound on the toroid (414, 714, 1014, 1114, 1214) configured to generate a current in response to an imbalance of magnetic flux in the toroid (414, 714, 1014, 1114, 1214), and a second secondary winding (421, 721, 1021, 1121, 1221) wound on the toroid (414, 714, 1014, 1114, 1214); a differential ground fault detector (404, 704, 1004, 1104, 1204) electrically connected to the first secondary winding (420, 720, 1020, 1120, 1220) of the differential current transformer (412, 712, 1012, 1112, 1212) and configured to compare the current generated in the first secondary winding (420, 720, 1020, 1120, 1220) to a predetermined threshold; a controller (406, 706, 1006, 1106, 1206) configured to control a test stimulus signal to be generated in the second secondary winding (421, 721, 1021, 1121, 1221); characterised in that the ground fault detection device comprises a DC power supply (410, 710, 1010, 1110, 1210) electrically connected to the second secondary winding (421, 721, 1021, 1121, 1221) wound on the toroid (414, 714, 1014, 1114, 1214) of the differential current transformer (412, 712, 1012, 1112, 1212), wherein the controller (406, 706, 1006, 1106, 1206) is configured to control the DC power supply (410, 710, 1010, 1110, 1210) to generate directly the test stimulus signal in the second secondary winding (421, 721, 1021, 1121, 1221) from the DC power supply (410, 710, 1010, 1110, 1210).