Surge protective circuit and surge protective device

A surge protective circuit connected between a three-phase AC power source and an apparatus that operates with electric power supplied from the AC power source to suppress an overvoltage applied from the AC power source to the apparatus, the surge protective circuit comprising:

• a first constant-voltage device and a first discharge device connected with one another between a first ground terminal and a first phase terminal of the AC power source;
• a second constant-voltage device and a second discharge device connected with one another between a second ground terminal and a second phase terminal of the AC power source;
• and a third constant-voltage device connected to a third phase terminal of the AC power source, wherein
• a midpoint between the first constant-voltage device and the first discharge device, a midpoint between the second constant-voltage device and the second discharge device, and a side of the third constant-voltage device opposite to a side connected to the third phase terminal are connected,
• the first, second and third constant-voltage devices do not conduct at a voltage lower than a rated voltage, but when an overvoltage of the rated voltage or more is applied due to a surge, the resistance decreases abruptly and the first, second and third constant-voltage devices becomes a conductive state,
• the first and second discharge devices are brought into a conductive state by being discharged by applying an overvoltage equal to or higher than the discharge voltage due to a surge,
• the first, second and third constant-voltage devices have a function of suppressing discharge of the first and second discharge devices by becoming an insulating state when the voltage drops to a predetermined range due to discharge of the first, second and third discharge devices,
• the midpoint between the first constant-voltage device and the first discharge device, the midpoint between the second constant-voltage device and the second discharge device, and the side of the third constant-voltage device opposite to the side connected to the third phase terminal are connected by an intermediate line,
• in an absence of a failure of the first constant-voltage device, the first discharge device, the second constant-voltage device, the second discharge device and the third constant-voltage device,
• a surge current on a first power line connecting the AC power source to the first constant-voltage device is discharged through the first constant-voltage device, the intermediate line, and at least one of the first and second discharge devices,
• a surge current on a second power line connecting the AC power source to the second constant-voltage device is discharged through the second constant-voltage device, the intermediate line, and at least one of the first and second discharge devices, and
• a surge current on a third power line connecting the AC power source to the third constant-voltage device is discharged through the third constant-voltage device, the intermediate line, and at least one of the first and second discharge devices,
• if a failure occurs in one of the first and second discharge devices, surge current is discharged by the other discharge device, and
• if a failure occurs in one of the first, second and third constant-voltage devices, surge current is discharged through at least one of the first and second discharge devices from at least one of the other constant-voltage devices.