Overvoltage protection of a.c. measuring devices

Claim 1

IN A A.C. CIRCUIT INCLUDING A FIRST PAIR OF TERMINALS ACROSS WHICH AN A.C. VOLTAGE IS TO BE MEASURED, AND AN A.C. MEASURING DEVICE HAVING A PAIR OF VOLTAGE INPUT TERMINALS ELECTRICALLY CONNECTED ACROSS SAID FIRST PAIR OF TERMINALS AND WHICH IS CAPABLE OF SAFELY HANDLING VOLTAGES BELOW A GIVEN MINIMUM UNSAFE VOLTAGE LEVEL, THE IMPROVEMENT COMPRISING OVERVOLTAGE PROTECTION MEANS ELECTRICALLY CONNECTED ACROSS SAID VOLTAGE INPUT TERMINALS, SAID OVERVOLTAGE PROTECTION MEANS COMPRISING A BI-DIRECTIONAL SEMICONDUCTOR CURRENT CONTROLLING DEVICE INCLUDING A SOLID STATE SEMICONDUCTOR MATERIAL AND ELECTRODES COUPLING THE SAME ACROSS SAID INPUT TERMINALS, SAID SOLID STATE SEMICONDUCTOR MATERIAL IN ONE STATE HAVING AT LEAST PORTIONS THEREOF BETWEEN THE ELECTRODES IN ONE CONDITION WHICH IS OF HIGH RESISTANCE AND SUBSTANTIALLY AN INSULATOR FOR BLOCKING THE FLOW OF CURRENT THROUGH THE CURRENT CONTROLLING DEVICE THERETHROUGH IN EITHER OR BOTH DIRECTIONS WHEN AN APPLIED VOLTAGE IS BELOW AN UPPER THRESHOLD VOLTAGE LEVEL WHICH IS IN EXCESS OF THE PEAK VALUE OF A NORMAL RANGE OF A.C. VOLTAGES APPEARING ACROSS SAID FIRST PAIR OF TERMINALS AND LESS THAN SAID MINIMUM UNSAFE VOLTAGE LEVEL, AND BEING DRIVEN SUBSTANTIALLY INSTANTANEOUSLY INTO ANOTHER STATE WHEREIN SAID AT LEAST PORTIONS THEREOF BETWEEN THE ELECTRODES ARE IN ANOTHER CONDITION WHICH IS OF LOWER RESISTANCE AND SUBSTANTIALLY A CONDUCTOR FOR CONDUCTING THE FLOW OF CURRENT THERETHROUGH IN EITHER OR BOTH DIRECTIONS WHEN THE PEAK VALUE OF THE APPLIED VOLTAGE THEREOF IS RAISED ABOVE THE UPPER THRESHOLD VOLTAGE LEVEL AND THE PEAK VALUE THEREOF REMAINS ABOVE A LOWER THRESHOLD VOLTAGE LEVEL WHICH IS ABOVE THE PEAK VALUE OF THE NORMAL EXPECTED VOLTAGES ACROSS SAID FIRST PAIR OF TERMINALS, AND REVERT IMMEDIATELY TO SAID BLOCKING CONDITION WHEN THE PEAK VALUE OF THE APPLIED VOLTAGE DROPS BELOW SAID LOWER THRESHOLD VOLTAGE LEVEL, AND THERE BEING IN SERIES WITH SAID SEMICONDUCTOR CURRENT CONTROLLING DEVICE A VOLTAGE DROPPING IMPEDANCE WHICH REDUCES THE VOLTAGE ACROSS SAID A.C. MEASURING DEVICE WHILE CURRENT FLOWS THROUGH SAID SEMICONDUCTOR CURRENT CONTROLLING DEVICE.