An electric quantity measuring device comprising: - two input clamps (2, 3) adapted to be electrically connected with two output terminals (4, 5) of an electric quantity transducer for receiving an AC current or an AC voltage in said input terminals (2, 3), - a processing unit electrically connected with said two input clamps (2, 3) and configured to generate at least one electric quantity measuring signal for measuring electric quantities related to said AC current and said AC voltage, - a switch device comprising one electronic switch and resistive means arranged in series, wherein one end of said electronic switch is electrically connected with one of said two input terminals (2, 3) and one end of said resistive means is electrically connected with the other of said two input clamps (2, 3), said electronic switch being operably configured to switch between an open configuration in which it prevents the passage of electric current between said two input clamps (2, 3) and a closed configuration in which it allows the passage of electric current between said two input clamps (2, 3), wherein - said processing unit detects a potential difference value at said two input clamps (2, 3) and, according to said potential difference value, said processing unit is configured to cause said electronic switch to switch to said open configuration when it detects that said two clamps (2, 3) receive said AC voltage at their inputs, and to said closed configuration when it detects that said two clamps (2, 3) receive said AC current at their inputs. - said electronic switch comprises two antiserially-connected N-type Mosfet transistors (11, 12) which are powered with a floating voltage, said processing unit is configured to control said floating voltage between the gate and the source of said two Mosfet transistors (11, 12) to thereby switch said electronic switch between said open configuration and said closed configuration, characterized in that - said electric quantity measuring device comprises a charge pump in signal communication with said processing unit, said processing unit is configured to generate a PWM control signal with such a preset duty cycle frequency that the voltage Vgs between the gate and source electrodes of said two antiserially-connected Mosfet transistors (8, 9) will be at least 5V. - two input clamps (2, 3) adapted to be electrically connected with two output terminals (4, 5) of an electric quantity transducer for receiving an AC current or an AC voltage in said input terminals (2, 3), - a processing unit electrically connected with said two input clamps (2, 3) and configured to generate at least one electric quantity measuring signal for measuring electric quantities related to said AC current and said AC voltage, - a switch device comprising one electronic switch and resistive means arranged in series, wherein one end of said electronic switch is electrically connected with one of said two input terminals (2, 3) and one end of said resistive means is electrically connected with the other of said two input clamps (2, 3), said electronic switch being operably configured to switch between an open configuration in which it prevents the passage of electric current between said two input clamps (2, 3) and a closed configuration in which it allows the passage of electric current between said two input clamps (2, 3), wherein - said processing unit detects a potential difference value at said two input clamps (2, 3) and, according to said potential difference value, said processing unit is configured to cause said electronic switch to switch to said open configuration when it detects that said two clamps (2, 3) receive said AC voltage at their inputs, and to said closed configuration when it detects that said two clamps (2, 3) receive said AC current at their inputs. - said electronic switch comprises two antiserially-connected N-type Mosfet transistors (11, 12) which are powered with a floating voltage, said processing unit is configured to control said floating voltage between the gate and the source of said two Mosfet transistors (11, 12) to thereby switch said electronic switch between said open configuration and said closed configuration, characterized in that - said electric quantity measuring device comprises a charge pump in signal communication with said processing unit, said processing unit is configured to generate a PWM control signal with such a preset duty cycle frequency that the voltage Vgs between the gate and source electrodes of said two antiserially-connected Mosfet transistors (8, 9) will be at least 5V.