Electrical protection device for alternating current and with limited power absorption, operating by detecting the earth resistance with elimination of the interference due to any voltage between earth and neutral

摘要

A description is given of an electrical protection device which operates with alternating current with limited power absorption, by the detection of values, which may be very small, of earth resistance. Starting from the phase voltage, the device supplies and charges a capacitor which reaches a voltage equal to a specified reference value whose size depends on the voltage between earth and neutral, is discharged through a controlled switch in a circuit consisting of the neutral, the earth connection, the earth resistance and a sampled resistance of any signal of the detection of the existence of an earth resistance exceeding an acceptable maximum value. The value of this sampled resistance is comparable to the aforesaid maximum value of the earth resistance, so that effective earthing can still be guaranteed. The pulsed voltage sampled across the terminals of this resistance is compared with a predetermined threshold value, the crossing of which in a downward direction causes the generation of a signal capable of operating the protection devices of the installation. IMAGE