Electrical network protection device with automatic switch over between protection criteria

Claim 1

An electrical network protection device comprising: a distance protection device (11) coupled to said electrical network and indicating a fault condition in said network responsive to voltage signals (U 1 and U 2 ) derived from said network and being a function of voltages in said network; an overcurrent protection device (X) coupled to said electrical network and indicating a fault condition in said network responsive to a current signal (I f ) derived from said network and which is a function of current flowing in said network; circuit breaker means (3) coupled to said electrical network; a threshold-value logic system coupling said distance protection device (11) and said overcurrent protection device (X) to said circuit breaker means (3) and including OR gate means (15) having an output operatively coupled to said circuit breaker means (3) and having two inputs, one of which is coupled to an output of said distance protection device (11) and the other of which is coupled through additional gating means to the output of said overcurrent protection device (X), said additional gating means comprising first AND gate means (16) having one input coupled to the output of said overcurrent protection device (X) and a further input gating means (16a) coupled with said first AND gate means and having inputs coupled to said voltage signals (U 1 , U 2 ) through respective first threshold switches (12, 13) which deliver output signals when the corresponding voltage signals (U 1 , U 2 ) exceed predetermined threshold values, the output signal of said first AND gate means (16) depending on at least one of said voltage signals (U 1 , U 2 ) failing to exceed the corresponding one of said predetermined threshold values.