Fault current interrupter including a metal oxide varistor

A fault current interrupter comprising mechanical switching means and current limiter means connected in an electric circuit between a source of electric power supply and a load, and switch actuating means for opening said mechanical switching means to interrupt the circuit when a predetermined time interval has elapsed after occurrence of a fault current, said current limiter means comprising:

• i. first current sensitive means exhibiting a resistance value which is negligible with respect to that of the load when a rated current flows into the circuit and, in the occurrence of a shorter circuit, a resistance value which rapidly increases during said predetermined interval, and of
• ii. second voltage and current sensitive means having a clamping threshold voltage whereby substantially no current flows therethrough when the voltage across its terminal is lower than said clamping threshold voltage, while, when an increasing current flows therethrough, the voltage drop across its terminal substantially keeps the said clamping threshold voltage value until the said increasing current reaches a predetermined intensity and when said increasing current exceeds said predetermined intensity, the voltage drop across said terminals rapidly increases substantially above said clamping threshold voltage value, wherein the said clamping threshold voltage is so predetermined at a value substantially lower than the supply voltage than, when a current transfer takes place in the occurrence of a short circuit during said predetermined time interval from said first current sensitive means to said second voltage and current sensitive means at the time when the voltage drop across said first current sensitive means reaches said threshold, the current flowing through the second voltage and current sensitive means will increase until it exceeds said predetermined intensity, whereby the voltage drop across the terminal of the second voltage and current sensitive means will rapidly and substantially exceed said clamping threshold voltage during said predetermined time interval, said second means being connected in series with said mechanical switching means to form a series combination and said first means being connected in parallel across said series combination and comprising a positive temperature coefficient resistor connected in series with a mechanical circuit breaker switch.