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Entry type: FAQ, Entry ID: 21656061, Entry date: 05/30/2005
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3VF: Time discrimination between upstream fuses and downstream 3VF Circuit-Breakers?

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QUESTION:
What is the time discrimination between upstream fuses and downstream 3VF circuit-breakers?

ANSWER:
Discrimination between upstream fuses and downstream 3VF circuit-breakers:--------------------------------------------------------------------------------------Overcurrents:Discrimination is provided if the tripping characteristics of both protection devices do not touch each other. The tripping characteristic of downstream circuit-breakers must be quicker than that of the upstream fuses.--------------------------------------------------------------------------------------Short-circuit currents:It must be taken into consideration that fuses are still heated up during the arc quenching process of the circuit-breaker. In practice it is sufficient if the tripping characteristic of the instantaneous release of downstream circuit-breakers is at least 70 ms quicker than that of the upstream fuses.--------------------------------------------------------------------------------------
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