High speed fuses


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In the product which i'm testing there are three 25A fuses provided in parellel on both Phase and Neutral lines. While doing short circuit test the external line Circuit breaker rated for 50A trips and the fuse is intact. Short circuit current seems to be of 800A and it is tripped within 3-5mS. Is there any high speed fuse available which blows before the Circuit breaker gets tripped and also holds good for tests like surge test(were high current is made to flow for very short duration of 8/20uS)or any other suggestion
Max current is 40A for the product..
Even with 20A fuses also Circuit breaker trips with fuse not blowing.