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Popping a 5000 amp fuse

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Brian Jojade:

--- Quote from: Stephen Swaffer on June 14, 2021, 01:13:06 PM ---My facility had a fused main disconnect-with 3000 amp fuses.  We replaced it with a breaker a couple of years ago.

Often, fuses are a better choice to reduce arc flash, because they can interrupt current faster than breakers.  Unfortunately, when you get into fuses that large, the available fault current may not be high enough the quickly blow the fuse.  IIRC the fault current I calculated was somewhere around 12,000-15,000 amps-which put the clearing time for the fuses in questions around 2-3 seconds.  I'm thinking it would seem like a lot longer than 2-3 seconds in the event of a fault like that!

--- End quote ---

The job of a fuse is to protect things.  In the case of a massive fault, 2-3 seconds would probably be fast enough that it would fail before any connecting lines heated enough to be damaged.  Just like the fuse, a piece of wire can handle a LOT more current than it's rated for for short durations.  It's a factor of time vs heat generated and dissipated.

Stephen Swaffer:

--- Quote from: Russell Ault on June 14, 2021, 03:48:55 PM ---This is fascinating to me; does this just apply to thermally-triggered breakers, or is this true for any reset-able over-current devices?

-Russ

--- End quote ---

I am sure it would apply for any resettable device-there is a finite inertia of the mechanisms that must be overcome and the larger the current the heavier those mechanisms are.  The disconnect in the fused switchgear was a massive switch.  I suppose I should dig back into my physics books and look at the formulas for momentum and acceleration-but you are looking at very small fractions of a second clearing time.

Brian-you are correct.  I was picturing in my minds eye an arc flash event lasting 2 or 3 seconds.

Jared Bartimus:

--- Quote from: Keith Broughton on June 11, 2021, 06:48:03 AM ---No Health and Safety issues with this :o ;D

--- End quote ---

He actually has some pretty decent Health and Safety practices.  Including using a pneumatic switch to activate his high voltage supplies.  He has a video where he went through his other "policies" when playing with electricity.

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