Why use a 400 amp fuse after a 200 amp breaker?
I'm trying to coordinate them so breaker not fuse opens in the event of any normal overload. But, in case of a dead short, fuse blows and prevents breaker from being destroyed. Also successfully stops current if breaker can't due to insufficient rating. It looks to me like smaller fuses 225A to 350A might blow too soon. So I'm selecting a fuse which should be slower under moderate overload and faster under extreme overload.
Trip curves for my 200A main breaker show it will hold 4x (800A) for at least 10 seconds and possibly as long as 80 seconds, but above 12x (2400A) will trip in under 40 ms.
Trip Curve: QOM2 200A 225A 2P - No. 736-07 (QOM2 QOM2-VH Main Circuit Breakers)
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400A Class T JLLN fuse will clear at > 2000A in less than 50ms, will hold 800A for 100 seconds.
At high currents such as 40,000A, the breaker magnetic trip would be open in less than one 17 ms 60 Hz power line cycle but possibly unable to clear the fault (current continues to flow in arc). Fuse will also blow in much less time and cut short the arc.
Breaker has 22kA interrupt rating. "current limiting" Fuse, if interrupting a fault with 200,000 short-circuit current, limits time so breaker only experiences 12,000A equivalent.
Same applies for DC, except the class T fuse is only rated 20kA interrupting up to 125VDC. (just a bit above my 400A AGM battery's short circuit current.) I want my breakers to trip for any overload, but they are at the far end of the inverter cables (built in to inverter). 350A class T fuse at battery protects 2/0 cable from shorts, but not from longer term overload.
They aren't cheap - cost me almost $200 for two fuses and two fuse holders.
