Time Current Curve Help

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mbrooke

mbrooke

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Why do fuse time current curves stop at 0.01 seconds or 6 cycles?
 

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paulengr

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mbrooke said:
Why do fuse time current curves stop at 0.01 seconds or 6 cycles?
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Not all of them do. Some semiconductor fuses trip in 1-2 ms. The thing to remember is that you hit peak current in 1/4 cycle or about 4 ms with 60 Hz power. After that it’s just a matter of things like melt time. R and E rated fuses are a lot slower and with many MV fuses like the popular SM-5s used indoor it takes a few milliseconds for the spring to pull the plunger after the fuse link goes off.
 
mbrooke

mbrooke

Batteries Included
Location
United States
Occupation
Technician
paulengr said:
Not all of them do. Some semiconductor fuses trip in 1-2 ms. The thing to remember is that you hit peak current in 1/4 cycle or about 4 ms with 60 Hz power. After that it’s just a matter of things like melt time. R and E rated fuses are a lot slower and with many MV fuses like the popular SM-5s used indoor it takes a few milliseconds for the spring to pull the plunger after the fuse link goes off.
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Good info, I didn't know that about the type Es.
 
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paulengr

Senior Member
mbrooke said:
Good info, I didn't know that about the type Es.
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MV fuses frequently use the “E” curves which are intended for transformers and general use. The “K” curves are popular with Duke and are just a steeper “E”. “R” are slower still and designed for motors. There are tons of other proprietary curves.

Physically one design is the Kearney or pigtail style which has a spring stretched out then soldered. When the solder melts the spring snaps and pulls a plunger that pulls the arc apart and drops the cutout open. Another uses boric acid that vaporizes and blows the fuse apart (expulsion fuse). This variation can also be connected to a muffler so it can be used indoors. The SM-4 and SM-5 style has a long body and a solder fuse link but the spring is mechanically set when it is installed. There are plenty of the bus bar and sand designs too just like low voltage fuses. The only ones I’m not real clear on are Bay-O-Net fuses that as I understand it have to be installed under oil. The ratings are very slow though, not at all standardized.
 
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paulengr

Senior Member
mbrooke said:
Bay-O-Nets are used in utility padmounts- so I could imagine them allowing for profound overloading.
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They do. The best features are they don’t corrode, no arcing (contained inside transformer), cheap. Performance though is that you get what you pay for.
 
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