It's not just about the breaking capacity. It's also about how fast it can act and if they are rated for DC. That Elmark fuse @brum posted for example: a 200A version of this at 1000A takes about a second or two to break, while a similar 200A class T fuse does that in 0.02 seconds.
These fuses are used with the main purpose to prevent a short circuit and its consequences (fire, destroyed wires, damaged cells). The wires will survive with 1000A for 1 second. Especially if you are creating a system that can operate at 200A continuously.
What matters is what is the short circuit current, the time that the fuse will take to trip under it and if your wires system will survive with that.
The BMS should trip on short circuit current in microseconds.These days, fuses are also specifically useful to protect semiconductors (google "semiconductor fuse"). With a fast acting fuse, your BMS will likely survive (I tested this). With a slower fuse, your wires will be fine, but you will likely fry the BMS. Keep in mind that short circuit current with a BMS that starts to smoke/burn might be much lower than a direct short on the battery terminals outside the BMS. LiFePO4 by the way also has a more limited short circuit current compared to e.g. LiPo or other chemistries.
The BMS should trip on short circuit current in microseconds.
The semiconductor fuse should blow in milliseconds.
Therefore is the semi fuse blows it means the BMS has already failed.
The question I have is, will the really fast blow fuses prevent the already failed BMS from causing a fire?
I found "Technical Data Sheet" at Eaton that has the DC Breaking Capacity rating: 50 kA at 500 V d.c.
What was the short circuit current duration set to in the BMS?So, from personal experimentation: I have used a 100A JK BMS, with a 225A class T fuse, and shorted the whole thing (48V LiFePO4 battery). The class T fuse blew before the BMS knew what happened. The BMS survived in all tests (I did this multiple times; this was when I could get a class T fuse for €20).
What was the short circuit current duration set to in the BMS?
Good to know. Thanks!These days, fuses are also specifically useful to protect semiconductors (google "semiconductor fuse"). With a fast acting fuse, your BMS will likely survive
I know there are already two sizes of Class T holders for <=200A and >=225A which seems to cause lots of confusion and ordering mistakes.
I mean that's one reason. It provides a small amount of protection from putting a wildly wrong sized fuse on a system, but you can still put a 400A fuse on a system designed for 225A. And there are actually 5 sizes in the Class T fuse lineup catalog. Blue sea systems only carries 2 of the sizes. On the smallest size they range from 0.5A to 60A so you could potentially fit a 60A fuse into a system designed for 0.5A. I think the primary reason they have different physical sizes is that the engineering requirements of the fuses demand different physical sizes of fuse. They struck a balance between having a different fuse holder for every amp rating and having one huge fuse size which would make all the smaller rated fuses bulky and overpriced.True, but for a good reason: so you can't put a 500A fuse in a system that needs to be protected under 225A.
It actually looks like the BS88 LET series (25A to 180A) would fit in the 101-200A class t holder. Am I missing something?As far as I know, those BS88 fuses (I have not checked/tried them all) don't fit a class T fuse holder. Even if the space between the holes matches on some (160 LET iirc), the size of the fuse itself makes it not fit in the Class T holder.
And I found a Mersen data sheet that gives the DC breaking capacity of 40 kA at 240 volts. Either way, on paper this is superior to class T.I found "Technical Data Sheet" at Eaton that has the DC Breaking Capacity rating: 50 kA at 500 V d.c.
It's possible that the Bussman fuse has superior performance to the Mersen one and yet they both meet the BS88 requirements set by the queen herself back in 1988. (/s) It's also possible that they are equivalent and were simply tested/qualified at different conditions.And I found a Mersen data sheet that gives the DC breaking capacity of 40 kA at 240 volts.
I find those time vs current breaking curves to be very interesting. People are talking about fast blow vs slow blow, but for example, in response to a 300A overload, a 150A fast blow might perform similarly to a regular 100A fuse. The Class T are described as "extremely" fast. But you also have to consider the Amp rating and look at the charts.On the Bussman spec, it's showing the 100 amp fuse breaking at over 150 amps after infinite time. The Mersen specs show the same thing. I had always assumed a 100 amp fuse would open up very close to 100 amps over a long time period.