LFP Available Short Circuit Current

[CaptainRivet]

Originally Posted by CharlieJ

@CaptainRivet #9:



No, fuses do not "blow at their rating", they blow in accordance with their load/time curves.



"Area" is computed by multiplying length x width. You have measured either length or width and multiplied it by thickness. The mathematical result is not area.

Yes Mr Professor.
Rating is load/time and much more...point here is what does your fuse at 80% of its rating, nowhere speced and my little test shows you significant differences. Yes not a scientifically proofed test but inline what i experienced using different fuses over the years. So for the users with electric galley and longer higher power draw thats a fact taking into concern too. Not the first time i see a 95sqmm=Gauge 2.0 cable running down the wall because it was fused with a 300A megafuse that overheated into the cabling and that installation ran at 250A for 15min straight because an inverter oven was making a roast of meat....part of stress test i am doing after finishing installation. Like this you find bad crimps, not correct torqued bolts and contact issues, a flir cam for your cell is very handy here but toaching and smelling at all connections does the trick too.

Regarding area, i think everybody got what i mean. You are requested to use 50sqmm cable minimum for 200A but all 200A rated fuses except of NH have significantly less square diameter surface of rheir contacts then 50sqmm resulting in heating up and transfer that heat into your cabling/installations causing all sorts of problems. Additional heat means raised resistance=more voltage drop=more current which is the start of the devils overheating cycle.

[Putt-Putt]

CaptainRivet.

Perhaps I don't correctly understand your issue regarding the lack of cross sectional area and/or surface area of the connection flange of a Class T fuse, but let's try this.

The link below should take you to the Littelfuse info. for their JLLN Class T fuse.

There you should find that the cross sectional area of the connection flange on the 200 Amp. fuse is (Dim. C ) 22.2mm x (Dim. G) 4.8mm = 106.5 sqmm. Thats about double the cross sectional area of your 50 sqmm. 200 Amp. supply conductor.

If you were to jump up to 400 Amps the fuse's connection flange cross sectional area goes to 162.6 sqmm, or about 50% larger than the required 4/0 (107 sqmm.) cable.

With respect to surface area for the conductor's lug on the 200 Amp fuse's flange, the area calculates to about 440 sqmm. (less the bolt hole) and in my limited experience exceeds that of the supply conductor's lug, every time.

This seems reasonable to me as how else would Littelfuse gain all of the certifications on the fuse?

In practice, with the 6 Class T fuses I have onboard I have yet to find an increase in temperature under near full load for a sustained period of time, of more than about 5º C. It's always less than the supply conductor's temp. rise.

Where is the problem?





https://www.littelfuse.com/media?res...fuse-datasheet

[CaptainRivet]

Originally Posted by Putt-Putt

CaptainRivet.

Perhaps I don't correctly understand your issue regarding the lack of cross sectional area and/or surface area of the connection flange of a Class T fuse, but let's try this.

The link below should take you to the Littelfuse info. for their JLLN Class T fuse.

There you should find that the cross sectional area of the connection flange on the 200 Amp. fuse is (Dim. C ) 22.2mm x (Dim. G) 4.8mm = 106.5 sqmm. Thats about double the cross sectional area of your 50 sqmm. 200 Amp. supply conductor.

If you were to jump up to 400 Amps the fuse's connection flange cross sectional area goes to 162.6 sqmm, or about 50% larger than the required 4/0 (107 sqmm.) cable.

With respect to surface area for the conductor's lug on the 200 Amp fuse's flange, the area calculates to about 440 sqmm. (less the bolt hole) and in my limited experience exceeds that of the supply conductor's lug, every time.

This seems reasonable to me as how else would Littelfuse gain all of the certifications on the fuse?

In practice, with the 6 Class T fuses I have onboard I have yet to find an increase in temperature under near full load for a sustained period of time, of more than about 5º C. It's always less than the supply conductor's temp. rise.

Where is the problem?





https://www.littelfuse.com/media?res...fuse-datasheet

Read my post, where did I write that cross section of class T is insufficent?...nowhere.
Class T and NH are the only one that have enough cross secrion.
The issue is with megafuse and ANL.

Class T is ok unless your LFP bank is bigger then 400AH as then the bank short curcuit is higher then the arc rating of a class T. The IR is high enough and it will blow. But the banks short curcuit current will raise further and arc over the 7mm distance of a blown class T. I had a blown class T arced due to a shortened bank, so thats not theory.

Only NH fuses have enough arc rating to act as main LFP fuse >400AH , they are speced and certified for that.

[Putt-Putt]

CaptainRivet.

Well, it might be an out of context or second language issue, but I could have sworn you said (by the use of the word "all") that a Class T has this inherent problem.

Originally Posted by CaptainRivet

but all 200A rated fuses except of NH have significantly less square diameter surface of rheir contacts then 50sqmm resulting in heating up and transfer that heat into your cabling/installations causing all sorts of problems.

With respect to the AIR/AIC rating issue, isn't it the conventional wisdom to partition, using multiple fuses, large Battery Banks?

[s/v Jedi]

AIC for a class-T fuse is 20kA at 125V

AIC for Megafuse is 2kA at 32V

AIC for ANL fuse is 6kA at 32V

AIC for AMI fuse is 5kA/2kA at 16V/32V

AIC for MRBF fuse is 10kA/5kA/2kA for 14V/32V/58V

AIC for NH fuse is up to 120kA very much depending on specific model fuse

It is impossible to create a short current that a class-T can’t handle because total circuit resistance is too high. Stating otherwise, without providing proper examples is fear mongering.

[CharlieJ]

@CaptainRivet #18

Read my post, where did I write that cross section of class T is insufficent?...nowhere.
Class T and NH are the only one that have enough cross secrion.
The issue is with megafuse and ANL.

There is no issue with Mega (AIC=2kA @ 52VDC) or ANL (AIC=6kA @32VDC) fuses as
they have insufficient AIC for all but the smallest of banks unless you use them to sub-divide a large bank as pointed out by @Putt-Putt #19. Then they may be acceptable.

This is all interesting but out in the field we are at a loss on specifying the correct main battery fuse unless the LFP battery/cell manufacturers provide, or we can derive (as pointed out by SV Jedi above), the ASCC which is Post #1 in this thread.

[CharlieJ]

S/V Jedi #20: We are in violent agreement!

[s/v Jedi]

Originally Posted by CharlieJ

S/V Jedi #20: We are in violent agreement!

Just to illustrate: the internal resistance of a Winston 400Ah cell is 0.3 mOhm. When you put 8 cells in series for 24V then this adds up to 2.4 mOhm. At 28V the short circuit current would be 28 / 0.0024 = 11.66kA which is well below the 20kA rating.

Here’s the thing: resistance for all the terminals, jumpers, cables, shunt resistor, fuse and main battery switch must still be added to those 2.4 mOhm!

The whole thing is ridiculous

[Statistical]

Originally Posted by s/v Jedi

Just to illustrate: the internal resistance of a Winston 400Ah cell is 0.3 mOhm. When you put 8 cells in series for 24V then this adds up to 2.4 mOhm. At 28V the short circuit current would be 28 / 0.0024 = 11.66kA which is well below the 20kA rating.

Here’s the thing: resistance for all the terminals, jumpers, cables, shunt resistor, fuse and main battery switch must still be added to those 2.4 mOhm!

The whole thing is ridiculous

Yeah the worst case scenario for fuse protecting battery is a short circuit across the terminals of the battery itself.

Even wiring adds significant resistance. 4/0 is ~0.5 mOhm per foot. So a short circuit 4 feet down the primary cable at the busbars (8 feet total) added another 4 mOhm which has cut current flow by a factor of 3.

28V / (.0024 + 0.004) = 4375A. That still ignores the resistance of lugs, terminals, and busbar so real number would be lower. Smaller gauge wiring would limit current even further. So while ANL fuses are insufficient on the battery itself for large LFP packs using it for anything from the primary busbar onward are fine.

[T1 Terry]

Just for the record, AC fuses can not simply be derated for DC use, switches relays and contactor all fit into the same category as far as this rule applies.
The reason is, AC current/voltage passes through 0 at 50 or 60 hertz (depending on the frequency used) and this acts as an arc quench because a new arc must be established every time.
This requires the wave to reach a higher and higher voltage each cycle to establish the new arc combined with the material used as a fuse melting at each end increasing the gap , eventually the required voltage exceeds the supply voltage and the arc is extinguished.
DC voltage/current flows constantly in the same direction and allows the air between the two arc points to ionise and this greatly reduces the current required to maintain the arc.
For this reason, DC fuses are physically longer than the same rated AC fuse, the gap needs to be larger between the terminal metal size and the the much smaller fuse material size. In larger capacity DC fuses the wave back and forth of the fuse section is further spaced to increase the speed the fuse material gap grows.
Switches, relays and contactors have a greater gap between the contacts and in some instances, a magnet sweeps across the arc field as the contacts are opened designed to pull the arc away before an ionised path can develop. Anyone who has used an AC light switch to try and cut a 10 amp or more DC circuit will have experienced the melted switch and even the the whole plastic piece catching fire ..... the arc gap is not large enough to extinguish the arc .... so there is a reason why DC switches are so much more expensive than AC switches.

T1 Terry

[CharlieJ]

@T1 Terry: #25:

[betwys1]

Originally Posted by CharlieJ

Since the OCPD ampere interrupt capacity (AIC) has to be matched to the battery available short circuit current (ASCC), does anybody know what the ASCC is for some of the more popular cells: Thundersky; CALB, GBS? How about for the Victron 330Ahr Smart LFP or the Lithionics family of batteries?

The NEC seems to propose matching the current conductor to be protected with an OCPD of at least 125% of that maximal conductor current in order to protect that conductor. Some batteries of Li-ion type are capable of VERY high max currents so that it would be unwise to attempt a battery match. But see what you think:
https://www.ecmweb.com/content/artic...e-calculations

[CharlieJ]

@betwys1
Two different topics: rating of the OCPD to protect the conductor (the NEC reference) and the ampere interrupt capacity (AIC) of that OCPD (the topic of this thread) so that the OCPD can clear a fault and not fail catastrophically because of the exceptionally high available short circuit current (ASCC) presented by LFP batteries in a crow bar fault.

[CaptainRivet]

Originally Posted by s/v Jedi

AIC for a class-T fuse is 20kA at 125V

AIC for Megafuse is 2kA at 32V

AIC for ANL fuse is 6kA at 32V

AIC for AMI fuse is 5kA/2kA at 16V/32V

AIC for MRBF fuse is 10kA/5kA/2kA for 14V/32V/58V

AIC for NH fuse is up to 120kA very much depending on specific model fuse

It is impossible to create a short current that a class-T can’t handle because total circuit resistance is too high. Stating otherwise, without providing proper examples is fear mongering.

As i said, i had a class T that blew correctly and then later got arced by a shortened (assume around 400AH) LFP bank which resulted in burning the car down...

Thats why i use only NH Fuses. The issue is not the AIC, there the class T blows. The problem is that doesn't stop the shortened bank and the current continue to rise over time. And well if its high enough it arcs over the blown fuse if its only the 7mm distance a 400A class T has...
NH are mainly/oeiginally made for grid stations where you have the same issue, eg the shortened trafo still produces more current when the fuse blew, the fuse just disconnected it from the rest of the grid and it must stay this way under all circumstances. Thats main reason why they use large ceramic bodies, 2nd they are optimised in resistance as this adds up on the monthly bill of the consumer.

[s/v Jedi]

Originally Posted by CaptainRivet

As i said, i had a class T that blew correctly and then later got arced by a shortened (assume around 400AH) LFP bank which resulted in burning the car down...

Thats why i use only NH Fuses. The issue is not the AIC, there the class T blows. The problem is that doesn't stop the shortened bank and the current continue to rise over time. And well if its high enough it arcs over the blown fuse if its only the 7mm distance a 400A class T has...
NH are mainly/oeiginally made for grid stations where you have the same issue, eg the shortened trafo still produces more current when the fuse blew, the fuse just disconnected it from the rest of the grid and it must stay this way under all circumstances. Thats main reason why they use large ceramic bodies, 2nd they are optimised in resistance as this adds up on the monthly bill of the consumer.

Your issue was errors made during installation, like position of the fuse block or wiring routing and connected wrong. Or maybe you even had a knock-off cheap fuse that did not conform to the standards.

But the standards and certification is correct; it has been tested too much for having any doubts, let alone your conspiracy theory about it being certified falsely because your car burned down. Your car burned down because someone did something wrong with the installation or cheaped out buying components