Large Server Rack fusing

[chicagoandy]

I'm in the process of acquiring the large Ruixu server rack. It's 10 48V 100Ah server racks assembled in parallel. It'll be paired with a Sol-Arc 15k.

I'll be attaching a fuse to the rack for safety. Currently I'm planning a 350A class-T fuse, the Bussman JJN-350. This has an AIC of 20 kAIC at 160 Vdc, 200 kAIC at 300Vdc. Presumably the AIC is less at 52V.

My question is, with 10 large batteries in parallel, is the class-T even sufficient? It seems each of these large batteries could push out 2,000 amps in a failure? Or do we trust the BMS to keep each battery current more reasonable?

Are there other fuse types with even higher AIC?

What is the point at which large parallel batteries can have inrush beyond the capability of Class-T fuses?

 

[SparkyJJO]

Especially after a recent thread I advocate a class T at every battery pack, as close to the pack as possible, and before the bus bar.

Yes, the $ add up. But better that than something going sideways and the whole thing burning down because those thousands of amps end up shoved through a failed cell in a pack unabated.

 

[Ampster]

What is the point at which large parallel batteries can have inrush beyond the capability of Class-T fuses?

I thought I read somewhere that the maximum capacity of a class T fuse was 10 or 20 kilo Amperes? This is called AIC (Amperage Interrupting Current). I thought the typical size was related to the current at which point the fuse opens?
EDITED since quote to clarify max capacity .

 

[chicagoandy]

How do we calculate the inrush capability of a battery pack?

 

[Brucey]

Is there a maximum capacity of a class T fuse? I thought the capacity was related to the current at which point the fuse opens?

I think they top out at 400A.

 

[DIYrich]

Since the max amps to ONE Sol-Ark 15k is 275a, I would use a 300a Class T fuse, on each rack. If you parallel a second 15k, then you can up the Class T size to the rating on the wire.

Will all 10 batteries be in one Rack? If so, one Class T is the best you can do. Otherwise, one for each rack.

>Or do we trust the BMS to keep each battery current more reasonable?

Don't trust the BMS. It can fail to interrupt a high current.

>Are there other fuse types with even higher AIC?

I believe Class T is 200,000 amps of AC, but maybe only 20,000 amps DC.

Class L fuse is rated for 200,000 amps of DC. The lowest I could easily find is 1,400a for a DC rated fuse, and it has a 4 second delay at 500%:

https://www.eaton.com/us/en-us/skuPage.KRP-C-1400SP.html

Maybe put the Class T before the Class L. Class T will fast blow above 300amp, but if it can't interrupt the 20,000+ amps, the Class L will blow (delay is probably much shorter at 1400%).

>What is the point at which large parallel batteries can have inrush beyond the capability of Class-T fuses?

The rack battery should have a pre-charge circuit to avoid too high an in-rush when the battery turns on. If not, hopefully the in-rush may be too short to blow a class T fuse.

 

[chicagoandy]

Is there a maximum capacity of a class T fuse? I thought the capacity was related to the current at which point the fuse opens?

Capacity of the fuse is measured in amps. 50a, 100a, 150a, etc. I would use a 350a. This value speaks to the rated amperage the fuse will blow, after a certain amount of time has expired.

My question here is about AIC. AIC, or Amerage Inrush Current, is something completely different. AIC speaks to the MAXIMUM amperage that the fuse will be able to handle. And this speaks to a very different problem.

Yes, fuses do have a maxiumum capacity, and this is measured in AIC, or Amperage Inrush Current. If you exceed the AIC, the fuse can fail in a manner where it continues to pass current. Often this is just plain melting - the metal in the fuse melts, turns into a pool of molten metal, which happens to touch the connectors, and continues to pass current. Often an arc will form, and the arc will bridge the gap opened by the fuse, and current will continue to pass.

Class-t fuses are packed with sand, which makes it difficult or impossible or very difficult for an arc to form.

But Class-t fuses do have a maximum AIC, somewhere around 10,000 - 20,000 amps.

And when we're wiring large 5,000wh batteries in paralell, it seems to be at least it's plausible that we could have 10,000amps of current if a number of batteries were to fail.

 

[Ampster]

I think they top out at 400A.

I changed my question to distinguish between when they open and max capacity. @chicagoandy answered the question. Class T fuses have a maximum Amperage Interrupting Current (AIC) of 10,000 to 20,000 Amps.

 

[chicagoandy]

>What is the point at which large parallel batteries can have inrush beyond the capability of Class-T fuses?

The rack battery should have a pre-charge circuit to avoid too high an in-rush when the battery turns on. If not, hopefully the in-rush may be too short to blow a class T fuse.

Sorry, I meant to ask - What is the point at which large parallel batteries can have a max discharge beyond the capability of Class-T fuses? - my question wasn't about inverter inrush, but just - "how big is too big" for a Class-T ?

 

[sunshine_eggo]

Sorry, I meant to ask - What is the point at which large parallel batteries can have a max discharge beyond the capability of Class-T fuses? - my question wasn't about inverter inrush, but just - "how big is too big" for a Class-T ?

Depends on the total system resistance, R - battery cells, internal connections, BMS, battery interconnects and main cables. I = V / R.

 

[DIYrich]

Sorry, I meant to ask - What is the point at which large parallel batteries can have a max discharge beyond the capability of Class-T fuses? - my question wasn't about inverter inrush, but just - "how big is too big" for a Class-T ?

At 50v, you need a resistance of less than 0.0025 ohms to allow 20,000 amps to flow.
5 feet of 2 awg wire has 0.001 ohms of resistance. Class T fuse adds another 0.001 ohm. Add BMS, switch, connections, etc. - getting pretty close to the point where it would be hard to flow much more than 20,000 amps, regardless of how many batteries you have.

 

[robbob2112]

My question here is about AIC. AIC, or Amerage Inrush Current, is something completely different. AIC speaks to the MAXIMUM amperage that the fuse will be able to handle. And this speaks to a very different problem.

You are mistaken - AIC is "Ampere Interrupting Capacity" ... This is the amperage of an arc it can extinguish

The maximum amperage a fuse can handle is variable based on time. For instance a 350amp class T fuse will continue to pass current for around 9 minutes before blowing if it is just above the amperage it is rated for. It generally takes 5 to 6 times the rated current to make a fuse blow "quick". In the chart below it takes 1837 amps to blow the fuse in 0.01 seconds. Basically a short.

This is why you will read a hundred times on this website that fuses protect wires not equipment. This is also why you won't blow a fuse from inrush current unless you are hooking to something with massive caps in it.

Another factor is the resistance of a fuse varies based on the temperature of the fuse. There are curves for that too.








Your server rack batteries can only push 100amps each because of the BMS inside them. Looking at them I do not see any breaker or other protection devices on them. So, I would put a 125amp class T at the positive bus bar end of the connecting wires.


Class T fuses are filled with a sand like mixture to break any arcs and the only failure mode is to stop passing any current.

Realize you are using the BMS as a fuse and they can and do fail as a short until they self destruct and blow open.

 

[hwy17]

It's not the battery rack overwhelming the Class T into an external short that really stands out to me. My gut feeling is that external shorts are rarely that good and low resistance to carry 10k amps. A short with that low resistance would probably require an intentionally designed shorting mechanism.

It's one pack inside the rack shorting internally, and then all the other packs feeding into that short that I'm thinking of. And in that case, the rack fuse does nothing. So it's really just a question of do you trust the breakers or not, and if you don't then the costly answer is every pack needs it's own Class T.

 

[sunshine_eggo]

At 50v, you need a resistance of less than 0.0025 ohms to allow 20,000 amps to flow.
5 feet of 2 awg wire has 0.001 ohms of resistance. Class T fuse adds another 0.001 ohm. Add BMS, switch, connections, etc. - getting pretty close to the point where it would be hard to flow much more than 20,000 amps, regardless of how many batteries you have.

Good points, but the last part gives me pause without a little qualification. Resistances in parallel result in a lower overall resistance, 1/R = 1/R1 + 1/R2 + ... + 1/Rn, so with 10 batteries in parallel, the "between bus bar" resistance of 10 batteries is effectively 1/10th that of a single battery; HOWEVER, if a fuse is on EACH battery, then you're only contending with the one battery. If a single fuse is used at the main bank (+), I would expect AIC would be exceeded.

 

[robbob2112]

It's not the battery rack overwhelming the Class T into an external short that really stands out to me. My gut feeling is that external shorts are rarely that good and low resistance to carry 10k amps. A short with that low resistance would probably require an intentionally designed shorting mechanism.

It's one pack inside the rack shorting internally, and then all the other packs feeding into that short that I'm thinking of. And in that case, the rack fuse does nothing. So it's really just a question of do you trust the breakers or not, and if you don't then the costly answer is every pack needs it's own Class T.

I would agree with this to a point. If the operator is extra careful and follows all the rules the odds of an external short are just about zero. It is when the OP does something silly like try and replace a class T while power is still applied. Yes there is a video with that happening and the poster got bit by the voltage

 

[DIYrich]

Good points, but the last part gives me pause without a little qualification. Resistances in parallel result in a lower overall resistance, 1/R = 1/R1 + 1/R2 + ... + 1/Rn, so with 10 batteries in parallel, the "between bus bar" resistance of 10 batteries is effectively 1/10th that of a single battery; HOWEVER, if the fuse is on EACH battery, then you're only contending with the one battery. If a single fuse is used at the main bank (+), I would expect AIC would be exceeded.

Ideally, you have a Class T on each battery. Absent that, you have a Class T on the Rack, and have to take the risk of a rack short.

 

[DIYrich]

It's not the battery rack overwhelming the Class T into an external short that really stands out to me. My gut feeling is that external shorts are rarely that good and low resistance to carry 10k amps. A short with that low resistance would probably require an intentionally designed shorting mechanism.

It is the nail that penetrates wires behind the wall and shorts Pos and Negative, or two wires where the insulation wore off, and shorted against each other.

 

[DIYrich]

It's one pack inside the rack shorting internally, and then all the other packs feeding into that short that I'm thinking of. And in that case, the rack fuse does nothing. So it's really just a question of do you trust the breakers or not, and if you don't then the costly answer is every pack needs it's own Class T.

I worry about the Positive Busbar somehow shorting to the grounded case.

 

[hwy17]

I hope we get someone on youtube eventually trying to intentionally overwhelm a Class T with Lifepo4.

I bet it would be very very difficult to do, and if it was achieved successfully, would involve an impressive amount of copper. 8x 2/0 feeding into dinner plate sized bus bars, and then a knife switch the size that you'd power an industrial building with.

The stuff we work with, I bet you couldn't do it.

 

[robbob2112]

I worry about the Positive Busbar somehow shorting to the grounded case.

If you can figure out a way to do that short then you aren't using a shield over your positive bus bar. It should be insulated and surrounded with the fingers and have a cover over it, or have a shield over it, or be in a locked/enclosed cabinet that is inaccessable so it can't be touched.