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Single vs. Multiple Class-T Fuses

John Frum said:
An MRBF fuse does not have the breaking capacity for your intended purpose.

Breaking capacity - Wikipedia

en.wikipedia.org en.wikipedia.org
Click to expand...
The article you referred to states the breaking capacity and the interpret rating are one in the same.
The fuse I mentioned has Interrupt Capacity: 2000A @ 58V DC.
Why wouldn't that work?
If it doesn't work, then what should I look for?
Sorry for the questions, I simply don't understand.
 
AHTrimble said:
The article you referred to states the breaking capacity and the interpret rating are one in the same.
The fuse I mentioned has Interrupt Capacity: 2000A @ 58V DC.
Why wouldn't that work?
Click to expand...
Because you need ~10,000 amps breaking capacity at 58 volts or better
AHTrimble said:
If it doesn't work, then what should I look for?
Click to expand...
Class t or BS88 or something else that meets or exceeds the specs above ^
AHTrimble said:
Sorry for the questions, I simply don't understand.
Click to expand...
Have my inline responses in this message cleared up your remaining confusion?
 
@AHTrimble have a look at the doco attached in this post.
diysolarforum.com

Single vs. Multiple Class-T Fuses

Technically true, but for our purposes here it's false. Yes, the resistance goes up, but each .17mOhm is matched by 3.2V from the cell: .17mOhm busbar +.19mOhm cell IR = .36mOhm/3.2V. Doesn't matter whether you've got one cell or one hundred cells. The fault current through that is the...
diysolarforum.com diysolarforum.com
 
From an earlier discussion:

diysolarforum.com

MRBF fuses - OK for 48V LiFePO4system?

Hi - I was thinking of using MRBF fuses on my busbar of my 2P16S off-grid system. I'm planning on charging the cells to a maximum of 3.6V for a total of 57.6V, which is just within the fuse's maximum rating of 58V. HOWEVER, product info also says "The breaking capacity meets the requirements...
diysolarforum.com diysolarforum.com
 
John Frum said:
Because you need ~10,000 amps breaking capacity at 58 volts or better

Class t or BS88 or something else that meets or exceeds the specs above ^

Have my inline responses in this message cleared up your remaining confusion?
Click to expand...
Yes, thank you.
I was unaware of the 10ka @58v breaking capacity. Never was exposed to the information/knowledge.
 
I have found something!

This report tests short circuit behavior of a 5Ah and 160Ah LiFePO4 cell. There is not enough detail of the test procedure used to be entirely conclusive. But It seems that the short circuit current for a single LFP cell could be in the range of 6C to 10C.

This should give people some idea of what they need to be prepared for.

I.e if you have a single string of 100Ah cells (12V, 24V, 48V it doesn't matter) you are looking at a short circuit current as low as 600A or up to 1000A. Therefore you need to make sure your fuse would trip for a fault that's as low as 400-500A in a reasonable time. And is able to handle well in excess of 1kA (I wouldn't go any lower than 4kA rated fuses or MCBs).
This is good because it means that the MCBs in the server rack batteries are probably fit for purpose.


Update: My friend contacted EVE (as we currently have a combined order in for 120 LF280Ks) and asked what the prospective fault current is. Their immediate response was 2000-4000A. I have asked if they have any documentation around this.
That works out to 7-15C
 

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John Frum said:
Because you need ~10,000 amps breaking capacity at 58 volts or better

Class t or BS88 or something else that meets or exceeds the specs above ^

Have my inline responses in this message cleared up your remaining confusion?
Click to expand...
DANG ! ! ! ! ! !
I just got it :(
I was sitting there watching TV and it finally dawned on me...it got through my thick skull.
Obvious I am an armature solar guy...I don't know all the technical terms for sure. But I understand when a fuse, fuse holder, CB, etc melts down and fails from too much current.
Thank you for educating me on the actual terms, specs, etc. I really appreciate it :)
 
upnorthandpersonal said:
It's standard, recommended practice by everyone: this is also what Victron recommends, and is the way it is shown in documentation from e.g. Mersen (page 19 for example), ETI (this doc, page 35), etc. Doesn't have to be a Class T, but a fast acting fuse is recommended.



Cells make up a battery, multiple batteries make up a bank. Doesn't matter how many you add, it's still a bank.
Click to expand...
Most time I see a battery BANK being built, people don't fuse each battery *at all*.
Signature Solar was (is?) selling "ready to go" rack + batteries with a busbar, and no fusing per battery besides the breaker. And I'm fairly sure I've seen similar offering elsewhere. Now, I'm not saying I agree, but I'm also not sure I agree with a Class-T per battery, especially the 100AH ones; since if you do have one per battery each fuse only needs to break the amps FROM a single battery (if you have a short or fault in a battery it is no longer the upstream device).

Personally what I would love to see in these server rack batteries is a mounting spot for a fuse directly on the battery with clear instructions to not connect on the unfused side. This would allow it to be user serviceable and readily visible. It would remove the clutter/annoyance when fusing each battery.
On a related note, are there are reputable "inline" style fuse holders for a fuse that has AIC enough for a 100AH 48V battery? My current setup does not bring joy and looks ugly :(
 
John Frum said:
To address the broader point of why I think its most important to have a high breaking capacity fuse as close as possible to the battery positive terminal is to keep the electrolyte inside the battery.
Click to expand...
I 100 agree with Keeping Fuses as close to battery/bank as possible. Seeing someone have a rack of batteries connected to a busbar and unfused wires going multiple feet before the first fuse makes me.... itchy.

I don't actually know if you need Class T per battery (since each fuse only break the amps for the battery it is on), but fully support Class-T or equiv (too bad group buys often go so poorly we could group buy some Class T fuses...) for banks, perhaps the 280+AH batts need class T as well?

Someone (in this thread?) was trying to calculate actual dead short amps but there seemed to be a lot of "assuming such and such" :-/
 
cods4 said:
I have found something!

This report tests short circuit behavior of a 5Ah and 160Ah LiFePO4 cell. There is not enough detail of the test procedure used to be entirely conclusive. But It seems that the short circuit current for a single LFP cell could be in the range of 6C to 10C.

This should give people some idea of what they need to be prepared for.

I.e if you have a single string of 100Ah cells (12V, 24V, 48V it doesn't matter) you are looking at a short circuit current as low as 600A or up to 1000A. Therefore you need to make sure your fuse would trip for a fault that's as low as 400-500A in a reasonable time. And is able to handle well in excess of 1kA (I wouldn't go any lower than 4kA rated fuses or MCBs).
This is good because it means that the MCBs in the server rack batteries are probably fit for purpose.


Update: My friend contacted EVE (as we currently have a combined order in for 120 LF280Ks) and asked what the prospective fault current is. Their immediate response was 2000-4000A. I have asked if they have any documentation around this.
That works out to 7-15C
Click to expand...


So at 7-15C, a rack of 6 100AH batteries is 4,500-9,000A. I'd personally even double that expectation for the bank fuse.
 
Mithril said:
Signature Solar was (is?) selling "ready to go" rack + batteries with a busbar, and no fusing per battery besides the breaker. And I'm fairly sure I've seen similar offering elsewhere. Now, I'm not saying I agree, but I'm also not sure I agree with a Class-T per battery, especially the 100AH ones; since if you do have one per battery each fuse only needs to break the amps FROM a single battery (if you have a short or fault in a battery it is no longer the upstream device).
Click to expand...

Because they consider that breaker good enough. I disagree, but that's just my opinion.
Even if it's 'only' a 100Ah 12V pack, if you put them in parallel, you should put a fuse (unspecified for the sake of this reply) per battery, because this protects potentially catastrophic results if something happens in one battery and the other parallel ones are dumping their energy into this one. For the fuse, the failed battery becomes the upstream device and sees multiple other batteries in parallel dumping their current towards it.

The same holds true for solar panels, but only when you have three or more parallel strings (because solar panels are limited by their very nature, unlike a battery). If you have three panels in parallel, and one fails (i.e. a short) the other two panels will dump their full capability into this short. A panel itself can handle being shorted, but it can not handle the current of multiple panels being shorted in it (which is why it's only needed with three or more). Therefor, you put a fuse per string before you combine.
 
Last edited:
Mithril said:
So at 7-15C, a rack of 6 100AH batteries is 4,500-9,000A. I'd personally even double that expectation for the bank fuse.
Click to expand...
Correct. Which is well within the capability of a Class T fuse or MCCB.

It also means that a half decent MCB should be suitable to protect a 100Ah battery.
 
Mithril said:
Someone (in this thread?) was trying to calculate actual dead short amps but there seemed to be a lot of "assuming such and such" :-/
Click to expand...
That was me.
If you see a problem with the calculations please be explicit.
 
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