NOW we know why there has been such a class T shortage...Gotta love these fuse discussions. “ opinions are like A…”
Me, I put a class T fuse between every fricken cell!
?
Single vs. Multiple Class-T Fuses
- Thread starter AHTrimble
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John Frum
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An MRBF fuse does not have the breaking capacity for your intended purpose.
Breaking capacity - Wikipedia
The article you referred to states the breaking capacity and the interpret rating are one in the same.An MRBF fuse does not have the breaking capacity for your intended purpose.
Breaking capacity - Wikipedia
en.wikipedia.org
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.
John Frum
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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?
Give it 1000a and see it it lasts as long!!
John Frum
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@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
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
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
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
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
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
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" :-/
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.
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.
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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.
John Frum
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That was me.
If you see a problem with the calculations please be explicit.
John Frum
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Their manuals include this disclaimer
"The included breaker is not a guarantee of battery protection. Size and install the correct over
current protection for conductors and battery if not included with the product." -- https://signaturesolar.com/content/documents/EG4/EG4-LL 48V-24V Manual-1.2.0.pdf
John Frum
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Agree.
Nice - so the breaker gives people a false sense of security, as I feared.
John Frum
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Is this cells or batteries?
Perhaps 2000 amps is empty and 4000 amps is full?
John Frum
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IMO it is worse than useless.
ah yes but they also market that same CB as and “added safety” feature. Almost seems like that needs a little astrick, or a question mark after it, lol.
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