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Single vs. Multiple Class-T Fuses

Mithril said:
Ok, so how do people feel about the need for T-class fuses "beyond" the main system fuse(s). Assume batteries and battery banks are all fused correctly (enough AIC, correct size to protect wires). Is there any actual NEED for T-class level AIC on branches? In theory if a 50A fuse blows and there's a 5000A arc the main T-class is ALSO going to blow and it has sufficient AIC. I think the only concern is if you used a fuse with such a low AIC that it could form an arc that would NOT kick the main fuse(s).
Click to expand...
High AIC fuses are only required on the battery(feeder) circuits.
The branch circuits can get away with lesser fuses.
The branch fuses can break on over-current, but are not guaranteed to break a dead short.
In a dead short on a branch the BMSs will likely cascade trip if the branch fuse sustains an arc.
If any of the BMSs fail closed then the high AIC feeder fuses are the last line of defense.
 
Also the further away from the battery, the higher the path resistance.
 
John Frum said:
High AIC fuses are only required on the battery(feeder) circuits.
The branch circuits can get away with lesser fuses.
The branch fuses can break on over-current, but are not guaranteed to break a dead short.
In a dead short on a branch the BMSs will likely cascade trip if the branch fuse sustains an arc.
If any of the BMSs fail closed then the high AIC feeder fuses are the last line of defense.
Click to expand...
Ok that aligns with my thinking, and how I've already designed my system. I do think I'm going to replace certain fuses with higher AIC for a bit of "belt and suspenders" but if 2000-4000AIC works for the non T-class fuses that does lower the cost.

Re: Path resistance; Very god point! It's sometimes easy to forget the *reason* for the high possible fault current is due to the insanely low resistance of the cells/batteries.
 
Mithril said:
Ok that aligns with my thinking, and how I've already designed my system. I do think I'm going to replace certain fuses with higher AIC for a bit of "belt and suspenders" but if 2000-4000AIC works for the non T-class fuses that does lower the cost.

Re: Path resistance; Very god point! It's sometimes easy to forget the *reason* for the high possible fault current is due to the insanely low resistance of the cells/batteries.
Click to expand...
I think its safe to say that the Victron lynx system is the most popular distribution system for experience people.
I just found these.
AIC of 2500 amp at 120VDC
From all the fuses that show their breaking capacity at different voltages, I infer that the AIC might be as high as ~5000 amps at 60VDC.
 
Last edited:
I think MCBs or MCCBs are the best option for branch circuits (assuming you mean to feed inverters or charge controllers etc). It's important to be able to isolate the device from the batteries if needed. I.e when working on them, replacing faulty units etc.

I'm going to be using 3 of these for my 3 inverters. (I'm currently using 3 MCBs for my current lead acid battery, but need something that can handle a constant 100A per breaker when i get my LFP batteries.)

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BENY BDM Series: DC Circuit Protection for BESS | BENY New Energy

BENY New Energy's BDM series, including BDM-125 and BDM-250, offer robust DC circuit protection for BESS, with a 500V rating and up to 250A current capacity.
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Mithril said:
Someone should convince Electroboom or styropyro to do an experiment and see the actual volts and amps required to sustain an arc for various fuses
Click to expand...

As I mentioned somewhere, something like this is on my todo list. I'll borrow some high current probes from my lab and do some dead short tests on a 48V pack. It takes some time to set this up, and I will do this outside, so once we hit spring. Not doing that in -20C. I also have some Class T and BS88 fuses (and a few BMS) that I can sacrifice.
 
John Frum said:
I think its safe to say that the Victron lynx system is the most popular distribution system for experience people.
I just found these.
AIC of 2500 amp at 120VDC
From all the fuses that show their breaking capacity at different voltages, I infer that the AIC might be as high as ~5000 amps at 60VDC.
Click to expand...
Since I am using a Lynx PowerIn as my busbar for my 200ah batteries, I could easily add the bolts for these fuses at a very reasonable price.
I found them here for a $6.27. [https://www.waytekwire.com/item/47255/Littelfuse-0888250-U-2M8-High-Performance-MEGA/]
What is your opinion on using the 250a version on each battery?
 
AHTrimble said:
Since I am using a Lynx PowerIn as my busbar for my 200ah batteries, I could easily add the bolts for these fuses at a very reasonable price.
I found them here for a $6.27. [https://www.waytekwire.com/item/47255/Littelfuse-0888250-U-2M8-High-Performance-MEGA/]
What is your opinion on using the 250a version on each battery?
Click to expand...
The lynx power in has to be modified to accept fuses.
Also, batteries should be fused as close to the positive terminal.
This minimizes the chance of an un-protected short.
 
John Frum said:
The lynx power in has to be modified to accept fuses.
Also, batteries should be fused as close to the positive terminal.
This minimizes the chance of an un-protected short.
Click to expand...
As he said, “ I could easily add the bolts for these fuses”
 
John Frum said:
The lynx power in has to be modified to accept fuses.
Also, batteries should be fused as close to the positive terminal.
This minimizes the chance of an un-protected short.
Click to expand...
Also I'm guessing that the breaking capacity is ~5000 amps at 60VDC.
 
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John Frum said:
The lynx power in has to be modified to accept fuses.
Also, batteries should be fused as close to the positive terminal.
This minimizes the chance of an un-protected short.
Click to expand...
I have already modified a couple of PowerIn's...really simple. I am using one in my upgrade as a distributor (inverters & charge controlers) without all the fancy LEDs and computer board of a Lynx Distributor, didn't need those features.
Yeah, I as afraid you would bring up with the "...close to the positive terminal..." thing. But, given the mission of the fuses, protecting batteries from each other, I was hoping it would be ok.

But what do you think of the 250a version for battery-battery protection?
 
John Frum said:
Also I'm guessing that the breaking capacity is ~5000 amps at 60VDC.
Click to expand...
Yeah, I get it. And the math indicates you would be right.
I am just trying to get something affordable to do the battery-to-battery protection.
The battery bank is protected right after the busbar (PowerIn) with a Class-T but after having read the posts in this thread I am think more might be better.
 
AHTrimble said:
Yeah, I get it. And the math indicates you would be right.
I am just trying to get something affordable to do the battery-to-battery protection.
The battery bank is protected right after the busbar (PowerIn) with a Class-T but after having read the posts in this thread I am think more might be better.
Click to expand...
I’m modifying my Victron Power In to take class T fuses (three) . My plan is to connect two batteries in parallel for each class T fuse on the Victron busbar. So six batteries per Power In.
 
AHTrimble said:
But, given the mission of the fuses, protecting batteries from each other, I was hoping it would be ok.
Click to expand...
That is only part of the mission.
The other mission is to protect the wire which means the fuse must be at the battery.
The most important mission is protect the battery from a dead short.
Nobody wants an electrolyte volcano.
AHTrimble said:
But what do you think of the 250a version for battery-battery protection?
Click to expand...
Should be fine.
Requires 1/0 awg wire or better for ABYC rules.
 
AHTrimble said:
Yeah, I get it. And the math indicates you would be right.
I am just trying to get something affordable to do the battery-to-battery protection.
The battery bank is protected right after the busbar (PowerIn) with a Class-T but after having read the posts in this thread I am think more might be better.
Click to expand...
IMO that is a waste of a class t fuse.
It seems to me, this whole thread and many others like it is about people trying to save money.
IMO over-current protection is the last place to try and save money.
 
Danke said:
I’m modifying my Victron Power In to take class T fuses (three) . My plan is to connect two batteries in parallel for each class T fuse on the Victron busbar. So six batteries per Power In.
Click to expand...
Very cool !
Pictures?????
 
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