Class T vs ANL fuse

[MarvRVHam]

I'm about to start my upgrade to our RV electrical system. It will be a Victron MultiPlus 3000 with 4 Battleborn Batteries and 700 watts of solar. I went and go everything from BattleBorn but noticed that they supplied me with a 400 Amp ANL fuse to place between the battery and the inverter (it would have been nice if they supplied the fuse holder as well). Prior research that I've done indicates that a 400 Amp Class T fuse would be a better choice. But since I'm sure that the people at BattleBorn know more than I do should I just go with the ANL type, or replace it with a Class T? The cost difference doesn't matter to me. My primary concern is keeping my RV electrical system and everyone in the RV as safe as possible at all times.. Thoughts?

 

[Hedges]

An ANL fuse can interrupt up to 2700 amps. Perfect for an automotive starting battery that sees a dead short.
Class T can interrupt up to 20,000 amps. Any idea what the short-circuit current of the Battle Born is?

I saw one video a guy did with a fuse and battery, but a quick back-of-the-envelope calculation told me it was the long, skinny wire used which probably saved the day. Resistance limited current.

I found one link that reported 4000A from a 100 Ah AGM battery. So I figure my 405 Ah is about 16,000A, and class T covers that but not much larger.

A Blue Sea class T fuse and holder may cost about $80 or so.

 

[Dzl]

Class T is the gold standard for interrupting a ton of current, making it great for use as a main battery fuse particularly for lithium batteries. MRBF and ANL also have moderately high interrupt ratings suitable for a main battery fuse but nowhere near as high as Class T.

Class T is also the most expensive.

 

[Zil]

Every engineering paper on LiFePo batteries I have read said use Class-T fuse.

 

[Hedges]

Every engineering paper on LiFePo batteries I have read said use Class-T fuse.

Certainly depends on the size battery. My Harbor Freight jumpstart pack is LiFePO. Bet it can only deliver a few hundred amps.

Did the papers say what short-circuit current capability was for a particular capacity battery?
I'm in the dark on that, and don't know if any BMS have interrupting capability.

I would expect some size pack to exceed Class-T capability.
For AGM, data I've seen indicates 500 Ah would have 20,000A output. So what do we use with 1200 Ah cells?
Or do we just carefully design in resistance in the form of battery cable length, and plan where shorts are possible?

The class T data sheet indicates 20kA and either 125VDC or 300VDC.
It's actual interrupting capability for a 48V battery about 60V unloaded might be somewhat higher, but not documented.

 

[Zil]

Hedges. Bet you a good beer even that HF jumpstart can dump 10's of thousands amperes. Maybe not for very long, but enough to melt wires and let the smoke out.
Of the fuses that show ratings at different dc voltages, seems the interrupt capacity goes up as the voltage goes down. I would guess ac voltage interrupt capacity would be higher than dc voltage. But if there is no separate rating on a fuse sold for dc voltage, the 20,000 ampere interrupt would be across the range?
I just except that published engineers may know more than me.
I don't know how amp hour ratings affect the short circuit amperes, except for how long the amperes can flow. . I would guess it is voltage making the difference.

 

[Hedges]

I do think at lower DC voltage, interrupt capability will be higher.
For some rotary cam switches, that was the case. Something like 40A 600VAC, 20A 24V DC, 1 amp or less at 600VDC.
Class T can interrupt 20kA DC, 200kA AC.

The lithium jump starter is going to be made as small as possible, sufficient to deliver cold cranking amps. Maybe 600A into 8V for a 12V starter? So that might be 1800A into a short. I doubt it is 10's of thousands, but I haven't worked with lithium. If it could do 10's of thousands, I think a smaller cheaper battery would have been used, still able to turn the car engine over.

I figure more amp hours is more plate area, but it does crowd down to the same terminal.
My 100 Ah 12V are about 60 lbs. 405 Ah 6V are about 120 lbs (L16 size).

Lithium batteries come in a variety of discharge rates, as well as capacity. The sheetmetal fuse per cell sheets to be spot welded on some cell types are sized according to intended current draw. I do wonder how they perform in case of a short circuit. For some faults they would only have 3V across them. Shorted pack would distribute the fault across all fuses in series as well as parallel. If one blows first but doesn't interrupt, the others might catch up and divide voltage among them. (breakers are ganged if used in series.)

 

[pablo fiasco]

Every engineering paper on LiFePo batteries I have read said use Class-T fuse.

I know this topic has been covered repeatedly. I’ve read the discussions on this forum and others. But I still don’t get it. I have a 2000W Gopower inverter and a 100Ah lithium battery. When I boil down the specs of my inverter I come to the conclusion that with a 3500W surge and allowing for inefficiency the maximum DC it will draw before it reaches its’ surge limit and shuts itself off is between 388A to 281A depending on the voltage. Everybody and his dog is telling me to use a (300amp slow blow) class-t fuse. Thing is that fuse and holder are going to cost me well over $100 CDN + mafia graft. On top of that I would feel impelled to carry a spare because they aren't commonly available. So now it’s up to well over $200 for a fuse that I will likely never see do its job - ie blow. Frankly that plan won’t bankrupt me but it would irritate me.
At the same time I know from experience that inverters don’t care if you use an external fuse or not and that Gopower is a reputable mfg and their equipment has internal fusing and will protect itself. And I know that the real use of the ‘catastrophic’ fuse is to protect the wiring. My wiring will be simple and short. 2 gauge battery cable. The battery and the inverter will be close together inside the living space. Skoolie. I’m much inclined to use a 300A MRBF fuse. That might clip my surge limit but I don’t expect to use that much power anyway. Alternatively Blue Sea makes a 400A ANL. That wouldn’t clip my surge and would still protect my wiring.
I’m really loathe to just follow advice I don’t understand the reasoning behind. If I short wires any fuse that represents a weak link in the chain will blow. What’s the big deal with Class-T fuses??

 

[Hedges]

I’m really loathe to just follow advice I don’t understand the reasoning behind. If I short wires any fuse that represents a weak link in the chain will blow. What’s the big deal with Class-T fuses??

You may be OK with 12V. Some fuses (e.g. ANL) are rated to interrupt as much as 6000A up to 32V.

For those of us with a 48V bank, above the voltage rating of ANL and above 6000 A short circuit current, ANL might just burn out the element and sit there with an arc giving off 10's of thousands of watts of heat. That could ignite anything nearby.

Class T fuses used in an AC application can interrupt 200,000 A of fault current from a utility transformer in an industrial neighborhood. It does that so fast that a main breaker only rated for 22,000 AIC doesn't explode. The Class T fuse is called "current limiting", but what it really limits is current X time due to its speed.

You've probably seen the tests of various lithium batteries. Some might ignite with a short circuit, some types may not.
The type of battery, what would be lost if it started a fire, your escape route all are factors to consider.

 

[pablo fiasco]

You may be OK with 12V. Some fuses (e.g. ANL) are rated to interrupt as much as 6000A up to 32V.

For those of us with a 48V bank, above the voltage rating of ANL and above 6000 A short circuit current, ANL might just burn out the element and sit there with an arc giving off 10's of thousands of watts of heat. That could ignite anything nearby.

Class T fuses used in an AC application can interrupt 200,000 A of fault current from a utility transformer in an industrial neighborhood. It does that so fast that a main breaker only rated for 22,000 AIC doesn't explode. The Class T fuse is called "current limiting", but what it really limits is current X time due to its speed.

You've probably seen the tests of various lithium batteries. Some might ignite with a short circuit, some types may not.
The type of battery, what would be lost if it started a fire, your escape route all are factors to consider.

This is all very confusing to someone with a very tenuous grasp of the principals involved. I pretty sure that the fuse does not protect the device at the end of the line. In this case the inverter. It would be great if people who should know better would stop misleading poor schmucks like me. When you go off on an explanation about industrial situations it doesn't help to clear my conceptual table much. It may be helpful if I ever get to the point of incorporating it in my understanding of basic principals but for now I have to read around that part of your reply. What is interesting is your mentioning the the possibility of the battery igniting in the case of a short circuit. It has occurred to me that the battery is the third part of the system and the one that nobody ever mentions. So: Is there a question I can ask the mfg that might shed some light? It will be a long while before this bus rolls so I'm taking time to sort things out.

 

[Sabre36]

The ABYC marine standards suggest the proper fuse for LiFePO4 is a Class T fuse. This is due to the 20,000A AIC rating of the Class T fuse. This 20,000A AIC rating is at 125VDC which means AIC is actually significantly higher at 13V. The Class T is also fully metal encased.

 

[Dzl]

The ABYC marine standards stipulate the only fuse for LiFePO4 is a Class T fuse. This is due to the 20,000A AIC rating of the Class T fuse. This 20,000A AIC rating is at 125VDC which means AIC is actually significantly higher at 13V. The Class T is also fully metal encased.

Can you point to this standard? I have been trying to look into this recently, but the latest publicly available version of the ABYC E-11 is 2008 when LFP was in its infancy. They do not differentiate OCP based on battery type, only on battery bank size.

This is all very confusing to someone with a very tenuous grasp of the principals involved. I pretty sure that the fuse does not protect the device at the end of the line.

Let me see if I can help (disclaimer, I will probably hurt not help) as I am someone that also likes to focus on the conceptual level before diving into the practical application. For now lets ignore the idea of 'slow blow' or 'quick blow' beyond acknowledging that different fuses have different 'trip curves' i.e. the time it takes for a fuse to blow for a given current.

Fuses have 4 ratings (well probably more)
1) AC or AC/DC
2) Continuous current
3) Voltage
4) Ampere Interrupt Capacity (AIC)

This last point is where a class T fuse shines. A class T fuse has the ability to reliably break a extremely high ampere current. Much higher than MRBF or ANL fuses. It is my understanding that this is much more important with lithium batteries because of the low resistance, a lithium battery can pour so much current into a short. With lead acid battery banks, ANL, MRBF, and Class T fuses are all permitted for main battery protection by the ABYC (US marine code), but with lithium's much greater ability to supply current, a fuse with a very high AIC becomes much more useful (or even mandatory--i'm not sure about this).

Any fuse should fail (what a fuse is designed to do) when its current rating is exceeded for a long enough period of time, but to fail safely and reliably the fuses AIC rating needs to be greater than the current flowing through it. At least that is my simplified understanding of the idea.

 

[pablo fiasco]

Can you point to this standard? I have been trying to look into this recently, but the latest publicly available version of the ABYC E-11 is 2008 when LFP was in its infancy. They do not differentiate OCP based on battery type, only on battery bank size.


Let me see if I can help (disclaimer, I will probably hurt not help) as I am someone that also likes to focus on the conceptual level before diving into the practical application. For now lets ignore the idea of 'slow blow' or 'quick blow' beyond acknowledging that different fuses have different 'trip curves' i.e. the time it takes for a fuse to blow for a given current.

Fuses have 4 ratings (well probably more)
1) AC or AC/DC
2) Continuous current
3) Voltage
4) Ampere Interrupt Capacity (AIC)

This last point is where a class T fuse shines. A class T fuse has the ability to reliably break a extremely high ampere current. Much higher than MRBF or ANL fuses. It is my understanding that this is much more important with lithium batteries because of the low resistance, a lithium battery can pour so much current into a short. With lead acid battery banks, ANL, MRBF, and Class T fuses are all permitted for main battery protection by the ABYC (US marine code), but with lithium's much greater ability to supply current, a fuse with a very high AIC becomes much more useful (or even mandatory--i'm not sure about this).

Any fuse should fail (what a fuse is designed to do) when its current rating is exceeded for a long enough period of time, but to fail safely and reliably the fuses AIC rating needs to be greater than the current flowing through it. At least that is my simplified understanding of the idea.

So what I'm getting around to understanding is that a blown MRBF or ANL could arc if the current (amps) interrupted were high enough to jump whatever gap the fuse created but that something in the design of Class-T fuses reduces that possibility. Is that the non technical explanation? So what I want to know is how many amps my battery could pour out under ideally catastrophic, perfect storm, conditions. Right? Is that number the AIC?

 

[Hedges]

AIC "Amps interrupting Capacity" is what the fuse can stop.
Short Circuit Current is what the battery (or grid) can deliver.

 

[pablo fiasco]

Thanks. I got through to that and have sent email to the mfg asking the question.

 

[Dzl]

So what I'm getting around to understanding is that a blown MRBF or ANL could arc if the current (amps) interrupted were high enough to jump whatever gap the fuse created but that something in the design of Class-T fuses reduces that possibility. Is that the non technical explanation?

Yes I believe your understanding is for the most part correct.

So what I want to know is how many amps my battery could pour out under ideally catastrophic, perfect storm, conditions. Right?

Yeah, me too. I would be interested to see some data, or some code references, or even a rule of thumb. All I know is that a typical lithium battery can sustain a lot more current into a short than a typical lead acid battery can. But I can't give you any numbers.

Is that number the AIC?

Sort of yes, the AIC is the maximium current that the fuse/breaker is rated to fail safely at. So you would want the AIC of the overcurrent protection to be higher than whatever your battery bank could pour into a short circuit.

Here is the NEC's dry definition:

“the highest current at rated voltage that a device is identified to interrupt under standard test conditions.” source

Here is an explanation from marinehowto, a resource that I have a good amount of respect for:

Spoiler: fuses

In this photo we can see the battery bank and the red 2/0 wire feeding the Class-T fuse holder. At a bare minimum you want to be using Class-T fuses as your main bank protection for an LFP bank.
This bank can easily throw 20,000A or more of current into a dead short and can damage and literally blow windows out of ANL fuses. I had this happen during the testing of some ANL fuses sent to me by a DIY LFP guy from Cruisers Forum, Thanks Bob E.. Class-T fuses are fully metal encased and are a very safe fuse.
All fuses have what is called an AIC rating or amperage interrupt capacity rating. This is the rating at which the fuse will fail safely. Class-T Fuses have the highest AIC rating of any fuse we use in the marine environment. There are fuses out there with higher AIC ratings but none of them have fuse holders available that are suitable for a marine application.

During the course of our testing & experimentation with LFP battery banks I blew approximately $400.00 worth of MRBF, ANL and Class-T fuses.
The only unsafe failures we had were off-brand el-cheapo car stereo type ANL fuses. As can be seen here the windows literally exploded out of the fuse when it tripped. I did not have a single unsafe failure of a Cooper Bussmann/Blue Sea Ignition Protected ANL fuse but I only blew about 10 of them. I suppose if you blew 100 you may have an unsafe failure on an LFP bank..
Still, I would strongly urge Class-T as the bare minimum for LFP bank main over-current protection.
NOTE: Class T fuses do not have an ignition protection rating. As near as I can tell, from speaking with Blue Sea Systems, as well as Cooper Bussmann, they have not been specifically tested for this. This only means that they’ve not been tested, not that they would necessarily be unsafe.
Remember an ANL IP rated fuse has an AIC of 6000A and a Class-T non IP fuse has a 20,000A interrupt rating.. If you have a gasoline powered vessel, which requires ignition protected devices, consider this when engineering the over current protection for your LFP system..

A similar (but shorter discussion here) including video of an MRBF fuse being tested (successfully):

LiFePO4 prismatic cell short circuit current and main circuit protection

I am building a LiFePO4 battery bank for my vehicle using 8x EVE 105Ah cells in a 2p4s configuration (12V, 210Ah). While not an EE, I am fairly comfortable with circuit design and protection. My question pertains to main circuit protection, specifically short circuit protection. I see a lot of...

diysolarforum.com

 

[timmeh]

If you have a class T as the main fuse. For al the smaller load bank and such does it then matter what AIC those fuses have? Like if you have a class t on battery to a fuse block with amg or anl or midi fuses? I'm now wondering if one of them pops will it arv untill the main fuse pops in the event of a dead short?

 

[Daveyclimber]

Great supplier for this stuff is waytek wire. I just bought a bunch of Blue Sea stuff from them. Awesome company and reasonable prices. Class T fuse is what you really want . My particular needs for fuses were on backorder. Nothing new in this day and age.

 

[A.Justice]

I saw a short circuit test of a CALB 180ah cell on YouTube. He pulled 2500 amps on a dead short with a wrench. That's about 13c. Round up to 20c for a safety margin. That's also an older model CALB cell, so newer cells could potentially push out way more than that, especially ones like headway and other lithium chemistries.

A 280 amp hour cell at 20c is 5600 amps. A 2p version of that (which a lot of people have, and more) is almost 12k amps, which is higher than most MRBF or ANL fuses can handle. If you have a large bank, you could potentially short with tens of thousands of amps.

It's a class T for me! Unless I had a cell bank that was under 100ah, then I would consider another, still high interrupt rated fuse type. Even so, I have a class T on a 25ah LiFePO4 power bank.

Fuses are the absolute last line of defense against catastrophic failure, don't skimp on them.

 

[upnorthandpersonal]

You know my thing with building a cheap 10kW system, right? Guess what I never, ever cheap out on: Class T fuses. I've witnessed tons of LiFePO4 shorts, and frankly, they're scary. I've also seen ANL fuses just sit there in a short, arcing, heating up and melting the plastic holder they're in. Class T is encased in metal and has silica sand to quench the arc.
Each one of my parallel batteries has a Class T fuse. The Inverter is preceded with a Class T fuse just in case some MOSFET blows out and creates a short. Yes, expensive, but absolutely worth it - especially with higher voltage systems where ANL is not an alternative at all.

 

[Hedges]

I saw a short circuit test of a CALB 180ah cell on YouTube. He pulled 2500 amps on a dead short with a wrench. That's about 13c. Round up to 20c for a safety margin. That's also an older model CALB cell, so newer cells could potentially push out way more than that, especially ones like headway and other lithium chemistries.

A 280 amp hour cell at 20c is 5600 amps. A 2p version of that (which a lot of people have, and more) is almost 12k amps, which is higher than most MRBF or ANL fuses can handle. If you have a large bank, you could potentially short with tens of thousands of amps.

It's a class T for me! Unless I had a cell bank that was under 100ah, then I would consider another, still high interrupt rated fuse type. Even so, I have a class T on a 25ah LiFePO4 power bank.

Fuses are the absolute last line of defense against catastrophic failure, don't skimp on them.

Nice enough demo, but I wouldn't use the raw numbers reported to select a fuse.

3.3V cell
2.2V under load
2220A

Not quite maximum power transfer, with 4880W delivered; max would occur at 1.65V (and same power dissipated inside cell)

He dropped a large steel wrench across a single cell (effectively).

Steel ranges from 13 to 70 micro-ohm centimeters http://www.rfcafe.com/references/electrical/resistivity.htm
I'll use 30
Consider a wrench 20 cm long, 1.5 cm wide, 0.5 cm thick.

30 micro-ohm cm x 20 cm / 1.5 cm / 0.5 cm = 800 micro ohms = 0.0008 ohms.
Compare to a cell with spec 0.00025 ohms, typical measurement 0.00017 ohms.
Wrench plus typical cell is 5.7x higher resistance than cell alone. (hmm, doesn't agree with my estimate of close to max power transfer)

Consider same wrench shorting out a 16s pack. Then, resistance would be just 1.3x the cells alone.

I don't have cell resistance figures for his cell, but I would base fuse AIC rating on cell resistance figures. Using What I have seen quoted:
3.4V/0.00017 ohms = 20,000A. So class T would be good for a single string of LiFePO4 cells.

 

[wiseacre]

Solar for Dummies
I want to use a 24v 200Ah LiFePo4 battery wired to a 3500w inverter. What size class T fuse should I use?

I don't know if it matters but the inverter is "oversized" so it doesn't get pushed to it's limits. I doubt I'll ever use more than 2000w at most.

 

[John Frum]

Solar for Dummies
I want to use a 24v 200Ah LiFePo4 battery wired to a 3500w inverter. What size class T fuse should I use?

I don't know if it matters but the inverter is "oversized" so it doesn't get pushed to it's limits. I doubt I'll ever use more than 2000w at most.

As a matter of policy I size the rest of the system components based on the max continuous rating of the inverter.

For a high frequency inverter
3500 ac watts / .85 conversion factor / 20 volts low cutoff = 205.882352941 service amps.
205.882352941 service amps / .8 fuse headroom = 257.352941176 fault amps.

That means a class t fuse of minimum 250 amps.
The wire that you use needs to be rated at 250 amps minimum.
The fuse is intended to stop the insulation melting/burning off the wire and protect the battery from venting toxic stuff.

 

[wiseacre]

The wire that you use needs to be rated at 250 amps minimum.

So, if I read an ampacity chart right I should use 4/0 wire
add a 250 amp fuse and I should sleep better at night ?

So, if I understand correctly the inverter is the deciding factor.
Although I don't ever plan on using the inverter to it's full potential I'll feel better using the "recommended" sizes. One never knows and should never say never.

 

[upnorthandpersonal]

Although I don't ever plan on using the inverter to it's full potential I'll feel better using the "recommended" sizes. One never knows and should never say never.

Inverters sometimes release the magic smoke in the mosfets and short out the input, so that's when you need the fuse to kick in. Remember: fuses protect wiring, not equipment.