Class T vs ANL fuse

Dzl said:

That is one way to look at it. Another way is that its just a statement made in recognition of the same problem that you stated--that AIC is often not listed by LFP manufacturers or assemblers--and guidance that can be followed without knowing that info would be useful to builders.

Click to expand...

A power of standards (which I don't believe TE-13 has the status of) is to compel people to list safety-critical specification information in order to be compliant. The manufacturers and suppliers "tail" should not be wagging the standards "dog".

Dzl said:

If each subdivision was properly fused, with properly sized AIC and current ratings, downstream would be protected wouldn't it? Only the first fuse for each subdivision would need to handle the full AIC of the pack I think. Beyond that, lower AIC fuses are okay I think. I may be misunderstanding something.

Click to expand...

I take TE-13 to mean that if you have say 4 parallel battery system branches, you could fuse each at say 200A, and each of those fuses would ordinarily have to deal with just the short circuit current of that string. (Exception: if that strings shorts, the other three strings fault into it. ABYC did not allow for that - another flaw in TE-13?). But after those fuses, the branches are combined, and now the available fault current is four times greater.

(Perhaps I'm misunderstanding TE-13 here, but if so I have to say it is unacceptably unclear. In constructing large battery banks you often have a single large load - a good example would be electrical propulsion - so parallel strings have to be combined. And there is often good reason not to subdivide an electrical system even when it might be technically possible.)

For example, if you have say a couple of inverters downstream, each might be fused with say 400A.

Now it is a basic principal of protection that downstream protective devices should operate first for faults that occur (immediately) downstream of them, and that they should be coordinated with upstream devices (in capacity and speed) so that the downstream device clears the fault without affecting (tripping or degrading) any upstream device. For instance, the 400A downstream fuse should melt and clear a fault in one of those inverters, without any upstream device (200A battery string fuses) getting to the melt point (after which it is suspect even if it did not rupture, and should be replaced). Class T fuses are capable of fairly close coordination, but I have no idea what level of selectivity you can achieve with lesser interrupting fuses (such as Mega fuses). As a corollary, you can mess up coordination by having say a fast, high current fuse upstream of a slow, lower current fuse of a different type.

Anyhow, that downstream device is subject to the full bolted fault current of the entire bank of 4 parallel strings (assuming the wiring resistance and inductance in between are small), so it is faced with a (nearly) 4 times greater prospective fault current than the individual string fuses are (for faults external to the battery bank). If it then failed in its duty, and the fault continued, eventually the per-string fuses would operate, but this is not what you want to happen. Among other things, then all power is lost to all consumers, and you need to replace 5 fuses - rather expensive fuses if they are Class T.

AC standards like the NEC deal with this sort of thing in great detail. They cut a certain amount of slack on protection coordination for ordinary house and industrial situations, but get rigorous for mission-critical applications like hospitals or elevators. For my boat, I want the fuses in the individual battery strings sized so they will not be affected by faults in downstream devices.

Dzl said:

Nice to have a little extra breathing room, particularly considering the unknowns. The 20kA stated in E-11 is referenced as the minimum if short circuit current is unknown, so exceeding it is reasonable. Was the fuse more pricey than other class T fuses you looked at?

Click to expand...

I think they were competitively priced.

alanapar said:

A power of standards (which I don't believe TE-13 has the status of) is to compel people to list safety-critical specification information in order to be compliant.

Click to expand...

alanapar said:

The manufacturers and suppliers "tail" should not be wagging the standards "dog".

Click to expand...

?

alanapar said:

I take TE-13 to mean that if you have say 4 parallel battery system branches, you could fuse each at say 200A, and each of those fuses would ordinarily have to deal with just the short circuit current of that string. (Exception: if that strings shorts, the other three strings fault into it. ABYC did not allow for that - another flaw in TE-13?). But after those fuses, the branches are combined, and now the available fault current is four times greater.

Click to expand...

Bank of N packs in parallel.

This (N-1) fault into 1 failure mode is something my gut tells me to prepare for seriously. But it says a lot of things.

The most extreme safety design I am considering is one contactor and fuse per pack, and a single main larger contactor and fuse to parallel. Gigavac makes some Normally Open Contactors with very low power consumption (1-1.5W holding closed).

e.g.

Pack 1 Positive -> Fuse -> Contactor -> MainFuse -> MainContactor

With MOSFET BMS on pack negative and straight to negative busbar.

Cell vendors publish internal resistance. With that and cell voltage, you can calculate an expected short circuit current.
From the numbers people have shown here, I get just about 20kA.
Maybe the chemistry can't produce current at that rate, but should be a good upper limit.

If four strings in parallel each with its own class T fuse, and one string shorts before the fuse, its fuse carries 60kA from the other three strings and probably sustains an arc. The other three strings blow their fuses, hopefully fast enough the one doesn't explode.
But that string is still shorted, because short was before the fuse. Try to not let that be possible (even if the boat capsizes)

If there is a short downstream you would like it to either trip its own OCP and interrupt whatever that current would be, or hold together long enough for the main fuse to blow. Don't want a minor appliance to disable power for propulsion and instruments.

In the AC world, you can choose coordinated breakers so branch circuit OCP stays closed long enough for main breaker to trip on a bonded fault, and everything can be reset. or you can choose branch breaker to trip faster, be rendered unusable, but leave the facility up.

Class T fuses have a "let through current" (really energy and time) such that a 200,000A AC fault is interrupted so fast that a 20,000A rated breaker downstream doesn't experience excessive energy. (we don't like things exploding). However, at 20,000A it doesn't protect others to 2000A. In DC application with 20kA interrupt rating, downstream circuits experience the full brunt of it.

Seems to me a "current limiting" fuse for DC applications would be good, but class T isn't it. Downstream loads carrying the fault will suffer, unless there is enough series resistance to reduce current.

Hedges said:

Cell vendors publish internal resistance. With that and cell voltage, you can calculate an expected short circuit current.
From the numbers people have shown here, I get just about 20kA.

Click to expand...

Could you please quote some actual data in support of that number?

BTW, a lot of vendors quote cell impedance at say 1kHz - that is a different animal from the effective series resistance seen with large DC currents.

To expand a bit further, Li-Ion batteries have a complicated variation of impedance as a function of frequency (often displayed on a so-called Cole-Cole plot). This data can be used to characterize various electrochemical and other aspects of the battery, but detailed measurements require special equipment and are time consuming, especially at the low frequency end of the spectrum (a few Hz down to DC). In a production environment, often a spot measurement at 1kHz is employed, and this serves to detect certain faults allowing sub-par batteries to be rejected (or sold on the cheap).

But the 1kHz data does not necessarily correlate well with the low frequency data. One reason is that when you model the battery impedance with an equivalent circuit (to greatly simplify) you find part of the circuit consists of a resistance (the reaction resistance) in parallel with a capacitor (the double layer capacitance). At 1kHz this capacitance effectively shorts out the resistance, so you do not see it in such a measurement. But this resistance is an important part of the low frequency impedance.

Correct, they take the measurement at 1 kHz. I don't have a way to translate that to DC.
But, people also have BMS reporting internal resistance which I recall were similar. I think that is delta voltage at some moderate current. (I'm not finding example posts at the moment.)
So it appears DC measurements at moderate currents match my V = I x R approach.

3.4V / 0.00017 ohms = 20,000A

"With an internal resistance meter like the yr1035+ you can test.

My 280's are 0.15 to 0.18 Mili ohms.
Most are 0.17"

Hedges said:

Correct, they take the measurement at 1 kHz. I don't have a way to translate that to DC.
But, people also have BMS reporting internal resistance which I recall were similar. I think that is delta voltage at some moderate current. (I'm not finding example posts at the moment.)
So it appears DC measurements at moderate currents match my V = I x R approach.

Click to expand...

I suspect your number of 20kA (which of course needs to be quoted for a particular cell) is well on the high side. I'm not seeing any real data to back this up.

Do you have discharge curves for this cell at different rates? That would provide some data.

I don't. I haven't used lithium cells. I'm just going by the IR numbers people report here, either from manufacturer or their own BMS or IR measurement device. I don't typically see anything like Peukert curves or other charts with voltage vs. current like we do for lead-acid. I expect IR figures to predict that (up to some point where chemistry of cell can't keep up with current, could be non-linear so you may be correct that 20kA is high.) It has been 280 Ah or so cells which had the 0.17 milliohm figure.

I saw one paper reporting 4000A from 100 Ah AGM. I assumed 16kA as a conservative figure for my 405 Ah bank, probably much less than 4x 100 Ah in parallel due to construction. We know lithium delivers higher current, can crank a car with a tiny jumpstart pack. People have welded contactors and blown fuses (including class T, I think) connecting an inverter to lithium batteries.

Of course, almost any battery included lead-acid could blow a class T fuse in some amount of time, if able to reach parts of its time/current trip curve. But blowing with inrush means the limited amp-seconds to charge capacitors have to occur in short enough time for the I^2 to overheat the fuse. My math said this might be feasible with the capacitor bank of some inverters - I think it was on the order of 100,000 uF.

On the other hand, my inverters are spec'd for up to 10,000 Ah battery bank, and no mention of precharge. For lead-acid & NiCd, wiring such a large bank might have more resistance and inductance (although it takes a lot of wire to reduce short circuit current much), but likely cells aren't optimized for low resistance high current.

alanapar said:

Ahttps://www.sbsbattery.com/PDFs/VRLAshortCurrentsStorageBatterySystems.pdfI suspect your number of 20kA (which of course needs to be quoted for a particular cell) is well on the high side. I'm not seeing any real data to back this up.

Do you have discharge curves for this cell at different rates? That would provide some data.

Click to expand...

Here's a paper that has some results:


This is one of the graphs, during short circuit of LiFePO4 160Ah battery (single cell):



This is at 3.6V with a single cell at 160Ah. Cells with a larger capacity will have a larger short circuit current. If you then make a 48V battery with them, and you parallel e.g. two of those, you're going to see some massive currents.

This paper confirms:


"A battery's short circuit current is typically estimated by dividing its open circuit voltage by its internal resistance. While the true DC internal resistance can be determined using a series of discharge tests, it is often simpler to directly measure the battery's impedance or conductance using an AC test signal".

So the internal resistance is a good estimate. Therefor, I believe that the current in the first paper and represented in the graph is on the low end. Calculating the internal resistance in that short gives a number of around 3mR. My guess is that the leads used were a large contributing factor. If this is the case, the current could be much larger in that case. The other reason I suspect this is because of this test by GWL:


The 160Ah cell in this case was overcharged and already had a higher internal resistance because of this. The short still generated 1000A.

More real data:


> 200C for 1/100th of a second, followed by 50C

Those weren't LiFePO4. If applicable, 280 Ah would deliver 50kA briefly. IR would limit it to 20kA. The time limit is on the order of class-T time to blow, so fuse should remain physically intact. But supports the idea of sufficient current to open the fuse in that time.

Full second at 50C (15kA) would best be protected by class-T.

Again, not LiFePO4 cells but worth considering.

upnorthandpersonal said:

This is at 3.6V with a single cell at 160Ah. Cells with a larger capacity will have a larger short circuit current. If you then make a 48V battery with them, and you parallel e.g. two of those, you're going to see some massive currents.

Click to expand...

I actually don't think I believe those results. 160Ah can only deliver 1000A into a short? A fraction of what a lead-acid battery can do? My truck battery can deliver 800A into 8.5V, 1/3 of a short?

I suspect they have way too much series resistance and it swamps out IR, limiting current to a fraction of what the cell can deliver.

Hedges said:

I actually don't think I believe those results. 160Ah can only deliver 1000A into a short? A fraction of what a lead-acid battery can do? My truck battery can deliver 800A into 8.5V, 1/3 of a short?

Click to expand...

See my updates (edits) as well.

My personal opinion on all this is to use Class T fuses, always. I've personally seen properly sized ANL fuses just sit there and get hot, melting the plastic case around them, in combination with LiFePO4 packs. At 48V, you can't use ANL anyway since they're only rated to 32V.

I just happened to notice, while shopping, that not all Class-T fuses share the same ratings.

For example, when looking at 300A Class-Ts:

1. Blue Sea: 20 kA @ 125VDC
2. Littelfuse: 50 kA @ 160 VDC, but with a lower rating product also available
3. Eaton Bussman: many options
4. Mersen/Ferraz Shamut: 50kA I.R. DC & 100kA I.R. DC

Maybe that was already known to most, but I'm less dumb today.

A.

upnorthandpersonal said:

My personal opinion on all this is to use Class T fuses, always. I've personally seen properly sized ANL fuses just sit there and get hot, melting the plastic case around them, in combination with LiFePO4 packs. At 48V, you can't use ANL anyway since they're only rated to 32V.

Click to expand...

That sounds more defective or poor installation vs failing to stop 1000+ amps.

carbon60 said:

I just happened to notice, while shopping, that not all Class-T fuses share the same ratings.

For example, when looking at 300A Class-Ts:

1. Blue Sea: 20 kA @ 125VDC
2. Littelfuse: 50 kA @ 160 VDC, but with a lower rating product also available
3. Eaton Bussman: many options
4. Mersen/Ferraz Shamut: 50kA I.R. DC & 100kA I.R. DC

Maybe that was already known to most, but I'm less dumb today.

Click to expand...

Furthermore, there can be differences within a line of fuses at different currents.
For instance the JLNN Class T Littelfuse from the link above is only rated for 160VDC under 60A, over 60A its rated for 125VDC. And AIC is 50kA under 30A and 20kA over 35A.
Class T is a class/type of fuse, there are still devil's in the details.

carbon60 said:

I just happened to notice, while shopping, that not all Class-T fuses share the same ratings.

For example, when looking at 300A Class-Ts:

1. Blue Sea: 20 kA @ 125VDC
2. Littelfuse: 50 kA @ 160 VDC, but with a lower rating product also available
3. Eaton Bussman: many options
4. Mersen/Ferraz Shamut: 50kA I.R. DC & 100kA I.R. DC

Maybe that was already known to most, but I'm less dumb today.

A.

Click to expand...

Looking at the Littlefuse data sheet


they list for voltage ratings of JLLN fuses (the 300VAC series):

Dc: 160 V (1–60 A)
125 V (70–1200 A)

and for interrupting ratings:

Dc: 50 kA (1–30 A)
20 kA (35–1200 A)


Mersen (Ferraz Shawmut) at


state for the corresponding A3T series:

DC: 1 to 1200A 160VDC, 50kA I.R.


BTW, there are two series of Class T fuses, generically identified as the 300VAC and the 600VAC series. It is the latter that has the 100kA IR for 300VDC (A6T series) in Mersen's range. I have not used the higher voltage series, but I suspect you will find they have significantly higher voltage drop at normal currents.

alanapar said:

I have not used the higher voltage series, but I suspect you will find they have significantly higher voltage drop at normal currents.

Click to expand...

The trip curves differ too. Littlefuse JJLN vs. JJLS 400A, 2500A vs. 5000A for 10 milliseconds to clear.

alanapar said:

BTW, there are two series of Class T fuses, generically identified as the 300VAC and the 600VAC series. It is the latter that has the 100kA IR for 300VDC (A6T series) in Mersen's range. I have not used the higher voltage series, but I suspect you will find they have significantly higher voltage drop at normal currents.

Click to expand...

The voltage drop is a concern to me,I didn’t realize. A Mersen was easily available locally while there is an undefined lead time for the Blue Sea, which I also ordered.

carbon60 said:

The voltage drop is a concern to me,I didn’t realize. A Mersen was easily available locally while there is an undefined lead time for the Blue Sea, which I also ordered.

Click to expand...

As long as you ordered the Mersen A3T (300VAC) series, you will be fine.

alanapar said:

As long as you ordered the Mersen A3T (300VAC) series, you will be fine.

Click to expand...

Just confirmed: A3T-300. Was $74 Canuckistan Kopecks!

FYI, These are still apparently available. The ad says "Slowblow" but the JLLN is the "Fast blow".

Cool. Thanks!

Posplayr said:

FYI, These are still apparently available. The ad says "Slowblow" but the JLLN is the "Fast blow".

Cheap Source LittleFuse Class T Fast Acting Fuse

Just an FYI for frugal shoppers. Littlefuse is a top name in fuse technologies which include Class T fuses. https://www.littelfuse.com/products/fuses/industrial-power-fuses/class-t-fuses/jlln/jlln110.aspx You can find them on Amazon for $48.99...

diysolarforum.com

Click to expand...

I found this https://www.invertersupply.com/media/data/Go-Power_SPC_FBL_vB.pdf
No sign of a trip curve plot though.

smoothJoey said:

No sign of a trip curve plot though.

Click to expand...

Zooming in on the picture, fuse says, "Buss"
With than name I find curves but it may be Buss has more than one model and trip curve for class-T

I have not read this yet but it looks like a great primer on fusing.


This Ohms law stuff is so eeaccy

I see comments regarding the class T open circuit being up to 20,000 amps. Some I've looked at state they will open up to 200,000 amps. Is there a misconception or just various differences depending on brand? Also I see comments regarding the need and purpose of class T over other types of fuses. Essentially, DC current can arc even after a fuse opens in some cases and with some types of fuses so that the current can continue to flow by arcing across the open fuse. That makes for a lot of danger of personal injury and possible property damage as well. The class T uses a type sand material around the element(s) inside it to absorb the arcing and make a more positive open when needed due to issues with the circuit including accidental shorts when working on these systems. And the manufacturers provide data that indicates the amount of current their product is capable of stopping when their device opens. I saw one comment that stated that their battery manufacturer provided one type fuse and was relying on that manufacturer's expertise. It's better to have what was provided with the product and add more. More is always better when it comes to circuit protection. Class T is the max available for DC circuits. You can go with lesser, but it might cost you in ways I don't want to mention.