LFP Available Short Circuit Current

[CaptainRivet]

Originally Posted by s/v Jedi

Your issue was errors made during installation, like position of the fuse block or wiring routing and connected wrong. Or maybe you even had a knock-off cheap fuse that did not conform to the standards.

But the standards and certification is correct; it has been tested too much for having any doubts, let alone your conspiracy theory about it being certified falsely because your car burned down. Your car burned down because someone did something wrong with the installation or cheaped out buying components

Maybe...at that time LFP was not officially avaliable, the scrapped cells where faulty or we draw too much current, didn't have specs or anything about them...was competition audio, cells delivered incredible power, so we went to limits and over...the class T fuse should protect and first did but then failed.

Was it a good knock off or a faulty fuse that wasn't according to spec or How do you know the current was not above spec? It melted 150sqmm/gauge 4.0 cables, isolation was dripping of the cable...
We definitly didn't buy cheap...all high quality and fuses, cables,amplifier, speaker all were considered wear and tear plus we were sponsored. We were regularly dealing with 1000A plus,did we made an installation mistake, maybe??

Class T was directly connected to plus terminal/300sqmm busbar of the bank, there you cannot make a mistake and i always check NM and with a Flir camera for hot spots. How it happenes, we were pulling around 800A, suddenly all the 3 class T 400A blew (3 gauge 4.0 with each a 400A fuse), then cells started to gas and then suddenly one off the class the class T got arced and the gauge 4.0 cables started to melt and somewhere else in the car it started smoking and then fire, all in 2min. It wasn't the bank and it wasn't our thick cables to the amps that burned, it started under dashboard and engine room after the class T got arced.

[CaptainRivet]

Originally Posted by s/v Jedi

Your issue was errors made during installation, like position of the fuse block or wiring routing and connected wrong. Or maybe you even had a knock-off cheap fuse that did not conform to the standards.

But the standards and certification is correct; it has been tested too much for having any doubts, let alone your conspiracy theory about it being certified falsely because your car burned down. Your car burned down because someone did something wrong with the installation or cheaped out buying components

I never said they are certified falsly. It interrupts till 25kA and disconnects, job finshed? No...
It is certified correctly but for what exactly?
Who really shortened a LFP bank and waited+monitoring whats happening? How low is the resistance of a shortened cell, we all talking about resistance of a working cell between what manufacturer states 0.12mohm till 0.5mohm but how much has a shortened cell? The only resistance that adds on top are the bussbars.

How is the certification process of the class T? Is it tested that after! the fuse blows you throw raising current 10000A+ at it for eg 1min and wait what happens?
I highly doubt they do simply because its made for a different purpose, short high current spike to protect and disconnect equipment is its major spec it was developed for.

But that a rasing current over time is ionizing the air and is then enabling to pass 7mm through air a (blown) 400A class T has above 10000A is physically possible. It simply arcs outside the fuse the 7mm gap...
Class NH are tested for that as its a common use case and thats a major reason they have ceramic bodies and fuse holder of these dimensions. Yes its pain in the ass to install this giant fuses but its worth doing it.
I know all i use has 80kA DC IR rating....and thats definitly enough.

[Statistical]

"knows" 25k AIC is not enough.
"knows" 80k AIC is enough.

Just happens to use 80k AIC fuse. Convenient.

[s/v Jedi]

Originally Posted by Statistical

"knows" 25k AIC is not enough.
"knows" 80k AIC is enough.

Just happens to use 80k AIC fuse. Convenient.

It’s all funny, isn’t it? I demonstrated that current will only be a fraction of what a shorted cell can do because of circuit resistance, but I guess Ohm ‘s law doesn’t always apply for everybody

[CaptainRivet]

Originally Posted by Statistical

"knows" 25k AIC is not enough.
"knows" 80k AIC is enough.

Just happens to use 80k AIC fuse. Convenient.

Never said that, just said 80kA AIC is definitly enough, overkill for sure yes but if its cheaper then class T why not?
As i don't exactly know how high is my internal cell resistance in operation and when shorted, i just calculate with the theortical maximum of my bank or if i help installing the owners bank. I only install NH as main battery fuse, if owner wants something else he can do it himself...
Yes i do mistakes as i am a human, till now the NH fuses done the job and saved my ass and the car/RV/boat i installed in last 20years, so why should i use something else? And thats why you do stress test at the end of install....to find your mistakes and faulty or not up to spec equipment.

[betwys1]

Originally Posted by CharlieJ

@betwys1
Two different topics: rating of the OCPD to protect the conductor (the NEC reference) and the ampere interrupt capacity (AIC) of that OCPD (the topic of this thread) so that the OCPD can clear a fault and not fail catastrophically because of the exceptionally high available short circuit current (ASCC) presented by LFP batteries in a crow bar fault.

I believe I am understanding that you are debating the qualities of a device specified to protect an associated conductor by failing, but a device which may fail catastrophically in some circumstances?
Perhaps because the conductor to which the protective device is connected is capable of carrying MUCH more than the load for device failure so that the protective device is ill-specified?

Would a useful rule of thumb be this: Do NOT connect over current protection
of a limit value which is much less than the actual current carrying capacity of the associated conductors. ??

[Statistical]

Originally Posted by betwys1

I believe I am understanding that you are debating the qualities of a device specified to protect an associated conductor by failing, but a device which may fail catastrophically in some circumstances?
Perhaps because the conductor to which the protective device is connected is capable of carrying MUCH more than the load for device failure so that the protective device is ill-specified?

Would a useful rule of thumb be this: Do NOT connect over current protection
of a limit value which is much less than the actual current carrying capacity of the associated conductors. ??

There are two different relevant values. The first is the OCP current rating. This is the one everyone knows. This is a 500A fuse. If current significantly exceeds 500A it will blow and open the circuit. How quickly it does depends on the time curve but in general it protects against currents over 500A and will allow currents less than 500A. If the wiring can safely handle 500A continuously then 500A is the appropriate fuse. You certainly can go with a lower fuse but not a higher one as a higher one would allow an unsafe amount of current to flow without blowing the fuse.

However a blown fuse could still transfer current if the current flowing across it is very large. There is a seperate value the AIC (Ampere Interrupting Capacity) which is the point where a blown fuse will no longer be able to prevent an arc forming across the two sides of the fuse the meaning is not protecting the wiring. This is the value far less known. It depends on the fuse type.

A mega fuse for example is 2,000A AIC. A class T fuse is 20,000. This is relevant with LFP packs especially large ones because a short circuit at the battery terminal could produce significantly more than 2,000A. If that happens your 500A (or 300A or whatever it is) Mega fuse will blow nearly instantly however the let say the over current is 8,000A. The current will simply arc across the fuse terminals meaning the circuit is still closed and as a result the wiring which can only handle 500A (or 300A or whatever) will very quickly melt or ignite.

So there are two different but equally important values. The current rating of the OCP (what current it blows at) and the AIC (what current it stops protecting at). There is no downside to having a larger AIC other than generally speaking fuse types with higher AIC tend to be bulkier and more expensive.

The AIC is usually (check spec sheets) not dependent on the OCP rating. A 100A and 500A Class-T fuse both have 20,000A AIC.

Note that switches (to include contactors and relays both mechanical and solid state) also have an AIC rating and should be appropriately sized but only need to be sized to the circuit amperage rating as there should also be a OCP (fuse or circuit breaker) which prevents current beyond that. However yes if you buy some relay with only 100A AIC and try to turn off a live circuit with 500A flowing across it the relay will throw open but the current will simply arc from pole to pole and the circuit will remain energized.

[CharlieJ]

@betwys1 #36
I thought I had clearly stated the two concepts of OCPD rating and AIC succinctly in post #28.

Let's try this:
1. An OCPD is designed to open under two different conditions, over current and short circuit. The OCPD trip curve illustrates this concept.
2. When an OCPD opens, either to clear a short circuit or because of over-current, it must have an ampere interrupt capacity (AIC) sufficient so that it successfully opens and clears the fault condition within the specified time and without further damage.

The time specification is dependent on how quickly the arc that forms when the OCPD opens is extinguished since it is conductive.

Further damage can include the destruction of the OCPD body mechanically and physically coming apart. In the case of a circuit breaker, ABYC Standards require that it still function after clearing a fault.

The AIC is a function, in the discussion in this thread, of the available short circuit current (ASCC) that can be produced by a battery/bank in a crow bar fault.

[Baronkrak]

Apparently the answer lies in various Standards.
Also.

https://www.researchgate.net/publica...ctric_Vehicles

[sailingharry]

First, let me say that I am NOT arguing, and do not disagree with the information above. Also, while I am a licenced engineer (mechanical) with a fairly solid electrical understanding, I not an expert. But I may have a better understanding than the average bear. I am posting because I don't understand and want to learn

I do understand that in electrical circuits, you can apply a voltage or a current -- but NOT both. An alternator can be set to do 13V or 100A -- but you only get to choose one. In sailboats, we generally "set" the voltage at 13V. Current is "available" -- an alternator can put out 100A, an LFP 2000A, but it only puts out what can be accepted by the circuit.

So, my ponder with AIC is the current. An LFP does not push out 2000A. It is a 13V voltage source with nearly unlimited current. But it is limited to 13V. In breakers, that initial very high current can weld the breaker shut. But in a fuse, the current path is an arc. I don't think CURRENT can support an arc -- I think an arc is supported by voltage. So, what is the voltage drop across an arcing fuse? In the classic "Jacobs ladder" arc across air, it is about 10,000 volts per inch.

My thoughts is that the AIC has to be related to voltage. A 1/8 inch long fuse may be able to support an arc on 12V, but it may take 30V to arc across 1/2 inch. DC stick welders, from some Googling, run around 30V to support what I assume is an arc under 1/8 inch.

So, it seems to me, a Class T may be able to interrupt a 10,000A arc at 100VDC, but at 12VDC the AIC may be closer to infinite, as the voltage drop is greater than 13V.

I guess what I'm asking is what is the physics of supporting a 1/2 long arc at 13V?

[Statistical]

Originally Posted by sailingharry

So, it seems to me, a Class T may be able to interrupt a 10,000A arc at 100VDC, but at 12VDC the AIC may be closer to infinite, as the voltage drop is greater than 13V.

I guess what I'm asking is what is the physics of supporting a 1/2 long arc at 13V?

You are correct that the AIC of a fuse in general would increase with lower voltage. The issue is one of certification. Mot OEMS simply provide an AIC with an upper limit voltage so 20,000A @ up to 125V. At 12V is the AIC really 40k or 80k, or 250k? Maybe but the OEM hasn't certified it to that level. I guess you could do your own testing but would you testing be sufficiently rigorous? How many $30 fuses are you going to destroy to ensure the results are consistent and repeatable?

It is largely moot though because while the internal resistance of LFP cells is low it is not zero so there is a maximum short circuit current and it is going to be <10,000A at the battery bank terminals. Further down the system the current is going to decrease rapidly. Even 4/0 cable is going to add not insignificant resistance.

On edit: Marine HowTo touches briefly on the idea that the AIC at marine voltages is actually probably higher. The 20k AIC for Class-T fuses is at 125V.

https://marinehowto.com/battery-bank...nt-protection/

It is a 13V voltage source with nearly unlimited current.

This is incorrect or at least ambiguous. It is a 12V/24V/48V source with HIGH but still limited current. V=I/R (or I=V/R). The cells do have internal resistance that puts an upper bound on peak current even in the highest current situation which would be a short circuit across the battery terminals.

[tanglewood]

Originally Posted by sailingharry

First, let me say that I am NOT arguing, and do not disagree with the information above. Also, while I am a licenced engineer (mechanical) with a fairly solid electrical understanding, I not an expert. But I may have a better understanding than the average bear. I am posting because I don't understand and want to learn

I do understand that in electrical circuits, you can apply a voltage or a current -- but NOT both. An alternator can be set to do 13V or 100A -- but you only get to choose one. In sailboats, we generally "set" the voltage at 13V. Current is "available" -- an alternator can put out 100A, an LFP 2000A, but it only puts out what can be accepted by the circuit.

So, my ponder with AIC is the current. An LFP does not push out 2000A. It is a 13V voltage source with nearly unlimited current. But it is limited to 13V. In breakers, that initial very high current can weld the breaker shut. But in a fuse, the current path is an arc. I don't think CURRENT can support an arc -- I think an arc is supported by voltage. So, what is the voltage drop across an arcing fuse? In the classic "Jacobs ladder" arc across air, it is about 10,000 volts per inch.

My thoughts is that the AIC has to be related to voltage. A 1/8 inch long fuse may be able to support an arc on 12V, but it may take 30V to arc across 1/2 inch. DC stick welders, from some Googling, run around 30V to support what I assume is an arc under 1/8 inch.

So, it seems to me, a Class T may be able to interrupt a 10,000A arc at 100VDC, but at 12VDC the AIC may be closer to infinite, as the voltage drop is greater than 13V.

I guess what I'm asking is what is the physics of supporting a 1/2 long arc at 13V?

You are generally correct, and the AIC rating, or at least it's capabilities will vary with voltage. But the rating is at the max allowed voltage, so is worst case. As you point out, the AIC capabilities of a particular type of fuse are really determined by it's physical dimensions, materials, and construction.

[s/v Jedi]

With DC arc’s itis about sustaining the arc, not starting one.

For AC, the arc has to be started 50 or 60 times a second as the voltage crosses through zero, while dc is continuous.

The start of the DC arc isn’t about jumping the air gap because the current is flowing through the fuse already. It starts to burn up on a hair width and when the current is high enough the metal will evaporate to aid the arc forming over a larger distance.

When you look at AIC ratings and compare to the physical fuses you will see that contact distance, not only within the fuse but also the fuse holder, is the main factor for their rating.

[more]

Originally Posted by s/v Jedi

Your issue was errors made during installation, like position of the fuse block or wiring routing and connected wrong. Or maybe you even had a knock-off cheap fuse that did not conform to the standards.

But the standards and certification is correct; it has been tested too much for having any doubts, let alone your conspiracy theory about it being certified falsely because your car burned down. Your car burned down because someone did something wrong with the installation or cheaped out buying components

if T-fuse good and reliable bee in all EU house,
simply NH is beter fuse capable Full load break
t-fuse is good for special purpose and location.(USA maybe)
all new EU house on conection with grid must have NH fuse with Full load break capable.
T-fuse is rare used in EU very hard find,conclusion skip

[s/v Jedi]

Originally Posted by more

if T-fuse good and reliable bee in all EU house,
simply NH is beter fuse capable Full load break
t-fuse is good for special purpose and location.(USA maybe)
all new EU house on conection with grid must have NH fuse with Full load break capable.
T-fuse is rare used in EU very hard find,conclusion skip

I have trouble following you, but I understand you describe house installations. What we discuss here is for boats and it is DC, not AC.

Also, I am in Europe and the first page that comes up when I Google it is like the attachment: class-T fuses are readily available, incl. the Blue Sea fuses and they are the same prices as in the US.

Conclusion: the #1 goto choice in Europe as well as in the US.