Dead short fuse trip experiment using an MRBF fuse
- Thread starter Johan
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Hedges
I See Electromagnetic Fields!
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Good that it interrupted the current, didn't sit there burning and ignite something else.
But blowing apart and splattering molten metal and plasma is not good. Human safety hazard, and could ignite flammable outgassing from LiFePO4 batteries.
Depending on how long it carried the current, it might coordinate with a Class T fuse, allowing high faults to open class T without the MRBF blowing up. e.g. MRBF on each of several battery strings, one Class T to inverter. (normally we want the best fuse closest to source, so try to make shorts impossible prior to Class T.)
But blowing apart and splattering molten metal and plasma is not good. Human safety hazard, and could ignite flammable outgassing from LiFePO4 batteries.
Depending on how long it carried the current, it might coordinate with a Class T fuse, allowing high faults to open class T without the MRBF blowing up. e.g. MRBF on each of several battery strings, one Class T to inverter. (normally we want the best fuse closest to source, so try to make shorts impossible prior to Class T.)
Brucey
Solar Wizard
I don't think there's any hydrogen outgassing unless they have been severely over charged and bloated and letting out the magic smoke and juices. Now a bunch of FLA....
“housing back wall blew out and landed ~10 ft away. Sparks of molten copper blew out and landed 1-3ft away.”
So much for marine ignition protection. The cover literally says “Ignition protected” see blowup
“The relay welded closed (this was expected) and had to be taken apart to break the contact points loose.”
I’ve even had Blue Sea rotary disconnects weld stuck even when used within their rated range and measured voltage loss across them. My opinion on them is not high.
“The MRBF fuse after blowing. The separation occurred at the plastic weld / glue seam.”
When operating outside of the MRBF specs, conservatively expect at least the housing to fail and molten copper and plastic debris to fly around.”
Well, the MRBF fuses are rated at 58VDC and your setup wasn’t at that level, so you are within the working environment for the fuses voltage.
As for your over current, obviously you exceeded the current protection but by how much is a mute issue. A dropped wrench or whatever is what it’s there for. This looks to be an authentic BUSS/Blue Sea fuse, not a knockoff. (See freeze frame back lit by flame
) So this leaves only one conclusion if this can be repeated, probably very likely in various current ratings; You sir may have brought to light 
a very serious flaw in MRBF fuses and I don’t believe that I’ll ever use them again. I have in several boats that I have previously owned. Thanks for sharing
So much for marine ignition protection. The cover literally says “Ignition protected” see blowup
“The relay welded closed (this was expected) and had to be taken apart to break the contact points loose.”
I’ve even had Blue Sea rotary disconnects weld stuck even when used within their rated range and measured voltage loss across them. My opinion on them is not high.
“The MRBF fuse after blowing. The separation occurred at the plastic weld / glue seam.”
When operating outside of the MRBF specs, conservatively expect at least the housing to fail and molten copper and plastic debris to fly around.”
Well, the MRBF fuses are rated at 58VDC and your setup wasn’t at that level, so you are within the working environment for the fuses voltage.
As for your over current, obviously you exceeded the current protection but by how much is a mute issue. A dropped wrench or whatever is what it’s there for. This looks to be an authentic BUSS/Blue Sea fuse, not a knockoff. (See freeze frame back lit by flame
) So this leaves only one conclusion if this can be repeated, probably very likely in various current ratings; You sir may have brought to light a very serious flaw in MRBF fuses and I don’t believe that I’ll ever use them again. I have in several boats that I have previously owned. Thanks for sharingFound a very interesting discrepancy. Ten year old packaging shows “32V DC Max”
Latest website site shows Maximum 58V DC.
I’m wondering how catastrophic the result would be at 24 and 12 volts with the higher current rated fuses. At 48 volts and higher current, there’s obviously a heck of a lot of energy/plasma/molten metal wanting to exit. I’m pretty confident that a T class fuse + quality holder won’t be squirting sparks.
Again, thanks to Johan for this test. Previously, I thought that MRBF’s were great if you weren’t concerned about “fast blow”.
Latest website site shows Maximum 58V DC.
I’m wondering how catastrophic the result would be at 24 and 12 volts with the higher current rated fuses. At 48 volts and higher current, there’s obviously a heck of a lot of energy/plasma/molten metal wanting to exit. I’m pretty confident that a T class fuse + quality holder won’t be squirting sparks.
Again, thanks to Johan for this test. Previously, I thought that MRBF’s were great if you weren’t concerned about “fast blow”.
Pi Curio
Sunlicious
How about we make some kind of fundraise and hire a professional to test all these fuses, maybe even find equally effective alternatives? Which profession works with this sort of thing?
Or, we keep buying whatever is in trend, MRBFs, T-class, and whatever fuse they throw at us and hope it works when shtf.
It's a bit strange that there's not a least one guy who works with this stuff in the DIY community who would shed light on these things once and for all time. At least I haven't read about one so far.
Or, we keep buying whatever is in trend, MRBFs, T-class, and whatever fuse they throw at us and hope it works when shtf.
It's a bit strange that there's not a least one guy who works with this stuff in the DIY community who would shed light on these things once and for all time. At least I haven't read about one so far.
Nice stuff. I requested T class testing a month or so ago and pretty much everyone just complained saying there's no reason to test lmao.
Gonna request they get tested too. I'll buy you a https://www.amazon.com/gp/product/B0BQZ8715R/ if you want because I don't think bluesea (little fuse) should be the only fuse anyone trusts.
Do you have a way to limit amperage to test like a 150amp fuse when it blows at.. say 200-250 amps etc?
Thus I don't believe these fuses, regardless of company, are going to also 100% always be 100% accurate.
Gonna request they get tested too. I'll buy you a https://www.amazon.com/gp/product/B0BQZ8715R/ if you want because I don't think bluesea (little fuse) should be the only fuse anyone trusts.
Do you have a way to limit amperage to test like a 150amp fuse when it blows at.. say 200-250 amps etc?
yep I'd like all of them tested. I don't think datasheets and curves are always accurate as they are not accurate for microchips, processors, micro controllers, mosfets, transistors, etc that I have used for decades.
Thus I don't believe these fuses, regardless of company, are going to also 100% always be 100% accurate.
Blue Seas put out some information a few years back that their 48 volt rating was a maximum, not an indication that it was for a 48 volt system. I think they recognized that 48 volt systems were being implemented more and more. The newer 58 volt rating likely included a change to their products to handle that voltage.
In my opinion the whole fast-blow "idea" of class-T is bit ill-informed.
Any decent inverter should have very fast overcurrent limiting built-in. Only if something goes terribly wrong in the inverter it will draw enough current to blow fuse. Fast fuses are typically too slow for protecting the transistors and inverter itself has so much input capacitance that if something goes wonky with the switching stage it will easily self-destruct irregardless of fuse selection. Fusing is only to serve as a fire protection.
I used some South Bend fuses simply because BUSS wasn’t available or some scalpers were flipping them for ridiculous prices during pandemic. So yeah I’d love to see if those spit lava when spanked.
Some bms’s have very fast over current protection as well. Now I’m just liking the T class for perhaps better containment of the boom. A good test might just be the last word thou.
I would never count on the BMS alone being able to survive short-circuit on the output. Disconnecting on overload is fairly easy but controlled disconnect during short-circuit on 48V system is not a trivial task as the current will rise to thousands of amps in less than 100 microseconds.
Saving the BMS could be actually one benefit of current-limiting fast acting class T or similar fuse. BMS typically has very hefty mosfet array that can handle thousands of amps in the on-state for a short perioid of time as long as fuse breaks the circuit. So the bms might survive fast-blow fuse but fail with slow-blow.
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Yea and I remember your several threads.. and I had threads on it too. I still use them. I have 125amp fuses and 125amp breakers on wire that can handle 200 amps. The breaker will trigger first and unless it's a dead short, the fuse probably won't ever trigger since breaker will first.
but they're way less money than bluesea so I don't think bluesea is worth it. I assume they work..
....but if testing shows they blow at like 200000 amps instead of 125 then I'd probably switch too LOL
oh yea I also have the JK bms with the over current protection so I guess all my batteries have 3 levels of protection
As it sounds like you're planning on repeating this test; a few suggestions for protection.
1. Have a loop of the wire away from the rest that you'd cut with the branch cutter
2. Swap the branch cutter to an insulated heavy gauge wire cutter
3. That loop should also have a bus bar so you can unbolt should cutting the wire not suffice to end the current flow.
4. A blade style disconnect? I think they make those for high current DC
1. Have a loop of the wire away from the rest that you'd cut with the branch cutter
2. Swap the branch cutter to an insulated heavy gauge wire cutter
3. That loop should also have a bus bar so you can unbolt should cutting the wire not suffice to end the current flow.
4. A blade style disconnect? I think they make those for high current DC
So to protect my system, I’ve got the inverter’s default over current, high voltage disconnect (thermal /magnetic breaker), Fast blow T class fuses at each BMS rated well below the cable and BMS ratings, the over current protection set well below the peak rating of each BMS. Not too much else can be done reasonably, however I figure that a massive, instantaneous short could still kill the system, but it won’t hopefully catch fire.
GXMnow
Solar Wizard
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Every over current protection device requires some time before it can shut off the current flow. An instant dead short can hit several thousand amps long before a 100 amp fuse or breaker can open. And a Mosfet based BMS is not going to save you. In fact, many Mosfet failures result in them being a dead short as well. So the BMS can lock on in a bad over current failure.
A worst case might even be the Mosfet H bridge in the inverter going dead short. So the inverter is not saving you. Then the BMS Mosfets also fail shorted, no help there. The fuse strikes a plasma arc, because you used a 12 volt Mega Fuse. The breaker welds shut because it was only rated for AC. Oops.
Protecting a high current DC system is not trivial. I have a 100 amp CNL fuse (125 volt rated) on each of my 4 battery strings, and then a Class T 225 amp fuse on the main run from the battery bank to the inverter. And I will soon, also have a 125 amp 400 volt DC rated breaker on each pair of battery strings.
While I hope to never have anything fail, I think I covered my bases if something bad does happen.
A worst case might even be the Mosfet H bridge in the inverter going dead short. So the inverter is not saving you. Then the BMS Mosfets also fail shorted, no help there. The fuse strikes a plasma arc, because you used a 12 volt Mega Fuse. The breaker welds shut because it was only rated for AC. Oops.
Protecting a high current DC system is not trivial. I have a 100 amp CNL fuse (125 volt rated) on each of my 4 battery strings, and then a Class T 225 amp fuse on the main run from the battery bank to the inverter. And I will soon, also have a 125 amp 400 volt DC rated breaker on each pair of battery strings.
While I hope to never have anything fail, I think I covered my bases if something bad does happen.
I use a 200 amp 400 volt DC breaker, thou I don’t think it’ll be the first thing to ever trip, but it at least won’t have a destructive arc show within when it gets used.
Yea I've tried to explain that to people before. People think without a fuse, without a breaker, the short protection of a BMS will save them...
Somehow software is going to save them from that type of failure? hahahahah
A BMS can save most of your stuff though, by dying before everything else does, which is what I'd expect. At least it's only a $80-$200 failure
I haven't seen anyone really thinking that here though, it's more random people on youtube / reddit that I've seen absolutely swear by the bms short circuit protection being enough
Steve Schofro
New Member
Very informative subject matter. Safety is my concern with any system or device. If a thermal event does occur, these are the gases released. Just a FYI for the forum. LFP is safer and I assume it predominates chemistry in most stored energy. However with the pouch style and lithium-ion chemistry, dramatic and fast-moving incidents occur. No news to most here.Introduction
Marine Rated Battery Fuses (MRBFs) are sometimes used to replace Class T fuses due to supply chain issues, ease of installation, space constraints, and cost. However, the breaking capacity of MRBF fuses is significanly lower than that of Class T fuses as shown in Figure 1.
View attachment 202479
Figure 1. Ampere Interrupting Capacity vs DC Voltage Rating
Problem
How will an MRBF fuse behave when subjected to a dead short outside of the fuse specifications?
Background
If the fuse does not quench the arc, then hot plasma could theoretically turn the fuse into a near-perfect conductor and cause a fire, instead of a near-perfect isolator that prevents fire.
In a previous experiment, an MRBF fuse was blown using an LFP battery, and a relatively small diameter wire was used, resulting in a relatively large circuit resistance, so it may not be a conservative representation of many battery installations.
Goal
The goal is to create a practical, realistic, and relatively safe near-worst-case dead short experimental setup using a relatively large capacity (Ah) battery together with large-diameter and short-length conductors that can be used to observe how MRBF fuses behave when a dead short occurs.
Experimental setup
See Figure 2 for the experimental setup.
View attachment 202470
Figure 2. Experimental setup.
Parts used:
Some of the safety aspects for this experiment:
- MRBF fuse block and MRBF fuse rated 150A (random rating choice).
- Battery: 4 x brand new Victron Smart Lithium 330Ah 12V LFP in series, 48Vnom; never charged or discharged; NO internal BMS; Battery voltage = 52.7V = 3.29Vpc (SoC unknown).
- Battery interconnects ("busbars"/ battery cables): 4/0 AWG UL-listed pure copper. Total cable length ~20" (500mm).
- Tinned copper lugs, hammer crimped on solid concrete substrate (one sample cut in half after crimping to verify quality which was good). Lugs degreased before mounting, torqued to spec.
- Dead short initiation device: 250A snowmobile winch relay (solenoid, approx 5Ω), triggered remotely with power supply set at 14Vdc.
Theory
- Backup circuit breaking methods in the order of prioritized use:
- Use extension pole tree branch cutter. This cutter was verified to cut like a knife through butter through copper cables. Once cut, the weight of the relay (black box with black tape in Figure 2) will pull down one cut end to create a gap large enough to quench the arc. Locally remove cable insulation to reduce cutting resistance.
- In case method (1) fails: Use a stand-by impact drill set to CCW rotation with socket wrench attached to disconnect interconnect bolt.
- In case methods (1) and (2) fail: Use manual cable cutter.
- Safety goggles; gloves; fire protective clothing.
- Fire extinguisher nearby especially for extinguishing burning insulation after breaking circuit.
- Second person with phone to call for help.
The initial calculated (estimated) theoretical dead short current is approx 14,000A (14kA). This is wel outside the MRBF specs as shown in Figure 1. The instantaneous theoretical dead short power is in the order of 700,000W (700kW), which roughly matches the maximum power of a Tesla Model S Plaid. Should we really do this? Just kidding - Curiosity wins
Results
View attachment 202467
- The fuse successfully quenched the arc in the order of one video frame rate interval, in this case 30fps, so 1/30 = 0.033s. See also Figures 3 and 4.
- The fuse polycarbonate (assumption; probably not acrylic) housing back wall blew out and landed ~10 ft away. See also Figure 5.
- Sparks of molten copper blew out and landed 1-3ft away.
- The relay welded closed (this was expected) and had to be taken apart to break the contact points loose as shown in Figure 6.
Figure 3. This is what the fuse blowing looks like in close-up (most dramatic video frame).
View attachment 202489
Figure 4. Fuse blowing process.
View attachment 202484
Figure 5. The MRBF fuse after blowing. The separation occurred at the plastic weld / glue seam.
View attachment 202493
Figure 6 - Welded relay. Only the right two studs were used.
Conclusions
When operating outside of the MRBF specs, conservatively expect at least the housing to fail and molten copper and plastic debris to fly around. This experiment is anecdotal only and needs to be repeated many times to show repeatability.
Recommendations
Ideas and criticism, preferably constructive with references when making claims, are welcome.
- Repeat this experiment to evaluate repeatability.
- Vary the fuse current rating: A 300A MRBF fuse could yield different results.
- vary the battery SoC.
- Etc.
