MRBF vs Class T

[Smoobly]

As it currently stands, I have two SOK24v100 batteries, wired in parallel. Each battery's positive terminal has an MRBF, and the combined output goes through a 200A breaker to the positive bus bar.

Lately I've been thinking it would be nice to be able to disconnect each battery independently, in times of trouble-shooting. What I'm thinking is to replace the MRBFs with 100A circuit breakers, which then feed into a class T fuse and then to the bus bar.

Will this work? Should the class T absolutely be the first thing the batteries connect to (other than each other), or will it work just as well with the CBs upstream of it? I know that any surge for which the class T is needed will likely fry the CBs, but that's still better than a fire, so I'm willing to put up with the possibility.

Are there any other gotchas I'm not considering? Any thoughts on the pros and cons of each approach?

Thanks for your consideration

 

[12VoltInstalls]

Should the class T absolutely be the first thing the batteries connect to

That is my opinion.

My strategy is that I believe they will NEVER blow- they are there as an insurance policy that prevents loss rather than pays me a portion if I lose everything.
“Fuses protect the wire” but I could care less about the wires- those are double sized anyway, and the Class T’s are ~130% of the bms output ratings: the wires(cables) are safe, and if something terrible happens to the batteries or cables the ClassT’s should stop a 10,000-Amp rapid discharge.

From the busbars I use an ANL fuse for the inverter, and I have a 30A ATC fuse protecting the 8AWG that feeds a bluesea ATC fusebox.

I have accidentally blown a couple ATC fuses (human error) and running the tablesaw or a welder I’ve blown a few 160A ANL’s…

But ClassT for the batteries always

 

[jcollman]

I'm using both: MRBF on the battery terminal and a 300A Class T for catastrophic protection. You can see a few photos here. By using the MRBF you reduce the length of unfused cabling to zero. The combination of the MRBF and Class T also works well since the MRBF is slower to blow, so if there's a catastrophic event (e.g., a dead short) the Class T with the much higher AIC will blow first without creating an inextinguishable plasma arc in the circuit and burning everything down.

 

[12VoltInstalls]

Hijack but not a hijack; I got thinking about my own situation relevant to this thread…

Each battery's positive terminal has an MRBF

By using the MRBF you reduce the length of unfused cabling to zero. The combination of the MRBF and Class T also works well since the MRBF is slower to blow

So I’m adding another 140Ah Btrpower 12V LiFePo to the two 140’s and one 200Ah I already have. All three are 100A BMS’s and are fused with ClassT’s directly on the battery sockets with copper adaptadoodads I made myself and bolted on. (Wrapped them a thousand rounds of electrical tape to ease delude my mind)

How much ‘slower’ actually are they? Does it matter? I’ve never blown a ClassT
And iirc MRNF are still 10kA safe-open.

Some say they are just as safe as ClassT @Hedges will probably have useful opinions.

I’m thinking I should be using a 125A MRBF bolted right on (with a longer battery bolt of course) at $10 each versus $40+ for the ClassT if smarter people than me think it’s not dumb!

 

[Drizzt321]

Actually Class-T is 20k AIC, so more protection (and better protection) than MRBF. Also faster acting, especially in dead-short scenarios.

https://www.bluesea.com/products/5112/Fuse_A3T___Class_T_110_Amp

I do wish Class-T had a form factor that'd be easier to put right on the terminal...but alas. Maybe someone reliable should start working on that? A good Class-T is holder is basically 2 small bus bars with 2 studs each. Could simply make, on one side, an open bottom with good spacing, plus an appropriately sized hole (maybe M8 & M10 as most common ones?) to bolt through, then the stud for Class-T, then the other bus bar is like it is now, with 2 studs, one for Class-T and other for the wiring heading out. Would increase the height by a fair bit, of course, even more than an MRBF terminal mount.

 

[outboard1196]

I’m thinking I should be using a 125A MRBF bolted right on (with a longer battery bolt of course)

You can't simply bolt a mrbf to the post, it needs an mrbf holder to make sure current passes through the fuse and not the bolt.

 

[Smoobly]

As I understand it, the class T fuse is there because it can handle, and blow on, an outrageous spike in current: 10k-20k amps. [I think a short in the BMS can cause one.] It's necessary because such a spike would arc through most other fuses and breakers, completely bypassing them. Such spikes are highly unlikely, but catastrophic if one occurs. So a class T does what a circuit breaker, or an MRBF cannot.

@12VoltInstalls, you've got the MRBFs upstream of the (multiple?) class T fuse. What happens to an MRBF in that rare instance of a huge current spike? Does the spike just arc through it to the class T, which interrupts it? Could the arc'ed MRBF cause a fire? Would it be safer to put the class T upstream of the MRBF? In that case, would the MRBF even be necessary?

I have an idea how all this works, but I'm short on the nitty-gritty details.

 

[robbob2112]

As I understand it, the class T fuse is there because it can handle, and blow on, an outrageous spike in current: 10k-20k amps. [I think a short in the BMS can cause one.] It's necessary because such a spike would arc through most other fuses and breakers, completely bypassing them. Such spikes are highly unlikely, but catastrophic if one occurs. So a class T does what a circuit breaker, or an MRBF cannot.

@12VoltInstalls, you've got the MRBFs upstream of the (multiple?) class T fuse. What happens to an MRBF in that rare instance of a huge current spike? Does the spike just arc through it to the class T, which interrupts it? Could the arc'ed MRBF cause a fire? Would it be safer to put the class T upstream of the MRBF? In that case, would the MRBF even be necessary?

I have an idea how all this works, but I'm short on the nitty-gritty details.

You have it mostly --- The class T is very fast and will blow on a dead short way faster than a MRBF --- but we are talking maybe 0.5 seconds verse 0.01 seconds --- If your wire is sized proper it will get red hot on the MRBF and faintly warm on the class T.... Does it get hot enough to damage the insulation? Unknown - depends on the size of the wire -- at 125% of expected current maybe -- at 200% of expected current probably not.

If you put fuses in series they can be unpredictable.... if they are the same size - say a class T and MRBF of 150amps - you would expect the class T to blow and the MRBF to not - but that just depends on the rise time of the current spike.... with oversized wired that rise time will be faster so you might blow both.... With right sized wires maybe only the class T blows because the resistance in the wires causes the rise time to be slow down before it hits the MRBF....

So in short, you might blow one or both ...

Now, lets say you think you are smarter than you are and you decide to put a 300amp class T and a 200amp MRBF under the theory the cheaper lower value is going to blow first .... Then you scratch your head when the class T blows and the lower value is still whole..... If you look at the time/current curves of both fuses you can probably figure how much bigger your class T needs to be to protect against a dead short and let the smaller fuse blow .... and it is still a tossup

 

[Smoobly]

If you put fuses in series they can be unpredictable.... if they are the same size - say a class T and MRBF of 150amps - you would expect the class T to blow and the MRBF to not - but that just depends on the rise time of the current spike.... with oversized wired that rise time will be faster so you might blow both.... With right sized wires maybe only the class T blows because the resistance in the wires causes the rise time to be slow down before it hits the MRBF....

So you're saying that if one were to have those two fuses in series, and a dead-short spike occurs, it would be so short in duration that the MRBF might not even notice, while the class T will blow? In this case, I'd think it wouldn't matter which order they were in.

Hmm, interesting. Thanks

 

[robbob2112]

So you're saying that if one were to have those two fuses in series, and a dead-short spike occurs, it would be so short in duration that the MRBF might not even notice, while the class T will blow? In this case, I'd think it wouldn't matter which order they were in.

Hmm, interesting. Thanks

That is a guess - if I had a handful of disposable fuses I could setup the Oscope and current monitors to see what happened in what time...

 

[LakeHouse]

In this case, I'd think it wouldn't matter which order they were in.

It would not matter at all what order they were in, or even where they are in the circuit. The only limitation is that the device has to be part of the circuit... You put them close to the battery to minimize the possibility of creating a short that doesn't pass through the fuse (or breaker).

 

[Smoobly]

It would not matter at all what order they were in, or even where they are in the circuit. The only limitation is that the device has to be part of the circuit... You put them close to the battery to minimize the possibility of creating a short that doesn't pass through the fuse (or breaker).

Good information. Thanks much

 

[12VoltInstalls]

You can't simply bolt a mrbf to the post, it needs an mrbf holder to make sure current passes through the fuse and not the bolt.

Yes, arc length crossed my mind.
They have a ~7/16 hole, and the bolts are 8mm- i was going to sleeve and isolate with a thick abs washer but have thought better of it.
The current ClassT’s are mounted on clean copper pipe flattened, drilled, and bolted through, many rounds of tape over. The MRBF holders would be a bit bulkier but just as simple.
Thank you for your input.

So a class T does what a circuit breaker, or an MRBF cannot

…probably because ClassT has an arc retardant in it I’d imagine

you've got the MRBFs upstream of the (multiple?) class T fuse

Not mine, that was someone else.
I have ClassT right on the battery bolt terminals (with a copper ‘bar to afford clearance)

Would it be safer to put the class T upstream of the MRBF? In that case, would the MRBF even be necessary?

Stacking a ClassT with an MRBF makes no sense to me. If you have a ClassT the problem is solved.

Does it get hot enough to damage the insulation? Unknown - depends on the size of the wire -- at 125% of expected current maybe -- at 200% of expected current probably not

Since I have three batteries parallel with 100A BMS’s, and 125A ClassT’s on each battery terminal, 2/0 from busbar to inverter and 1ga from battery fuse to busbar, and an inverter (3kW) that at most will ‘see’ a 250A load I don’t ever expect to be able blow a 125A ClassT. They are only there for the absolute maximum disaster aversion.
FWIW the inverter cable is fused 160A ANL and I haven’t blown one of those since last summer (using 120V Lincoln welder). I have some 200A ANL’s on hand and a fourth 140A battery on its way, so in “normal” use that would theoretically be 50A max from each battery.
Thank you for your responses.
••••••••
In conclusion, I am going to use ClassT not MRBF. I thought of the arc/flame suppression media in the ClassT in bed this morning and decided superior is actually superior.

 

[Drizzt321]

In conclusion, I am going to use ClassT not MRBF. I thought of the arc/flame suppression media in the ClassT in bed this morning and decided superior is actually superior.

Yup, exactly.

 

[robbob2112]

ANL fuses are something I won't ever use.... they originated in car audio and there are many mnay fakes out there. If they receive a large overcurrent jolt they tend to explode into a firey shower of sparks... say you are hooking up the inverter and slip and the positive and negative short and weld for a second until you can perk a wire free... now the ANL is subjected to minimum of 300amps.... but in that first 0.1 second the impulse of current is a lot higher... the class T will let through 6x current for 0.1 or so... it takes time for the BMS to turn off...

So the ANL could be subjected to 600 to 1000a for an instant... not sure if it would be or not...

 

[12VoltInstalls]

tend to explode into a firey shower of sparks

I haven’t done anything bad other than blow a few 160A ANL fuses. Welding.

I feel ok for this application