MRBF vs resettable breaker

[bborzell]

Wondering about the factors at play in deciding to install a MRBF that blows when needed rather than a resettable breaker on the positive post of (in this case) my #1 lithium battery.

ETI installed the MRBF. This is a stock photo of the MRBF, not necessarily how mine is rated.

[Centex]

Originally Posted by bborzell

.... ETI installed the MRBF....

^That^ mention caught my eye. I'm just curious, was the MRBF installed by ETI as a 'standard' item or was that a customization you requested?

I ask because one never knows about ETI's always-evolving build / component details.

[tdf-texas]

Originally Posted by bborzell

Wondering about the factors at play in deciding to install a MRBF that blows when needed rather than a resettable breaker on the positive post of (in this case) my #1 lithium battery.

If a storage battery (lead acid, lithium, etc) shorts positive to negative, the current flow can melt the wiring and/or catch fire to surrounding material. A fuse installed at the positive terminal would blow open stopping the current flow and prevent this from happening. Except.... if the fuse is not rated to interrupt the current that battery can produce, the fuse material can vaporize inside the fuse, coating the inside of the fuse with metal, and the current would continue to flow.

Ok, long story over now... A MRBF fuse is designed to have a high interrupt current capacity. A resettable breaker would just melt and the current would continue to flow.

So MRBF fuse on lithium battery terminal - GREAT! Resettable breaker - BAD IDEA!

and the fuse is required by code.
https://www.rvia.org/standards-regulations

[bborzell]

Originally Posted by Centex

^That^ mention caught my eye. I'm just curious, was the MRBF installed by ETI as a 'standard' item or was that a customization you requested?

I ask because one never knows about ETI's always-evolving build / component details.

Good question. I had had several conversations with our config person about whether there would be a positive post fuse on the lithium battery. She finally got back to me with a response from the tech guy who said they had been discussion using a fuse at that location and that, yes, I would get a fuse.

I did not ask for a MRBF specifically, but that is what came on it (75 amp/58 volts).

[bborzell]

Originally Posted by tdf-texas

If a storage battery (lead acid, lithium, etc) shorts positive to negative, the current flow can melt the wiring and/or catch fire to surrounding material. A fuse installed at the positive terminal would blow open stopping the current flow and prevent this from happening. Except.... if the fuse is not rated to interrupt the current that battery can produce, the fuse material can vaporize inside the fuse, coating the inside of the fuse with metal, and the current would continue to flow.

Ok, long story over now... A MRBF fuse is designed to have a high interrupt current capacity. A resettable breaker would just melt and the current would continue to flow.

So MRBF fuse on lithium battery terminal - GREAT! Resettable breaker - BAD IDEA!

and the fuse is required by code.
https://www.rvia.org/standards-regulations

Good to know. Perhaps ETI is upping their game with the lithium/solar installs.

[hugh]

Originally Posted by tdf-texas

Ok, long story over now... A MRBF fuse is designed to have a high interrupt current capacity. A resettable breaker would just melt and the current would continue to flow.

So MRBF fuse on lithium battery terminal - GREAT! Resettable breaker - BAD IDEA!

and the fuse is required by code.
https://www.rvia.org/standards-regulations

That's true for the cheap breakers you find on Amazon without an interrupt capacity specified. But there are good quality DC breakers that match or exceed MRBF's 10,000 amp interrupt capacity.



That said, even 10,000 amp interrupt capacity is considered marginal for short protection on a lithium battery systems, so YMMV.

[Centex]

Originally Posted by hugh

That's true for the cheap breakers you find on Amazon without an interrupt capacity specified. But there are good quality DC breakers that match or exceed MRBF's 10,000 amp interrupt capacity.

That said, even 10,000 amp interrupt capacity is considered marginal for short protection on a lithium battery systems, so YMMV.

Not at all to argue, just genuinely curious and seeking education ...

Can you (or anyone) help me understand what's considered an appropriate interrupt capacity for the main protective device on a given lithium array (how to determine that value)?

Can you provide a link to resettable DC breakers in the 50A-250A range with an interrupt rating @~14VDC approaching the 10k Amps of an MRBF?

My search of all of the (IMO reputable, YMMV) Blue Sea DC resettable breaker series' yields nothing over 7.5k Amps for that specification, and most series' are rated much lower than that.

There is of course the Class T fuse alternative for this application, most rated for ~20k Amps DC interrupt and having a trip curve very similar to MRBF's.

But the 'packaging' of a Class T fuse/holder is not as compact as the MRBF and doesn't lend as readily to mounting directly on the battery terminal before any cable is involved. It seems to me that mounting the main protective device very intimate to the battery is advantageous, but I've wondered if the difference between 10k and 20k Amps interrupt capacity rating is meaningful for our situation.

(I'm viewing this protective device as primarily for the purpose of personal protection against the worst-case and hopefully very rare accidental 'shorted to a tool' contact which might occur in the RV environment; perhaps I'm missing a more important mode of failure / short?)

[tdf-texas]

Originally Posted by Centex

Not at all to argue, just genuinely curious and seeking education ...

Can you (or anyone) help me understand what's considered an appropriate interrupt capacity for the main protective device on a given lithium array (how to determine that value)?

Can you provide a link to resettable DC breakers in the 50A-250A range with an interrupt rating @~14VDC approaching the 10k Amps of an MRBF?

My search of all of the (IMO reputable, YMMV) Blue Sea DC resettable breaker series' yields nothing over 7.5k Amps for that specification, and most series' are rated much lower than that.

There is of course the Class T fuse alternative for this application, most rated for ~20k Amps DC interrupt and having a trip curve very similar to MRBF's.

But the 'packaging' of a Class T fuse/holder is not as compact as the MRBF and doesn't lend as readily to mounting directly on the battery terminal before any cable is involved. It seems to me that mounting the main protective device very intimate to the battery is advantageous, but I've wondered if the difference between 10k and 20k Amps interrupt capacity rating is meaningful for our situation.

(I'm viewing this protective device as primarily for the purpose of battery personal protection against the worst-case and hopefully very rare accidental 'shorted to a tool' contact which might occur in the RV environment; perhaps I'm missing a more important mode of failure / short?)

Lead acid RV battery bank systems should have an interrupt capacity of 5000 Amps interrupt capacity minimum. 10k amps is usually what is recommended for lead acid battery banks and is the application for which MRBF fuses are designed. For Lithium battery systems the highest AIC available is usually required. Class-T fuses have an AIC of 20,000 amps.

To answer your question as to what interrupt rating is needed requires some electrical engineering calcs on the exact battery installation being evaluated. Since vendors of lithium batteries won't have that information, they take a worst case synario and recommend (require) a class T fuse install.

That Escape is installing MRBF terminal fuses is a step better than they have done before (no fuse) but still not recommended for lithium battery installs. I wouldn't bet that they did the EE calcs to determine that a MRBF fuse was adequate but maybe they did.

Oh, most lithium battery manufacturers now include a BMS inside each battery that should limit short circuit current on a fault. I would still install a class T fuse to be safe.

https://www.bluesea.com/support/arti...t_Installation

[tdf-texas]

Originally Posted by Centex

Can you provide a link to resettable DC breakers in the 50A-250A range with an interrupt rating @~14VDC approaching the 10k Amps of an MRBF?

My search of all of the (IMO reputable, YMMV) Blue Sea DC resettable breaker series' yields nothing over 7.5k Amps for that specification, and most series' are rated much lower than that.

A number of breakers are rated at 10k interrupt - but at AC voltages, not DC. when using a AC device (fuse,breaker,etc), it must be derated if used in DC service as DC current is more difficult to interrupt.

https://www.electricaltechnology.org...c-circuit.html

The only breakers I have found that are 10k rated are solar service breakers.
https://www.midnitesolar.com/product...tName=Breakers

I used one of these on my trailer for the battery to solar controller wiring - but I have lead acid batteries not lithium. 10K is fine for that application.

[Centex]

Originally Posted by tdf-texas

.... For Lithium battery systems the highest AIC available is usually required. Class-T fuses have an AIC of 20,000 amps.
.....
Oh, most lithium battery manufacturers now include a BMS inside each battery that should limit short circuit current on a fault. I would still install a class T fuse to be safe.
.....

Thank you sir for both posts, educational as always!

Building a system essentially from scratch, my approach to protection devices has been as follows - described here in hope you may offer critical comments /suggested revision as appropriate:

1. determine the amp load for a circuit
2. size the circuit's wire to cover the amp load and voltage loss - whichever rules
3. size the circuit's protection (fuse or breaker) to the lower of the two amps above

I'm relying on the Lithium BMS and a Class T fuse at the battery before all other distribution to address worst case AIC. My BestGo 400Ah battery BMS has an ultimate limit max discharge of 650A @ 0.5 seconds.

I'm using the rated short-duration peak load presented by my inverter to size the 'quick-blow' Class T fuse (20k AIC); that's 250A in my case. There's 6" of 3/0 Class M UL/CSA cable between the battery terminal and the Class T fuse bolted directly to the (+) battery buss.

The battery master switch is rated at 300A continuous / 900A 30-seconds. That's on the negative side between the battery and the shunt (so MUST be "ON" whenever underway).

For branch circuits off the battery buss that require switching I'm using (Blue Sea) breakers for combined protection and switching; those range from 15A-200A with a slower trip curve and a range of AIC's, all below 10k Amps, some much below that.

I only have one circuit off the battery buss that's un-switched (breakaway brake), for that one the circuit protection is a 30A Type 1 auto-resetting breaker.

All my branch circuits off of subpanels are fused (30A down to 2A).

So, insofar as circuit, device, and personal protection, everything steps-down (including AIC) as one moves farther from the battery. At the battery where high AIC seems critical, the protection (fuse) is separate from the switching to meet that AIC goal.

I'd appreciate your thoughts on where I am 'in trouble' or could 'do better'?

[tdf-texas]

Originally Posted by Centex

I'd appreciate your thoughts on where I am 'in trouble' or could 'do better'?

You've done everything to code or better.

There is only one thing I would check. Asuming that the lithium batteries are not enclosed, the only way you are going to get a catastrophic arc flash is if something fell directly across your battery terminals. If this is the case, maybe some terminal caps or covers over the battery terminals?

You should be proud of what you have done with your battery system.

[Centex]

Thanks, almost all of this based on the many posts here by you and other helpful experienced folks, I'm relieved to have apparently applied the lessons learned appropriately.

I'm still wrapping-up the installation, lots of tweaks to the 'final plan' during the fitting of things in the trailer, I'll post the final as-built schematic and pics in my mods thread when fully complete and through shakedown.

Yep, those terminals are covered!

[hugh]

Alan, looks like everyone else answered most of your questions.


In regards to DC breakers with relatively large OC and AIC ratings, there are a number of options, although most of them are either DIN rail or panel mount. You might have some luck looking for "UL 489A" breakers as those are specifically designed for DC circuit protection for telecom applications, so they are temperature stable, and offer a number of sensitivity options.

Just an an example, Sensata has a breakers certified for 80V DC with 10k-50k AIC, and OC rating up to 125A:

https://www.sensata.com/sites/defaul...-datasheet.pdf


That said, for battery protection, unless someone has a hobby of juggling crowbars over battery banks, it would seem that fuses are more cost effective.

[hugh]

Hmmm, looks like Amazon has a few cheap DIN rail breakers as well.


60VDC, 100A, 10k AIC:

https://www.amazon.com/dp/B089TNNRS6

[hugh]

Originally Posted by tdf-texas

Lead acid RV battery bank systems
Oh, most lithium battery manufacturers now include a BMS inside each battery that should limit short circuit current on a fault. I would still install a class T fuse to be safe.

Yeah, I haven't found a BMS spec sheet that lists an AIC rating for short circuit protection. For certain, many of the the mechanical contact BMS systems have fairly low AIC's, as the relays just aren't designed to connect / disconnect under high load. For the MOSFET BMS systems, as MOSFET's fail conductive, I'd be wary to trust them as well.