If I have Class T Fuses and a Large Switch do I need a DC Battery Bank Breaker?

[MrM1]

I am installing 32 280Ah EVE cells in a 16S2P configuration. I have 2 225 Class T fuses on each battery bank wired to the parallel basbar with 2/0 wire. From the busbar I am running 4/0 to the inverter. I will be installing a Marinco Dual Bank Control Switch with a precharge resistor. The inverter is the big 12kw 240v split phase Growatt that has an on/off switch in it too.

Questions is, with the Class T fuses and the Switch, Do I Need a large 300 amp DC breaker? Seems the breaker would be redundant to the Fuses and Switch.

 

[BentleyJ]

Yes, the breaker is redundant. You only need 1 type of properly sized Over Current Protection Device.
I generally prefer a breaker because it serves both purposes OCPD and a means of disconnect. Fewer components and fewer connections with a breaker.

 

[MrM1]

That's what I thought, but just wanted to be sure. Thanks

 

[chess-equality]

I will be installing a Marinco Dual Bank Control Switch with a precharge resistor.

Does it come with the precharge already installed, or separate?

 

[Samsonite801]

I am installing 32 280Ah EVE cells in a 16S2P configuration. I have 2 225 Class T fuses on each battery bank wired to the parallel basbar with 2/0 wire. From the busbar I am running 4/0 to the inverter. I will be installing a Marinco Dual Bank Control Switch with a precharge resistor. The inverter is the big 12kw 240v split phase Growatt that has an on/off switch in it too.

Questions is, with the Class T fuses and the Switch, Do I Need a large 300 amp DC breaker? Seems the breaker would be redundant to the Fuses and Switch.

I'd rather prefer to do it the other way around. Have a breaker on each battery pack (for each battery to disconnect independently, break it out of the battery array to service a single battery pack at a time), and then put the Class-T fuse(s) on the other side of the bus bars, one going to each inverter, to protect that wire run.

 

[MrM1]

Does it come with the precharge already installed, or separate?

Separate. I'll be installing a resistor in the 2 position switch.

 

[MrM1]

I'd rather prefer to do it the other way around. Have a breaker on each battery pack (for each battery to disconnect independently, break it out of the battery array to service a single battery pack at a time), and then put the Class-T fuse(s) on the other side of the bus bars, one going to each inverter, to protect that wire run.

Great idea and that works well. But I can also shut down each bank via the BMS. Not ideal, but it works.

 

[JoeHam]

I'd rather prefer to do it the other way around. Have a breaker on each battery pack (for each battery to disconnect independently, break it out of the battery array to service a single battery pack at a time), and then put the Class-T fuse(s) on the other side of the bus bars, one going to each inverter, to protect that wire run.

Honest question. What good is the additional breaker (fuse) after the busbar?

Trying to imagine a scenario where even a dead short, post busbar, wouldn’t pop the individual battery breakers.

 

[Quattrohead]

Great idea and that works well. But I can also shut down each bank via the BMS. Not ideal, but it works.

But what if the BMS is having a mental breakdown or has shorted MOSFETs ? Then a mechanical breaker is great for disconnecting the battery safely.
Fuses are 150yr old technology and the larger ones discussed here are only for "oh shit" moments.

 

[acdoctor]

If the sum of the batteries OCPD exceeds the inverter OCPD rating.
An example is 6 of the rack batteries with 100 breaker each and a 5kw inverter.

 

[chrisski]

Trying to imagine a scenario where even a dead short, post busbar, wouldn’t pop the individual battery breakers.

If you fuse the line into a busbar from a battery and then each individual line from the busbar or the same thing from the electric utility feed to the circuit breaker box, why would you not fuse the line from individual batteries to where they are combined?

I understand what you're saying about not popping the individual breaker. If the individual battery pack can run the entire system if all other battery packs fail. For me the question would be, is that to code and if not can you accept the risk? Not just for your build but for when this is sold. I have problems with guests and family members exceeding limits, never mind a future sale.

I only have two batteries 8S and I fused each battery individually and after combined. It was a pain both because of the space it took up and the expense. Although not a "best practice" I bet most installations here do not have the additional overcurrent protection after the device.

 

[MrM1]

I only have two batteries 8S and I fused each battery individually and after combined. It was a pain both because of the space it took up and the expense. Although not a "best practice" I bet most installations here do not have the additional overcurrent protection after the device.

For my house system, my 3 8s batteries are each fused between the busbar and battery at the battery box. Then 8 ft away and thru a wall via conduct I have a DC breaker for next to the inverter with a push button resistor bypass. For start up.

This 48v system is going in a cargo trailer and will be built for mobile use and our off grid farm. I have BMSs and fuses between the batteries and parallel busbar. On one side of the trailer, and the inverter and switch on the other side of the trailer.

But what if the BMS is having a mental breakdown

??? Yeah that could happen. I will be able to disconnect via the switch

 

[Samsonite801]

Honest question. What good is the additional breaker (fuse) after the busbar?

Trying to imagine a scenario where even a dead short, post busbar, wouldn’t pop the individual battery breakers.

It's about protecting all the cable runs from an overcurrent situation. The wires from the bus bar to the inverters are one size, and the wires from the batteries to the bus bars are one size. If each battery has a fuse/breaker to protect its wire-run from battery to bus bar, that wire is protected from overcurrent, but then say, if you had a short occur on the inverter to bus bar run, it would have to draw the full sum of all parallel breakers on each battery pack (added up together) in order to blow just the first one (which then leads to cascading trips on all the rest of the battery breakers), meanwhile the inverter-to-bus bar wire melts and starts on fire. That's why it's important to fuse or breaker on both runs (on each side of the bus bar), whenever you have parallel inverters or parallel batteries.

If you only had the batteries fused but not the inverters, you'd have to have a complete circuit wire size that could handle the current it would take to trip all the parallel battery breakers on the inverter runs (the more batteries in parallel, can lead to an intense amount of current potential), and then the problem becomes that most inverters won't even accept a wire that large.

 

[Skypower]

But what if the BMS is having a mental breakdown or has shorted MOSFETs ? Then a mechanical breaker is great for disconnecting the battery safely.
Fuses are 150yr old technology and the larger ones discussed here are only for "oh shit" moments.

The T class in the fast or ultra fast option will let go one heck of a lot faster than a common high quality breaker. Yes they make supper fast electronic sensing breakers, but they get pricey. I was until recently, “I got a breaker/switch and it’ll work crowd” till I seen how long a breaker can be over the rated current till something finally happens. And going to a lesser breaker to trip earlier gets warm and is voltage looser.

 

[John Frum]

On a dead short the class-t fuse will blow in milli-seconds.
A FET based BMS will typically trigger in micro-seconds.
That makes the fuse the first runner up in this beauty contest.

 

[RCinFLA]

Circuit breakers generally have two methods for breaking connection. One due to thermal heating of a bi-metal strip latch that sets breaker current rating and one due to instant current flow for high short circuit current levels.

The short circuit current trip is an electro-magnet latch trip with the solenoid coil in the series path of breaker.

There are a few relatively small gauge, short brad wires spot welded between the functional sections of the breaker to provide flexible connection to the moving contact breaker. The inline coil for the solenoid high current instant trip is a few turns of relatively small gauge wire.

All this adds up to significant series resistance.

A typical 200-amp DC breaker will have 4 to 5 milliohms of series resistance. At 100 amps that is 0.4v drop across breaker and >40 watts of breaker heating. For a 48v system, this will likely be close to a third to half the total series resistance from battery to inverter and the most significant single component of total path series resistance. For a 12v high wattage inverter it is a significant voltage drop.

This is not intended to advocate to use or not use a breaker, just pointing out its side effects. Should preferably use a fuse for high current 12v systems.

 

[JoeHam]

If you only had the batteries fused but not the inverters, you'd have to have a complete circuit wire size that could handle the current it would take to trip all the parallel battery breakers on the inverter runs (the more batteries in parallel, can lead to an intense amount of current potential), and then the problem becomes that most inverters won't even accept a wire that large.

Thanks for the complete and thorough answer.

In my particular case the wire from the busbar to my Schneider inverter can handle more than the sum current that my 2 batteries can put out. Each of those batteries have individual appropriately sized breakers.

Should I add more batteries I will have to add a class t fuse from the busbar to the inverter.

 

[wheisenburg]

Thanks for the complete and thorough answer.

In my particular case the wire from the busbar to my Schneider inverter can handle more than the sum current that my 2 batteries can put out. Each of those batteries have individual appropriately sized breakers.

Should I add more batteries I will have to add a class t fuse from the busbar to the inverter.

I have a dual Schneider system with three batteries. When designing it I was planning to add three more batteries. I'm still planning on that when I get my solar credit refund. Part of my reason for adding the fuse was the possibility of shorting something in the DC bus in the PDP or even the cabling. The PDP has a 250 amp DC breaker for each inverter. I have dual 4/0 cables between a battery combiner bus and the PDP DC bus.

I have a 400 amp "Oh, shit" fuse on each 4/0 cable. In theory these should never blow, but the inverters combined can draw up to 500 amps. If one of the 4/0 cables had a bad connection, the other cable could then be carrying the full 500 amp load. So the protection is not just for shorts. It could conceivably protect from other malfunctions.

You are correct though that the breakers in the battery should limit the current to 100 amps per battery. The things is that if something goes wrong and even one battery breaker fails, you have a big problem. Fuses just seem more reliable and fool proof.

For me a dual inverter system is way over kill. The only reason I went that route is that I have around 9000 watts of AC coupled PV. To properly handle all this AC coupled power I really need two inverters and 6 batteries for grid down operation.