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Do I need T class fuse if I have the MCB?

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BartonB

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25.6v - 200A (5kw x 2 parallel, there is a mistake in the specs)

This breaker is probably not DC rated, but every server rack battery has one of those non dc rated stuff

Found this for the breaker

Should I put a 200A T class fuse at the cable going to the inverter?
MRBF 200A at the terminal goingong to the inverter?
150A MRBF at both positive terminals?
(I need a 200A fuse anyway to the inverter)
I prefer MRBF in general, I like how easy they are to use, less connections less resistance less crimping
I don't know :(

IMG_20240416_164816.jpgIMG_20240702_160516.jpgIMG_20240404_165241.jpg
 
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Reason we use t-fuse is against a arc over the cable from the lifepo4 if the fuse burn.
But for 12/24 volt systeem its really over do it .
So i will set a cube fuse (mrbf )on both battery pools on the plus side .
If one break means a full shutdown even if the battery are in serie.
From 48volts and up yes please use t-fuses.


One thing more i forget so it set it now.
Its all about what the max output is from the bms
So if you use multi 12 or 24 volt in parallel.
And the bms are from 1 battery 200a fine a mrbf can be used .
Do you have 100x battery x 200A it go over that mrbf max icc 10.000a 14 volts / 7500a 24 volts / 5000a 32volts .
Than you go for a t-fuce .
So you do have a arc spark and burn your house down

So for small beult a mrbf will do the job just fine.


 
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Those MRBF holders mount to 3/8 battery terminals, I think you have 5/16.

Also the way you have your batteries “cross paralleled” with only one set of leads to your load, it would be ideal to run a fuse per battery then individual leads to a bus bar and loads tapped off the bus bar. That way if one battery fuse operates, you’d still be able to carry load.
 
740GLE said:
Those MRBF holders mount to 3/8 battery terminals, I think you have 5/16.

Also the way you have your batteries “cross paralleled” with only one set of leads to your load, it would be ideal to run a fuse per battery then individual leads to a bus bar and loads tapped off the bus bar. That way if one battery fuse operates, you’d still be able to carry load.
Click to expand...
8mm so its 5/16 of the cube blok its self .
And the m10 what you connect on the battery .
Or a battery car pool model for lead battery .

I have it connect difrent .

In my case the cube blok is direct connect on the battery it self .
Its really easy to do
Only you need this part on the top so the cable can not connect it against the threaded .
So i use a
threaded end cut that what i need and set it on my battery direct.
This way its the same like the hella connect see picture
Than use that helle part and it works the same.. (soliermutter)

See attack files.

 

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MRBF are great for the group style battery posts. They are not typically used on the server rack form factor. With just two you are fine with Z wiring the cables and using the built in breakers as your OCPD. If you add more batteries that may need to be revisited.

I would add a class T rated 20% over the max current pull if the inverter in the line connecting it. You can use a DC breaker here too, but they respond about 100 times slower than a fuse.

MRBF and class T have similar time/current curves but the T has a higher AIC and if there is a short the class T is more likely to contain any sparks.

Post a diagram of what you are doing and we might have more suggestions.
 
robbob2112 said:
MRBF are great for the group style battery posts. They are not typically used on the server rack form factor. With just two you are fine with Z wiring the cables and using the built in breakers as your OCPD. If you add more batteries that may need to be revisited.

I would add a class T rated 20% over the max current pull if the inverter in the line connecting it. You can use a DC breaker here too, but they respond about 100 times slower than a fuse.

MRBF and class T have similar time/current curves but the T has a higher AIC and if there is a short the class T is more likely to contain any sparks.

Post a diagram of what you are doing and we might have more suggestions.
Click to expand...
That's my current plan
Can you review?
Please forgive me for this horrible diagram :(

1721465298842.png
 
You can use drawio to do diagrams.. free download...takes 15 minutes of watching YouTube to learn the basics. Once you attach wires to a box you can rearrange thing to look better.

I can't tell, do you have 2 solar panel strings? One to the MPPT and the other to an inverter? The 25 amps fuses seem high depending on the layout. Somewhere on the panel or in the specs it will tell what size fuse to use. Also if you use fuses you need a dc breaker to disconnect them, opening fuses with current through them will cause sparks.

The fuse for the batteries should be 250amp class T and close to bus bar where the 2/0 cable attaches to the bus bar.

MRBF fuses have a higher AIC than Mega fuses but only slightly.

There should be another class T fuse near the bus bar on the line that connects to the inverter.
 
robbob2112 said:
You can use drawio to do diagrams.. free download...takes 15 minutes of watching YouTube to learn the basics. Once you attach wires to a box you can rearrange thing to look better.

I can't tell, do you have 2 solar panel strings? One to the MPPT and the other to an inverter? The 25 amps fuses seem high depending on the layout. Somewhere on the panel or in the specs it will tell what size fuse to use. Also if you use fuses you need a dc breaker to disconnect them, opening fuses with current through them will cause sparks.

The fuse for the batteries should be 250amp class T and close to bus bar where the 2/0 cable attaches to the bus bar.

MRBF fuses have a higher AIC than Mega fuses but only slightly.

There should be another class T fuse near the bus bar on the line that connects to the inverter.
Click to expand...
1. Yes I have 2 strings, 4 panels 610w + 1 610w
2. Manual says fuse max 30amp but I guess I can go 20amp
3. For disconnect I have this, it will blow the fuse?? solar -> fuse -> disconnect -> inverter
4. To check if I understand - 250A T fuse between pack and cable, 200A T fuse in the fuse block to the inverter?
 
Good disconnect, no trouble.

You can always fuse lower and risk nuisance blows. Technically you don't need to fuse only 2 strings at all. Especially with them connected to two different devices. No chance they can dump into each other.

The class t between battery and bus bar is optional but you are depending on the breakers to do the job. Maybe a MRBF wttached to the bus bar. The other class t is from the bus bar to inverter in case that shorts it won't take anything else with it. Should be near the inverter.
 
robbob2112 said:
Good disconnect, no trouble.

You can always fuse lower and risk nuisance blows. Technically you don't need to fuse only 2 strings at all. Especially with them connected to two different devices. No chance they can dump into each other.

The class t between battery and bus bar is optional but you are depending on the breakers to do the job. Maybe a MRBF wttached to the bus bar. The other class t is from the bus bar to inverter in case that shorts it won't take anything else with it. Should be near the inverter.
Click to expand...
Solar fuse - only way they can dump into each other is if the disconnect device itself has a fault, that's why I wanted to put the fuse above it...will they blow?

The battery itself has overcurrent and short circuit protection in the bms (pace), is it not fast enough?
for the t fuse to the inverter - mrbf won't do the job?, I currently have the MEGA, it's gonna be some work to re do it the inverter is 24V BTW
 
You can't trust a BMS. The MOSFETs typically fail shorted when they fails bad juju.

The MRBF can work, but the AIC isn't as good as the class T and when a MRBF failed from a hard high current short the plastic shell can crack. If you happen to have a battery vent at the same time it can ignite hydrogen. The class T is better at containing any sparks.

So the thing is the panels in series will never generate more than their Isc. Your other panels will generate the Isc. If you had a third string you could get the other two to dump into a single at twice the Isc. This is why fuses aren't required until you have 3 or more strings. You can use them but they will never blow because the panels can't generate more.

One thing to thing about is a SPD to protect against lightning. Won't stop a direct strike, but a nearby strike should be supressed.
 
robbob2112 said:
You can't trust a BMS. The MOSFETs typically fail shorted when they fails bad juju.

The MRBF can work, but the AIC isn't as good as the class T and when a MRBF failed from a hard high current short the plastic shell can crack. If you happen to have a battery vent at the same time it can ignite hydrogen. The class T is better at containing any sparks.

So the thing is the panels in series will never generate more than their Isc. Your other panels will generate the Isc. If you had a third string you could get the other two to dump into a single at twice the Isc. This is why fuses aren't required until you have 3 or more strings. You can use them but they will never blow because the panels can't generate more.

One thing to thing about is a SPD to protect against lightning. Won't stop a direct strike, but a nearby strike should be supressed.
Click to expand...
Thanks I'll take a look at the SPD

Correct me if I'm wrong but I'm thinking - if I put the class t between battery and wire i'll probably be fine between upper busbar to inverter wth mega
 
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that is the rating of max output current, it isn't ever expecting a dead short across it for a length of time. It would try to limit the current to 100 and might not have enough voltage in from the panels to produce it. But with the short it would be trying very hard to make the output connections be battery voltage which it can't.
 
robbob2112 said:
that is the rating of max output current, it isn't ever expecting a dead short across it for a length of time. It would try to limit the current to 100 and might not have enough voltage in from the panels to produce it. But with the short it would be trying very hard to make the output connections be battery voltage which it can't.
Click to expand...
Honestly didn't understand
But you're saying that the 200a fuse will protect everything so that's what i'll do
 
Do the math - if instead of an inverter you have a dead short what happens? Then do the same thing at each point in the circuit. Each positive wire needs a fuse or breaker someplace along the line in case of a short to prevent overcurrent.
 
robbob2112 said:
Do the math - if instead of an inverter you have a dead short what happens? Then do the same thing at each point in the circuit. Each positive wire needs a fuse or breaker someplace along the line in case of a short to prevent overcurrent.
Click to expand...
That I get
I just don't get why a 200a will protect the SSC / orion, I figured they will die before the fuse pop in a short circuit
 
the 200amp fuse will pop because it is sucking all the battery voltage and all other sources through it. Once that cuts off the Orion is protected.

We repeat over and over that fuses are to protect wires and not devices, but sometimes they do both.
 
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