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Blowing Fuses in the system

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gblcia

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Nov 14, 2022
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Hi,

Can someone help me to understand following

i got battery 280ah lifepo4. Its recommended to use T-class fuse. Ok - cost about 45 pounds 300A. Now after i use busbar where i want to have midi fuses for SCC, inverter, DC appliences, etc... these midi fuses cost about 5 pounds or more depends on type(blueSea they about 15pounds) My concern is Short circuit. If i short circuit after midi fuse, what will be blown T-class after battery, that would cost a lot if i think about it if this happend accidently or how is this?

And one more question, it mentioned everywhere that you should connect your battery negative to chassis(my case is caravan), why this is so?
 
Anything can happen with a dead short. Try to avoid it. Branch circuits have smaller wires that may limit the dead short amperage and probably save the Class-T.

Could run with isolated ground. Generally the ground is to provide a path back to the battery to power small accessories or provide a path during a failure or fault. The vehicle start battery is frame grounded by the factory for the same reason.
 
gblcia said:
If i short circuit after midi fuse, what will be blown
Click to expand...
The "midi" fuse.
gblcia said:
And one more question, it mentioned everywhere that you should connect your battery negative to chassis(my case is caravan), why this is so?
Click to expand...
For safety.
Like a positive conductor rubbing on part of the frame. And exposing the conductor.
 
timselectric said:
The "midi" fuse.

For safety.
Like a positive conductor rubbing on part of the frame. And exposing the conductor.
Click to expand...
what positive conductor you mean...?if midi blows first, good.

Btw if i got T class its fast reacting. Then what amp should i go for. Say there is inverter 2kw and gets surge so it will be over 200A(battery peak is 400A and 200A continous) -> so what rating should be used so it wont be blown by surge of inverter or combination of all appliences and inverter just to avoid accidental blowing?
 
timselectric said:
Fuse can be sized for 125% of conductor rating.
Conductor should be sized for maximum possible current.


Any positive conductor. (Battery positive)
Click to expand...
okay, but if i get surge its can go over 300A right, there is no cable with this AMPs unless i take 2 4/0 cables and put them together, but that seems bit accessive, anyway i will likely be using only 1kw inverter so it should be fine, with 4/0 cable. so if max Amp of cable is 230A i can go by 230*1.25 = 287A, so i would go for 250A fuse here or if there is 275A i go 275A will check.

Okay i understand concern here about battery positive would get to chassis and can become potentially leathal(can Lifepo4 kill you even if you dont arent properly grounded?). Yet once this happend you would get potentially Short again and T-class would blow(still better lost 40 pounds rather than electric shock though)

Just to understand good connection -> i can clean chasis, but do you put smth inbetween lug and chassis like paste for 100% conductivity and also for rust prevention -> looking for best practice here.
 
gblcia said:
Btw if i got T class its fast reacting. Then what amp should i go for. Say there is inverter 2kw and gets surge so it will be over 200A(battery peak is 400A and 200A continous) -> so what rating should be used so it wont be blown by surge of inverter or combination of all appliences and inverter just to avoid accidental blowing?
Click to expand...
That is a 12v battery right? I recommend a 200 amp Class-T and 2/0 or 4/0 wire. Class-T is very fast with a short but will pass 2x rating for almost 50 seconds. This gives plenty of headroom for surge performance.

ClassT_Fuse.jpg


https://www.bluesea.com/products/5116/Fuse_A3T___Class_T_200_Amp
 
gblcia said:
okay, but if i get surge its can go over 300A right, there is no cable with this AMPs unless i take 2 4/0 cables and put them together, but that seems bit accessive, anyway i will likely be using only 1kw inverter so it should be fine, with 4/0 cable. so if max Amp of cable is 230A i can go by 230*1.25 = 287A, so i would go for 250A fuse here or if there is 275A i go 275A will check.

Okay i understand concern here about battery positive would get to chassis and can become potentially leathal(can Lifepo4 kill you even if you dont arent properly grounded?). Yet once this happend you would get potentially Short again and T-class would blow(still better lost 40 pounds rather than electric shock though)

Just to understand good connection -> i can clean chasis, but do you put smth inbetween lug and chassis like paste for 100% conductivity and also for rust prevention -> looking for best practice here.
Click to expand...
A surge is only for a few seconds.
Size everything for expected continuous draw.
Fuses protect the wire. To avoid a fire.
 
timselectric said:
A surge is only for a few seconds.
Size everything for expected continuous draw.
Fuses protect the wire. To avoid a fire.
Click to expand...
understood. Just one more question why do you put fuse between battery and SCC? i suppose its not about SCC amp rating as this would be limited by max Amp SCC can give, but somehow battery can do it? i can imagine again short after busbar to SCC so basically fuse should be on Busbar end, not SCC end...Right?
 
timselectric said:
A surge is only for a few seconds.
Size everything for expected continuous draw.
Fuses protect the wire. To avoid a fire.
Click to expand...
And is there alternative to T-class fuses price wise? i suppose no as folks would be talk about it...right? just if i should google more or just get T class fuse without further search
 
gblcia said:
understood. Just one more question why do you put fuse between battery and SCC? i suppose its not about SCC amp rating as this would be limited by max Amp SCC can give, but somehow battery can do it? i can imagine again short after busbar to SCC so basically fuse should be on Busbar end, not SCC end...Right?
Click to expand...
Fuses are used to protect wiring.
And yes, close to the larger source. (At bus bar)
 
gblcia said:
And is there alternative to T-class fuses price wise? i suppose no as folks would be talk about it...right? just if i should google more or just get T class fuse without further search
Click to expand...
People recommend a T class fuse because it is fast acting. And a high current disconnecting rating. Other fuses or breakers can do the job. But a second or two later. And could have trouble disconnecting at higher current.
 
timselectric said:
People recommend a T class fuse because it is fast acting. And a high current disconnecting rating. Other fuses or breakers can do the job. But a second or two later. And could have trouble disconnecting at higher current.
Click to expand...
Hm, best would be to mount holder straight to battery i suppose, but holders are m10 and battery terminals m8, then i suppose will need cable connection, BMS should have short protection, i mean if i would place fuse say within 0.5m from battery would that be fine, really have no clue what can happend with cables before fuse, before fuse going to blow....
time2roll said:
That is a 12v battery right? I recommend a 200 amp Class-T and 2/0 or 4/0 wire. Class-T is very fast with a short but will pass 2x rating for almost 50 seconds. This gives plenty of headroom for surge performance.

ClassT_Fuse.jpg


https://www.bluesea.com/products/5116/Fuse_A3T___Class_T_200_Amp
Click to expand...
yeah 12V. thanks for graphs useful
 
timselectric said:
Fuses are used to protect wiring.
And yes, close to the larger source. (At bus bar)
Click to expand...
What can happend if you place fuse say 20cm from Busbar and get overload on wire? i mean what can practically happend and when it is problem i suppose when we deal with high AMP, but how long does take to damage cable placed before fuse blows? say i got busbar and wouldnt put fuses holders straight behind each stud, how big problem this can be or when this can go wrong? trying to understand, this is my first setup and i am limited by time to get all parts on spot, rather i would even buy smth what will work temporarily(but work safely)
 
timselectric said:
Fuse can be sized for 125% of conductor rating.
Conductor should be sized for maximum possible current.


Any positive conductor. (Battery positive)
Click to expand...
about if you size 125% fuse and get say 120% for long time, wont this going towards damaging cable insulation etc.? this looks weird to me, i would be thinking opposite - cable has max Amperage and would be aiming for less than that with fuse...please dispell my myst :)
 
gblcia said:
What can happend if you place fuse say 20cm from Busbar and get overload on wire? i mean what can practically happend and when it is problem i suppose when we deal with high AMP, but how long does take to damage cable placed before fuse blows? say i got busbar and wouldnt put fuses holders straight behind each stud, how big problem this can be or when this can go wrong? trying to understand, this is my first setup and i am limited by time to get all parts on spot, rather i would even buy smth what will work temporarily(but work safely)
Click to expand...
The fuse will protect against overcurrent from anywhere in the circuit. But it will only protect against a short on the opposite side from the source.
 
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