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Fuse or Dc isolator?

Ginja003

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Joined
Jan 30, 2023
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7
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Middelburg, South Africa
Hi everyone.

I need some help with fuses and dc isolators. I have a 3kw hybrid inveter and 2 x 12v 200ah agm batteries, I'm trying to find some information on how to size the fuses and dc isolators i need? This is probably a basic calculation and very simple? But i really just want to make sure i use the right one for my application.

And lastly can I use a dc isolator instead of a fuse? Or should I use both?
 
First need to know if your system is 12V (two batteries in parallel) or 24V (two batteries in series.)
After that the math for current draw is the same.

Fuses ought to be used at the battery to protect wires against short circuit.
They need to be able to interrupt battery short-circuit current at battery pack voltage. I don't know the exact short-circuit capability of your batteries but would guess somewhere between 4000A and 8000A per battery. There are some fuses rated for 32V and 6000 AIC, probably suitable for 24V pack. If a 12V pack, could use one of those fuses per battery.

A single MRBF may be good for either 12V or 24V configuration:


MIDI can only handle the current at 12V


Mega may be similar to Midi, but I don't see a higher current for 12V


ANL would be good for 24V


ANL and MIDI, I would suggest separate fuse for each 12V battery in parallel. Unlike BRBF which is good for 10,000 AIC at 12V


"DC Isolator" (switch) is convenient as a disconnect, could be used in addition to fuse. Maybe not needed so long as inverter can be turned off.

Beware of cheap switches and breakers that can't carry full current.
 
Thanks very much for the response, my inverter is a 24v system, sorry forgot to mention that. I posted below a fuse I have been looking at, would this be any good? I see the fuse options you have suggested we're 200amp to 300amp about, this 1 I posted is a 200amp. So my question would be what is the equation for sizing the Fuze? So my 24v battery bank is 200ah so a 200 Amp fuse would be OK right? And if I had a 24v battery bank that was 100ah I would need a 100 Amp fuse correct?

I have added an attachment for my battery data sheet if that helps?

And with regards to the dc isolators I was more asking in terms of maybe a cheaper way as a fuse, you need to replace but a fuse, but a dc isolator just trips (open circuts). And if that is the case, does the dame formula apply? 200amp circuit breaker?

Check this out on takealot: MEGA-fuse 200A/32V (1pc)
 

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You should never have to replace a fuse. It is only for a fault, a short circuit.
It needs to be sized for maximum operating load, with margin.
We usually recommend class T fuses or similar for the best protection, but your system is lower voltage and has limited fault current so cheaper options should be good enough.

3000W / 20V low battery / 90% efficient x 1.25 margin x 1.12 ripple factor = 233A recommended fuse

Round up to 250A

For 90 degree wire in a bundle, 4/0 awg


90 degree single wires in free air, 1/0 awg


Probably use 2/0, maybe 105 degree battery cable. Keep wires short.

From Blue Sea link for Mega fuse,
Interrupt Capacity
information.png
2000A @ 32V DC
Your 200 AH AGM battery can put out at least 4000A into a short. Well, 3300A: "Short Circuit Current 3300A"
Consider one of the others.

There are some quality breakers that will work, but they are in the $150 range.
Best not to use the cheap flat lever breakers, which are thermal only (no fast trip)

Make sure you don't over-discharge battery. But more difficult to distinguish < 50% SoC at high current vs. low current.
Detailed battery specs provided. Program the inverter/charger with those and use temperature sensor.
AGM should give long life if properly charged and not over-discharged. They do have a limited number of cycles, e.g. 700 cycles to 50% DoD vs. 1500 cycles to 30% DoD, so best to enable loads when battery is charged and surplus PV is available.
As a hybrid, the inverter should be able to maintain optimum charge rate while delivering power from PV to loads. (only some component systems can do that.)
 
I am seeing that the Go Power T fuse is not UL listed - is that correct? Also the above blue sea. Where can I buy UL listed T fuse 200amp? Having a difficult time tracking that down.
 
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