What do our experts think of this of this?

[fishplug]

I'm sure you could add extras at the bus bar. They also do fuse holders which will hold more than one fuse - which would probably be the way to go.

However, I don't see any advantage to using MRBFs further down the circuit as the reason for having one is to stop a failing battery from cooking stuff downstream. You only need one fuse to do that.

Further down the line you'd be running circuits through fuse boxes with standard blade fuses that would protect the remaining cabling.

Caveat emptor: I know nothing about inverters. They may well bring different considerations into the mix.

 

[fishplug]

The low cost versions of those are notorious, versions that do work to their spec, and less likely to open when there's a bit of vibration cost a lot more.

Well, I've just been out in the van planning the electrical routes, cable sizes etc and putting together my shopping list and....

...my terminal post bolts on the new battery are a whopping M10. Cross checked with the Blue Sea Systems holders and while they have an mate bolt, the mounting hole will attach to an M10 terminal. Phew!

.....Fogstar, however, appear to sell a fuse holder with an mate hole as the recommended option for their battery with an M10 bolt.....

I'll definitely not be buying the fuses from them then!

 

[growly_bear]

Is it advisable or even if possible to incorporate the MRBF fuses at the Bussbars for each circuit away from there?
Mike.

That's exactly what I did. I was tight for space and they were a good solution to that. Hidden bonus for what we are discussing here.

 

[EuroTrotters]

This has all become very confusing, even for a mildly, nerdy trainee geek !

SO.....

I have FogStar 105 Lithium, soon to be 210 (2 x 105) and Everything Victron . That is Solar Charger, B2B and Inverter, with mega fuses !.

Do I need Class T Fuses or some kind of DC RCD?

I have lost track on this thread.

 

[Gromett]

This has all become very confusing, even for a mildly, nerdy trainee geek !

SO.....

I have FogStar 105 Lithium, soon to be 210 (2 x 105) and Everything Victron . That is Solar Charger, B2B and Inverter, with mega fuses !.

Do I need Class T Fuses or some kind of DC RCD?

I have lost track on this thread.

I personally would only use a Mega perhaps on each one but using a bus bar rather than a cabled interlink.

Others may have a different opinion. A 100Ah battery will only give a short circuit current of 1000A so well within the 2000A limit of a mega fuse.

BUT. I would buy Littel fuse as cheaper ones are not reliably rated.

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[Ray-Vic]

This has all become very confusing, even for a mildly, nerdy trainee geek !

SO.....

I have FogStar 105 Lithium, soon to be 210 (2 x 105) and Everything Victron . That is Solar Charger, B2B and Inverter, with mega fuses !.

Do I need Class T Fuses or some kind of DC RCD?

I have lost track on this thread.

Mega fuses are fine with your battery size, but you should fuse each battery separately as close to the post as possible with its own fuse rather than a single fuse on the paralleled output of the batteries.

As noted by Gromett I also believe bus bars are the way to go and is what I have done and all works lovely.

 

[Gromett]

Trying to find a real test on youtube of the current that flows when a direct short circuit is applied.

This was interesting as it showed the BMS cutting off prior to the current ramping up to stupid levels?

Cutoff time is 200-500µs

 

[Gromett]

An interesting experiment.

 

[Gromett]

I finally found the source of the data for that video I saw a long time ago. Page 5. I am glad I was able to find it as I was starting to doubt my conclusion.

I have attached the file rather than giving a link as some may not be able to access the link possibly?

This is where I got my 1,000A for a 100Ah battery and my thoughts on 10x the capacity for short circuit current.
As you can see this was a short circuit of a 160AH battery which produced almost 1100Amps of short circuit current for 200 Seconds.

This is why I am comfortable with a Littelfuse MegaFuse rated at 2000A when used on up to 300Ah 12V batteries.
BUT, I would use one on each battery if adding multiple batteries connecting to a bus bar.

 

[Gromett]

Oh and the 8Ah test on the next page. 80A short circuit current for 125 seconds aprox.

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[Ray-Vic]

Trying to find a real test on youtube of the current that flows when a direct short circuit is applied.

This was interesting as it showed the BMS cutting off prior to the current ramping up to stupid levels?

Cutoff time is 200-500µs

I think the problem I see with the signal these kind of videos relay, is that some are led to believe that they can rely on the BMS to provide all the protection for a short circuit scenario. Will Prowse has done some videos where he had BMS's fail to operate at all as the settings were far too high from the factory. I would also be concerned that should a BMS fail, this may not necessarily fail safe, whereas that is unlikely with the correct fuse (albeit not impossible, granted).

Personally I would just completely ignore the BMS as far as short circuit protection is concerned and fit correctly rated fuses of the proper type.

 

[Raul]

An interesting experiment.

That test proves nothing, Meter can't read the short amps, and the cable is grossly undersized for a max amp test. The resistance is too high.
All he proved the fuse blown in time for the cable to survive. Thats NOT the point for max short interruption capacity of the fuse.
Just a thought.

 

[Ray-Vic]

I think as a rule of thumb it is generally accepted that LiFePO4 batteries can supply up to 10x their rated capacity under a short circuit condition and, in some cases, quite a bit more. Using this as a guide a single 300Ah battery could be well over 3,000A, so way above the ICC of a Mega fuse.

The MRBF's are not hugely costly when looking at holder and fuse against a Mega fuse and offer protection of 10kA @14v, so well within range.

 

[Ray-Vic]

That test proves nothing, Meter can't read the short amps, and the cable is grossly undersized for a max amp test. The resistance is too high.
All he proved the fuse blown in time for the cable to survive. Thats NOT the point for max short interruption capacity of the fuse.
Just a thought.

I assumed that was the purpose of the video, to show how quickly MRBF blows? If he ran that test with the circuit loaded to 300A for 20 minutes, he could have lit a cigarette off that cable!

 

[Gromett]

That test proves nothing, Meter can't read the short amps, and the cable is grossly undersized for a max amp test. The resistance is too high.
All he proved the fuse blown in time for the cable to survive. Thats NOT the point for max short interruption capacity of the fuse.
Just a thought.

I think he was just demonstrating the suitability of the MRBF fuse? but good point about cable.

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[Gromett]

I think the problem I see with the signal these kind of videos relay, is that some are led to believe that they can rely on the BMS to provide all the protection for a short circuit scenario. Will Prowse has done some videos where he had BMS's fail to operate at all as the settings were far too high from the factory. I would also be concerned that should a BMS fail, this may not necessarily fail safe, whereas that is unlikely with the correct fuse (albeit not impossible, granted).

Personally I would just completely ignore the BMS as far as short circuit protection is concerned and fit correctly rated fuses of the proper type.

Completely agree. I was not suggesting that the BMS should be relied on, just that they are an added level of protection. I like multiple levels of protection

 

[Gromett]

I think as a rule of thumb it is generally accepted that LiFePO4 batteries can supply up to 10x their rated capacity under a short circuit condition and, in some cases, quite a bit more. Using this as a guide a single 300Ah battery could be well over 3,000A, so way above the ICC of a Mega fuse.

The MRBF's are not hugely costly when looking at holder and fuse against a Mega fuse and offer protection of 10kA @14v, so well within range.

Good point. The cost of the MRBF over a mega fuse is not excessive. I think I was more thinking that for vans and 12V batteries going for a T class in most cases is not necessary as per the original video.

My thoughts were a 160Ah battery giving <1100A of short circuit current doubled would be 2200A for a 320Ah which looking at the comparison with the 8Ah would mean <2000A for a 300Ah battery.
But to be honest with only a small increase in cost of an MRBF I would go with that. The T Class are very expensive and not necessary for most 12V van leisure batteries.

I would still avoid the ebay/amazon BOJACK specials though. Even from an expensive supplier like RS the LittelFuse MRBF's are under £12 (just the fuse).
example: https://uk.rs-online.com/web/p/car-fuses/3377993

I think the thread is fantastic. Really helped clarify a lot of things for me and refreshed my memory from last time I looked at this.

I am going to stop using Mega fuses and move over to MRBF.

Thanks All

 

[Raul]

I assumed that was the purpose of the video, to show how quickly MRBF blows? If he ran that test with the circuit loaded to 300A for 20 minutes, he could have lit a cigarette off that cable!

In order to test for short circuit current:
1- you have a meter capable to read it
2- you have a very low Zs for the circuit you test, to allow the door wide open( min resistance) for the current to flood out.
Otherwise is just a overload test, and as expected it will blow.

Now swop the 8awg to a 50mm2 and get a meter capable reading 6000A, and repeat, that would give a a better answer, what kind of amps the fuse was capable to interrupt.

 

[Ray-Vic]

In order to test for short circuit current:
1- you have a meter capable to read it
2- you have a very low Zs for the circuit you test, to allow the door wide open( min resistance) for the current to flood out.
Otherwise is just a overload test, and as expected it will blow.

Now swop the 8awg to a 50mm2 and get a meter capable reading 6000A, and repeat, that would give a a better answer, what kind of amps the fuse was capable to interrupt.

Couldn't agree more.

I don't think the content maker was intending to demonstrate the ICC of the fuse, just that it would blow quickly in an overcurrent situation which he created. I think the underrated cable was just to demonstrate the fuse blowing before even this cable overheated, even under 'short circuit' conditions. Obviously I don't know for sure but that's how it came across to me.

I agree, a full ICC test should be carried out completely differently to this, as they are undoubtedly done when obtaining type certification, but I doubt that was the intent of this video (but I could be wrong of course).

 

[Raul]

Couldn't agree more.

I don't think the content maker was intending to demonstrate the ICC of the fuse, just that it would blow quickly in an overcurrent situation which he created. I think the underrated cable was just to demonstrate the fuse blowing before even this cable overheated, even under 'short circuit' conditions. Obviously I don't know for sure but that's how it came across to me.

I agree, a full ICC test should be carried out completely differently to this, as they are undoubtedly done when obtaining type certification, but I doubt that was the intent of this video (but I could be wrong of course).

100% agree

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[Raul]

Just one clarification just to be sure we are talking from the same text book;
ICC is the fuse rating capability, so to chose the right one you still need to find the PSCC from supply. In our case the battery, this is what we chasing, finding out what the source is capable, aka max Prospective Sort Circuit Current. Once you know this, then you can make an informed decision about the protection fuse rating to cover that PSCC value or exceed it.

 

[autorouter]

The 300A fuse is suggested for the 200A/250A BMS (I think mine has a 300A BMS as it's rated to that as a max for both charging and discharging current?)

The 150A fuse is suggested for the 100A BMS.

They do also warn in red letters on both: "Please ensure cables are appropriately sized. Otherwise, an additional fuse may be required for cable protection."

To take an extreme example, suppose you were using a 300Ah LiFePO4 battery to power a 50W street light, and nothing else. 50W is about 4A, so you could decide to wire it with say 10A cable. A 300A fuse would not prevent a 10A cable from glowing red hot and starting a fire if the wires were shorted at the lamp. So in that case a 10A fuse would be appropriate.

If the battery cable went to a busbar, and was thick enough to carry 300A, then a 300A MRBF fuse would be required at the battery in case of a short of the thick wire to negative. You would then take a 10A cable from the busbar, and fuse the 10A cable at the busbar with its own 10A fuse, to protect that 10A cable.

 

[Gromett]

To take an extreme example, suppose you were using a 300Ah LiFePO4 battery to power a 50W street light, and nothing else. 50W is about 4A, so you could decide to wire it with say 10A cable. A 300A fuse would not prevent a 10A cable from glowing red hot and starting a fire if the wires were shorted at the lamp. So in that case a 10A fuse would be appropriate.

If the battery cable went to a busbar, and was thick enough to carry 300A, then a 300A MRBF fuse would be required at the battery in case of a short of the thick wire to negative. You would then take a 10A cable from the busbar, and fuse the 10A cable at the busbar with its own 10A fuse, to protect that 10A cable.

In that case a standard 10A fuse (blade or glass tube type) would not necessarily protect against a FULL short? It may arc and sustain the connection? You still need a fuse capable of cleanly breaking the arc at the 2-3000A capability of the battery.

 

[Raul]

In that case a standard 10A fuse (blade or glass tube type) would not necessarily protect against a FULL short? It may arc and sustain the connection? You still need a fuse capable of cleanly breaking the arc at the 2-3000A capability of the battery.

Not necessarily Gromett, a ATO fuse will be fine on a 10A circuit.
Here is why, each circuit will have different Zs values, in relation to their size. A 10A circuit with 2.5mm2 will have a higher Zs ( resistance ) compared to the main distribution 50mm2 circuit.
That 2.5mm2 circuit, or even 1.5mm2 will be man enough to carry enough fault current to blow a ATO fuse. The ATO fuse can break 1kA, and you will not have a large fault current on small wires.
The fault current will not be the same in the 1.5-2.5mm2 circuit, compared to the 50mm2 distribution.
So, downstream on smaller circuits it is ok to fuse with conventional fuses.
I hope that makes sense.

 

[BouncyBoatFolk]

If it takes a pig a week to walk a fortnight, how many apples in a bunch of bananas???
Mike.

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