What are we fusing for? Really

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kibirad asked Feb 26, '24

What are we fusing for? Really

Hi,

currently, I'm thinking about fuses in the Victron system and I'm still unable to find the answer. The question is about fusing 5kVA Multiplus and Bluesolar 150/45. I will start with MPPT but the answer is the same.

The stated MPPT has specified fuse rating in a range of 50A to 63A as per documentation. I'm not talking about fuse voltage rating, breaking capability etc. those are obvious things. My question is what type of fuse should be used because I couldn't find anything in official Victron documentation. And to the answer is tied to the question of for what are we really fusing.

The MPPT can produce 45A so the 50A fuse of any kind won't blow. So we aren't fusing the MPPT in a sense of protecting the load (batteries) from overcurrent. The only scenario I expect the MPPT to produce higher current is in a case of severe failure and in that case, there is something wrong with the MPPT. The same can be said about fusing "from the other side". If we are fusing against some kind of short circuit in MPPT there is again something wrong with the MPPT.

Why I'm saying this? Because it will directly lead to the selection of fuse type. If we are fusing only for the short circuit scenarios in case something goes really wrong we can use slower fuses. Basically what I'm getting at is that maybe a type gG fuse is enough for fusing MPPT and we don't need to use faster-acting fuses because the only time they will get blown will be a short circuit scenario, not an overload. The other variant is to use aR or gR fuses intended for fusing semiconductors. Those fuses aren't cheap, especially with multiple MPPTs (at minimum 10x the price of gG), have bigger losses and I was told they age and someday they will most likely just blow because of the derating by the age and use. Blowing the fuse at the output of MPPT under operation isn't really something I want because I want my MPPTs to stay alive. Is there any recommended type of fuse? If I remember correctly the Lynx system uses midi fuses which is a slow-blow type of fuse.

So in short: Is the only case we are fusing for the short circuit scenario? In that case is the slow blow fuse with a lower limiting factor still okay because in that case there is something really wrong with MPPT and the fuse is used mainly to stop setting the MPPT on fire etc. not to protect it because, in a case of internal failure resulting in short circuit, there is already something wrong. Also in Victron manuals, there is only a fuse drawn on positive wires. So does Victron officially recommend using fuses on only one pole?

Multiplus-II MPPT Controllers fuses

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Answer 1 · 2024-02-26T22:34:44Z

Justin Cook answered · Feb 26 at 10:34 PM

Short circuit and over current (and over-temperature in a roundabout way). Have a read through Wiring Unlimited for a discussion on fusing:

https://www.victronenergy.com/upload/documents/The_Wiring_Unlimited_book/43562-Wiring_Unlimited-pdf-en.pdf

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kibirad commented · Feb 27 at 06:22 AM

Generally yes and it is the correct answer, but I was trying to make the statement that the over-current and in that sense, even the overtemperature isn't something we are fusing for in this exact case.

Again, there is no way that 45A MPPT unit will consume more than 50A of current from the battery side under normal operation. Especially not 70A for 50 minutes what is exactly the edge value that the 50A gG fuse blows at. So in a sense, the MPPT isn't the consumer and if it is and can blow the recommended fuse then there is something happening and it is something entirely different than just overcurrent.

On the other side, the MPPT is a producer. That is the normal operation mode. The mentioned MPPT has a maximum charging current of 45A and that is the current under ideal conditions - no temperature derating which is most likely to happen after some time at those charging values. But let's say it can hold this charging current entire day. The minimal recommended fuse is 50A. As I stated above this fuse just starts to "work" at 70A of current and that current does need to flow for 50 minutes. So the MPPT does need to produce 150% of the rated current for almost an hour to blow that fuse. If it does produce as much as 150% rated current I don't think it will stop at that value. If the MPPT produces for some reason so much higher than rated current it will most likely be some kind of fault and it won't stop at 150% of rated current and most likely will just blow any fuse. Not to mention I don't think the MPPT is physically able to produce such current.

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Answer 2 · 2024-02-26T23:08:05Z

Michelle Konzack answered · Feb 26 at 11:08 PM

If use only my 4 Battery Sets (each 735Ah @24V) with each a NH0 Fuse of 250A to protect the Battery for short circuits.

My MultiPlus-II 24/5000 go a 3-phase NH-Lasttrenner size NH1 from ABB with currently 200A each since I have no intention to overload the MultiPlus-II.

Then I have three NH0 Fuses with 2x 200A and 1x 250A and my 12kW Motor got a NH3 with 630A.

Previously I used the ANL fuses, but the 200A ones from my MultiPlus-II got so hot while charging 24hours my Batteries with 3,8kW that the 1,3m long Cable (H07V-K 50mm2) got so hot the first 50cm (Copper is a very nice heat conductor) that the plastic was melting. Now I use H01N2-D 70mm2 Welding cables which work MUCH better.

Switching from ANL to NH was the best thing I could do.

All 4 MPPTs (VE.Can versions) are not fused at all.

Dito for my 3 WindMax2500 and 1 WindMax1000.

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kibirad commented · Feb 27 at 08:21 AM

I'm planning to use NH fuses for the battery as well. The thing that I'm trying to solve is basically what fuse to use based on their speed. In a sense, the gG fuse and gR/aR fuses have really different time current curves.

Of course the faster, the "better" at least if properly chosen. But the chance of prematurely blowing the semiconductor fuse because of its degradation by use on the output side of MPPT isn't worth blowing MPPT by suddenly cutting it from the load (battery). So I'm asking if it is somewhere recommended to use a fast blow fuse because the recommendation of manifacturer stands above the electrical code. At least in my country. And if so... do we really gain something by using the fast fuses over the slower ones? The slower ones are a better option in my mind because of their lower losses, which means less heating and less degradation, the price is really different, availability etc.

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Answer 3 · 2024-02-27T07:31:59Z

delf67 answered · Feb 27 at 07:31 AM

The reason for fuses in most situations it to prevent a fire in the event of a fault. That's why your wiring should be rated at upto (an a bit beyond) the fuse rating. So the fuse blows before the cable melts.

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kibirad commented · Feb 27 at 08:00 AM

Yes, of course. I'm planning to use the cable from MPPT with a current rating of up to 109A so I don't really take that under consideration because under normal conditions there is no chance of overloading the cable. Basically what I'm getting at is if in any document Victron recommends any specific type of fuse in the sense of its speed. And if why. As I stated, recommended fuses won't blow from overcurrent in normal operation, there is the only chance of blowing a fuse under abnormal operation and that will be almost every time short circuit and then there is really not much of a difference in using slow or fast fuse because the semiconductors that should fast fuse protect are in short circuit.

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