Hi Peet, Fuses on the MPPT’s will have very little use, just like fuses for each PV string. The PV string can for argument sake deliver 10 amp, so you need to install a 12 or 15amp fuse. Even if you short the panel, it cant produce more than the rated short circuit current, thus it can never exceed the rating of the fuse and the fuse can theoretically not blow. If however you have more than 2 strings in parallel you can in theory h\have more current flowing into a faulty string and then you want to isolate that string by relying on the fuse to blow, disconnecting the faulty string from the rest.
The same with an MPPT, lets say its a 250/100, its can not produce more than 100amps, not even during a short circuit on its output. Because it needs to run at or very close to the maximum rating of 100 amps you would need to install a fuses of at least 125 amps to prevent nuisance blow outs… If the MPPT senses a short circuit, it will switch off and the current can not by design exceed the rated output current, so the fuse will not blow in normal operating conditions.
I guess the only time a fuse will work is if something in the MPPT goes wrong and causes a short circuit and the current start flowing into the fault (either from the battery or from multiple other mppts connected to the same DC bus). If you have a fuse between the battery and the MPPT that fuse should blow, if not and you are using lead acid banks, I guess you will have some serious issues. With lithiums and BMS protection, you should be okay, although I would not rely on a BMS for protection.
In short, I dont think installing fuses on Victron MPPTs will add any real benefit or protection.
Well, it kinda depends what side you are talking about.
The battery side needs some kind of overcurrent protection, either a breaker, or a fuse. The rule is that the fuse/breaker is there to protect the cable. So generally I tell people to put a fuse/breaker on the output side of the MPPT, sized for the cable.
Regulations require the PV side to have fuses too, and you need to fuse both sides (positive and negative). Jaco is right that in a single string, the PV modules will deliver a maximum of around 10A or so (for the typical 72-cell panels we install nowadays), so you will put a 12A fuse inline. Do remember though that fuses derate when they get hot, so in my own system I had to resort to 16A fuses because they kept blowing every few months. Again, it has to be sized for the cable.
Now on this topic (the PV side): the PV modules can never deliver more than their short circuit current, so under normal conditions, a fuse should never blow, since even a short circuit on the PV side will not exceed the size of the fuse, the normal operating condition of the panel is pretty close to the shofrt-circuit current anyway. But… you also want to protect against abnormal conditions. Imagine the MPPT is damaged and it allows battery current to flow backwards through a dead short somewhere on that cabling… you want the cable to be protected.
Furthermore, if you have multiple parallel strings on a single MPPT, a fault in one string could channel current from the other parallel strings into the faulty one. This usually only becomes a problem once you have 3 strings, but again, for multiple parallel strings you definitely want to fuse the strings individually before combining them.
Long story short: The fuse/breaker is there to protect the cable. Always put one in, unless you can mathematically prove that the cable is already sufficiently protected by existing overcurrent protection.
You use fuses specifically for high fault current protection. For example you would want it to protect cables from the battery or busbars as the fault current on a battery is huge and a minture circuit breaker is not rated high enough. As was mentioned a PV panel has a tiny short circuit current so a DC circuit breaker will work perfect.
For example the fuses on my battery disconnector are fault current rated 120 000A but the DC breaker I have is only fault current rated 10 000A, even thought they both for 125A protection.
Well, there would be two ways of making a counter-argument. At least the two I can think of.
I only have one string and it cannot possibly deliver more than 10A, which the cable is rated for. So I will not fuse it (despite what the regulations call for).
I only have one string and it is a single circuit, so I will only fuse the positive side (despite what regulations may call for).
The trouble with the first one is a blown MPPT. Imagine the upper MOSFET (if you think of a buck converter) fuses (as MOSFETs do) and you have a short circuit in the PV wiring. The full 100A+ from the batteries can now burn up the PV cabling. Better have a least one fuse sized for the wire in there, right?
The second one (two fuses) has to do with a double-earth fault. You always fuse both sides if the system is floating (ie one side is not earthed) to protect against double earth faults. If earth faults are extremely unlikely (eg small camping system), then maybe you can get away with just one fuse.
The thing is, I have seen regulations where the fuse rating is also prescribed as 125% of Isc.
Caveat: I am not sure if ZA regs prescribe this, but I wouldn’t be surprised if nowadays ZA officials just adopt policies.
Fuses are temperamental. Ratings that are too close to the mark will cause nuisance interruptions to the supply. You can’t have those issues on your roof.
So for practically reasons the strings have to be fused at ground level.
Well the “so what” still means you have nuisance issues, but now your cabling cost has also increased so you can indulge that pleasure. You could have paralleled strings on the roof and brought that single cable down, now you have to bring each string down. Not only has the cost increased but now there is more equipment (cabling) that can go wrong.
So in short, the regulations followed to the letter cause more issues than they address.
A double pole DC MCB at ground level can be practically rated not to be a nuisance, can be a marshalling point, a point of PV isolation, a visible status indicator and afford practical protection to multiple strings in parallel without being sacrificial.
It is a far superior solution, but it won’t conform to regulation.
From test data I have seen: A PV panel will only pass current when reverse biased with a voltage higher than its own Voc. It is apparently a technique that has been used to melt snow off panels in colder climes, and so there are several independent accounts of this.
As parallel strings can only ever attain Voc, that voltage necessary to pass current through a parallel string by other strings will never present. Not to mention that at Voc there is no current available to blow a fuse by definition.
I suppose one could dream up a theoretical scenario, but it would have to have several “if this and if that as wells”.
But as far as offering any practical protection in this scenario, nope a fuse won’t work here either.
The only real fault is sourced from the battery side in simultaneous combination with a PV cable fault. That will be a fairly substantial fault, so absolutely no need for nuisance 125% ratings.
Damaged cells in one of the strings could yield that one “shorter”, which means the other panels now have a higher voltage than that one. This is one dreamed-up solution.
I get the feeling we may be discussing different things though. I think we are all agreed that overcurrent protection is absolutely necessary. It seems we are discussing what kind, where it ought to be installed, and so forth.