Fuses for individual solar panels?

[GrantRobertson]

On Jack Danmayer's website, he recommends putting a fuse or breaker on each individual solar panel. I assume this is so, if a panel shorts internally, the other panels don't pump all their current through said short and cause overheating. Now, Jack recommends these huge breaker boxes that look like they are the size of a small loaf of bread. And they are not cheap. I'm looking for a smaller, less expensive solution.

First, I assume the fuses need to be rated above the maximum output current of that one panel, but lower than the average output of two other panels. Let's just assume that comes out to be 10 amps, for purposes of discussion.

Second, I assume all fuses are designed to have the least voltage drop while still accomplishing their function. Do some types of fuses have lower a voltage drop than others?

Third, is it not true that fuses are designed for a particular current AND voltage, thus depending on the power (watts = volts x amps) to heat the fusible link enough to melt it.

Fourth, once one has determined the necessary voltage and current rating for the fuse, does it matter what style (form factor) of fuse one uses?

Finally, let's say I have fuses available that are designed for 12 volt systems (cars). If my solar panels put out more than 12v, as most do, let's say 15v (for easy math), can I just use a fuse rated for a higher current to arrive at the same power level at which the fuse will blow? In other words, my 10 amp, 15 volt panel would be outputting 150 watts. If you divide that 150 watts by the 12 volt rating of the fuse, you arrive at >12.5 amps for the desired rating of said 12v fuse. Will this be an acceptably safe alternative?

 

[Optimistic Paranoid]

First of all, the common fuses used in cars are rated for up to 32 volts dc.

Secondly, are you trying to run your panels in series or parallel?

I've got 140 watt panels that I plan to run in parallel. Max power output is only 7.91 amps - short circuit amps are 8.68. Since I can only get fuses of either 7.5 or 10 amps, I don't see how I can make it work. The 7.5 amp fuses will be blowing all the time and the 10 amp fuses will never blow. And 8.68 amps isn't enough to overload the wire and cause a fire, so I don't see why I would need fuses, anyway. Battery powered circuits need fuses cause batteries can put out damn huge amounts of amps in a short circuit. Solar panels can't.

If you're going to go series, then someone like Sternwake is gonna have to jump in here and enlighten both of us.

Regards
John

 

[GrantRobertson]

@OP,

You are thinking of only one solar panel, all by itself. That is rarely the case. The fuses I am talking about would be on one of the wires from each panel, before they were all combined. Assume parallel connections. Now, if one panel shorts INTERNALLY, all the other panels are still connected to it and all their current will be dumped through that internal short. Lets say you have 6 panels, at 8 amps each but one of them shorts. That will mean the remaining 5 panels will be dumping a total of 40 amps through that one shorted panel. This will A) cause heating of the damaged panel and B) drop the current to the batteries to almost zero.

So, a 10 amp fuse would not blow under normal operation but it would quickly blow when those 40 amps started going through it. This would effectively remove the shorted panel from the circuit.

Now, all this is moot if you only have 2 panels. In that case, if one shorts, then the other would dump its current through said short but said current would not be enough to blow either fuse. So my question only applies for three or more panels in parallel.

 

[highdesertranger]

interesting I don't have an answer for you but I want to see what others say. I don't run a fuse on my panels. highdesertranger

 

[Spaceman Spiff]

On Jack Danmayer's website, he recommends putting a fuse or breaker on each individual solar panel. I assume this is so, if a panel shorts internally, the other panels don't pump all their current through said short and cause overheating.

All current solar panels that I am aware of have diodes (electrical one way valves) in the junction box to guard against this.

Second, I assume all fuses are designed to have the least voltage drop while still accomplishing their function. Do some types of fuses have lower a voltage drop than others?

Yes:  http://www.pbase.com/mainecruising/fuse_voltage_drop

Fourth, once one has determined the necessary voltage and current rating for the fuse, does it matter what style (form factor) of fuse one uses?

Yes:  http://www.pbase.com/mainecruising/battery_fusing

Finally, let's say I have fuses available that are designed for 12 volt systems (cars). If my solar panels put out more than 12v, as most do, let's say 15v (for easy math), can I just use a fuse rated for a higher current to arrive at the same power level at which the fuse will blow?

Not sure, but I wouldn't bet my system on this.

If one panel drops out (shorts, open, no output) in a parallel system, the rest of the panels feed your charge controller uninterrupted.
If one panel drops out in a series system, the whole system goes down.

In my system (200W solar, 208 AH battery), I have a 60A ANL fuse going into the charge controller and a 60A fuse off the positive terminal of my battery for 4 AWG wire.

-- Spiff

 

[GrantRobertson]

All current solar panels that I am aware of have diodes (electrical one way valves) in the junction box to guard against this.

Jacks site is a bit "rambley" and had been updated in bits and pieces over the years. He did mention protective diodes at one part of the site, without fully describing how they function within the panel. So, it is entirely possible that his recommendation of using the fuses/breakers was based on older designs of panels and was never taken out. That is the problem with trying to figure out things from poorly-written, rambling web sites.


If one panel drops out (shorts, open, no output) in a parallel system, the rest of the panels feed your charge controller uninterrupted.

... unless the panel were shorted "outside" of the protection diode. In other words, if the wires coming out of the panel short, then the protection diode cannot help. It only prevents current from flowing backwards through the panel itself. Though, I admit, I was specifically talking about INTERNAL shorts in my original post.


If one panel drops out in a series system, the whole system goes down.

Well, only if every panel in the system is connected in one long series circuit, which is highly unlikely.

 

[GrantRobertson]

Third, is it not true that fuses are designed for a particular current AND voltage, thus depending on the power (watts = volts x amps) to heat the fusible link enough to melt it.

Finally, let's say I have fuses available that are designed for 12 volt systems (cars). If my solar panels put out more than 12v, as most do, let's say 15v (for easy math), can I just use a fuse rated for a higher current to arrive at the same power level at which the fuse will blow? In other words, my 10 amp, 15 volt panel would be outputting 150 watts. If you divide that 150 watts by the 12 volt rating of the fuse, you arrive at >12.5 amps for the desired rating of said 12v fuse. Will this be an acceptably safe alternative?

OK, I have done some more research and have found some information from LittleFuse, a major manufacturer of fuses. They have a .PDF file at http://www.littelfuse.com/~/media/e.../littelfuse_fuseology_selection_guide.pdf.pdf (yes, .pdf appears twice in the URL). In it they state:

To summarize, a fuse may be used at any voltage that is less than its voltage rating without detriment to its fusing characteristics.

So, my above assumption about it being the power through the fuse that causes the melting was incorrect. If I had thought through the Ohms and Power laws more carefully, I wouldn't have made that mistake. It has been a very long time since I was an electronics technician in the USMC.

Apparently, voltage only comes into play during the brief instant after the fuse blows and a small arc forms. If the voltage is too high it will cause a large arc and cause the fuse to explode. So the voltage rating only applies to how resistant to exploding the fuse is when it blows...

So if, as Optimistic Paranoid states, all car fuses are rated at 32 volts, then a basic car fuse - inline with each solar panel - could be used to protect against the situation where the leads from a solar panel short together. It would prevent those leads from overheating due to over-current. Remember, shorting of those lead wires would be like laying a screwdriver across the output of your entire solar array. Considering that the maximum current rating of the 10 gauge wire (usually supplied with panels) is only 15 amps, and two panels at 8 amps each is 16 amps total; It is possible for a short between the lead wires from a panel to cause overheating in said lead wires, even if the larger wires they all connect to are safe.

Though my original question is moot because my original assumptions were wrong, it does turn out that these fuses could help in a very specific situation. Of course, now one has to decide for oneself how likely it may be for these two lead wires to short together. If they are run in such a way that they are likely to never short, then these fuses would be unnecessary. There is no point in going overboard on fuses.

 

[Spaceman Spiff]

[font=Verdana, Arial, sans-serif]If one panel drops out (shorts, open, no output) in a parallel system, the rest of the panels feed your charge controller uninterrupted.
[/font][font=Verdana, Arial, sans-serif]... unless the panel were shorted "outside" of the protection diode. In other words, if the wires coming out of the panel short, then the protection diode cannot help. It only prevents current from flowing backwards through the panel itself. Though, I admit, I was specifically talking about INTERNAL shorts in my original post.[/font]

[font=Verdana, Arial, sans-serif]You are correct.  If the wiring downstream of the panels shorts, you loose all output (to the short), but you do not damage the panels.  And if my hastily scratched schematic is correct no fuses will blow, no matter where in the circuit you put them.[/font]

[font=Verdana, Arial, sans-serif]If one panel drops out in a series system, the whole system goes down.

[/font][font=Verdana, Arial, sans-serif]Well, only if every panel in the system is connected in one long series circuit, which is highly unlikely.[/font]

Just a semantics misunderstanding.  You loose output from all the panels connected in series.

-- Spiff

 

[Spaceman Spiff]

Remember, shorting of those lead wires would be like laying a screwdriver across the output of your entire solar array (that is how you measure Isc - by connecting the leads togather and measuring current). Considering that the maximum current rating of the 10 gauge wire (usually supplied with panels) is only 15 amps (20' of AWG 10 wire capacity by AYBC standards is 60 amps), and two panels at 8 amps each is 16 amps total;
Those same two panels connected in series will have 16 amps of current flowing through the 10 ga wires (and 32 amps for 4 panels in series).

--Spiff - tired and going to bed.

 

[GrantRobertson]

Spiff,

So maybe I read the chart wrong. I'll have to double check. If you are right, which I'm guessing you are, then the fuses would be totally unnecessary unless one had over eight 8 amp panels in parallel.

 

[Optimistic Paranoid]

Spiff,

So maybe I read the chart wrong. I'll have to double check. If you are right, which I'm guessing you are, then the fuses would be totally unnecessary unless one had over eight 8 amp panels in parallel.

So far, I haven't seen anything in this thread that invalidates the other point I made in my post:  since you can only buy 7.5 amp and 10 amp fuses, on an 8 amp panel, the 7.5 will be blowing all the time and the 10 amp will never blow, so installing fuses would be pointless.

Maybe Jack is one of those guys who's using those big residential panels with a much higher individual output?

Regards
Joihn

 

[ccbreder]

Here is an explanation by Windy Nation. They at least are in the solar business.

https://www.windynation.com/jzv/inf/how-properly-fuse-solar-pv-system

 

[Optimistic Paranoid]

Damn.  I learn something new all the time here.  Thanks.

Regards
John

 

[blars]

Any design is a trade-off, but adding fuses on individual panels adds more potential points of failure and voltage loss to protect against a very unlikely shorted panel. In a larger array, it might make sense to fuse each group of a dozen panels or so.

 

[GrantRobertson]

Any design is a trade-off, but adding fuses on individual panels adds more potential points of failure and voltage loss to protect against a very unlikely shorted panel. In a larger array, it might make sense to fuse each group of a dozen panels or so.

Thanks. I'll just assume Jack Danmayer is going for a bit of expensive overkill. I mean, he bought a semi tractor to pull his 5th-wheel trailer.

 

[MikeRuth]

Blars makes a great point, multiple points of possible problems and voltage drop.

I will say given a single panel rated to 7 or 8 amps max out put I'd be will to bet the cost of a box of 7.5 amp fuses they wood not blow under normal operating conditions. It would be so rare to see that full output.

Like others that have posted I have no fuses in my panel side. I do have one cut off switch near the CC.
Typically were using pretty stout wire from CC to panel(s). Unless you can really foresee somewhere other than the CC shorting I seriously would have my doubts about a dead short.

In my case only thing I worry about is some  A-hole hitting me right in the side where the two GC's are~! There fused but that could get real interesting fast~!

 

[akrvbob]

There is some misunderstanding here about amps and volts in Series.

When you wire in series, you double the volts and you half the amps. So two 8 amp, 18 volt panels wired in series is now 36 volt at 8 amps. That's why you can use a smaller wire, it has less amps.

Where did my amps go?!!   :huh:  I want them back!!   

This is why panels in series require a MPPT controller, the controller will get back your amps. It will step down the voltage to whatever your battery can take at that moment and increase the amps to the maximum the panels can put out, in this case 16 amps.

Here's proof.

You take two 6 volt, 210 ah golf carts and wire in them in series. You now have one 12 volt battery, but it does not have 420 ah, it only has 210 ah, half of 420. 

When you double the volts, you half the amps. 
Bob