Purposes of fuses on solar panel array

[chadmichael]

I understand that fuses protect wires themselves from currents higher than they can carry. This prevents a possible over heating of the conductor which could cause a fire. I also understand that a fuse can be thought of as protecting a device from burning up or being damaged. Mostly, I have thought about this in the context of circuits attached to the battery. As I understand it, my 340A/12v LIFEPO array could discharge a lot of current if a circuit attached to it were to undergo a short circuit. This makes sense to me.

With regard to the panel array, my reading suggests that they can't discharge all that much power. My battery specs show "Short Circuit Voltage(Isc): 6.10 A". Since I have four 100 W panels, configured in two serial strings of two, I calculate the total maximum short circuit current to be 12.20 A. My solar panel connector wires, running between my panels and my charge controller, are 10 feet of 8 AWG. This wire can easily carry the 12.20 A of a short circuit scenario.

So, what would I be protecting with a fuse in this case?

The battery isn't involved in this equation is it? I mean, it's on the other side of the charge controller and there is a fuse on the wire between the controller and the battery array. I guess I've been assuming the charge controller prevent current from traveling upstream from battery side of controller to panel side of controller.

 

[Brucey]

I believe up to two parallel strings doesn't require fuses/breakers. Once you get to three strings or more, you need the protection for a short from another string

 

[timselectric]

OCP on solar circuits is only required when the available current exceeds the rating of the panels.
This usually happens when 3 or more panels/strings are in parallel.
For example:
Let's say that the panels/strings are rated for 15a. And have a maximum output of 12a.
When three are placed in parallel. A short in one won't be rated to handle the available 24a from the other two panels/strings.
This can cause the one shorted panel/string to catch fire.

 

[Robert Mate]

OK ... now how about the fuse between the scc & battery?

_What conditions would cause fuse between scc & battery to blow?

 

[chadmichael]

OCP on solar circuits is only required when the available current exceeds the rating of the panels.
This usually happens when 3 or more panels/strings are in parallel.
For example:
Let's say that the panels/strings are rated for 15a. And have a maximum output of 12a.
When three are placed in parallel. A short in one won't be rated to handle the available 24a from the other two panels/strings.
This can cause the one shorted panel/string to catch fire.

For clarification to my question about what the fuse would be protecting ... you are suggesting that a fuse might be need to protect the panels from a short in other panels? This is one of the scenarios I was reading about. My math says my array can generate 12.2 short circuit amps, but how do I know what the rating of the panels is? I suppose it's about the ampacity of the wires on the panel itself; they are smaller guage than my extension wires. The panels have a specification "Maximum Series Fuse Rating: 10A" ... is that the panel capacity? If so, sounds like I need a fuse.

 

[timselectric]

OK ... now how about the fuse between the scc & battery?

_What conditions would cause fuse between scc & battery to blow?

No fuse is required at that location.
But if overpaneling, it's a good idea. In case there is a short inside of the SCC.

Correction:
I misread the question.
This fuse is rated by the conductor size used.

 

[timselectric]

but how do I know what the rating of the panels is?

It's on the panel specs.
Maximum fuse rating.

 

[timselectric]

The panels have a specification "Maximum Series Fuse Rating: 10A" ... is that the panel capacity?

Yes

If so, sounds like I need a fuse.

Maybe
It depends on how the array is configured.
Is 12.2a the full array short circuit current? Or is 12.2a available to a shorted string.
Note:
The shorted string is not calculated in this.

 

[chadmichael]

OK ... now how about the fuse between the scc & battery?

_What conditions would cause fuse between scc & battery to blow?

The wire between battery and SCC is straight forward to me. Your charger needs to push a certain amount of amps to the battery; I have a 40 amp controller for instance. I need a wire that can carry 40 amps and I need a fuse to protect against over current. The obvious over current here is a short that pulls from the battery. The battery could obviously deliver a ton of current, but your wire only need to carry the 40 amps, and your fuse needs to prevent more than that; probs a 50 amp fuse.

Unlike a battery bank, the panels don't have a well of power to dump into the circuit because they rely on real time conversion of sun to power.

Note: I'm not an expert.

 

[timselectric]

No fuse is required at that location.
But if overpaneling, it's a good idea. In case there is a short inside of the SCC.

Correction:
I misread the question.
This fuse is rated by the conductor size used.

 

[chadmichael]

Yes

Maybe
It depends on how the array is configured.
Is 12.2a the full array short circuit current? Or is 12.2a available to a shorted string.
Note:
The shorted string is not calculated in this.

Configuration as I understand it... I'll start by listing the values for one panel. I have four identical panels, configured as two series strings that are then wired in parallel.

A Panel​

100 W
Optimum Operating Current: 5.7A
Short Circuit Voltage: 6.10A (I don't know why the specification lists this as voltage, but gives the value in amps, but that's what it says)
Maximum Series Fuse Rating: 10A

Series String​

Two of the above together, will double my voltage, but leave current unchanged, so the following still apply. Note that voltage has doubled but it's not important to this math as I understand it.

Optimum Operating Current: 5.7A
Short Circuit Voltage: 6.10A (I don't know why the specification lists this as voltage, but gives the value in amps, but that's what it says)
Maximum Series Fuse Rating: 10A

Parallelize the Two Strings​

Joining the strings together produces doubled Amps, so downstream of the branch connectors, i.e. the cable to the charge controller, will have:

Optimum Operating Current: 11.4 A
Short Circuit Voltage: 12.20A

A Picture​

In this pic, the green box shows that at Point A, the series will be producing those values. Point B is after the two strings are in parallel, and that's the cable to the SCC.


QUESTION

My question is whether a short in one of the panels causes 6.10 A of short circuit current to pass through the cable at Point A, or if it includes the 6.10 A from the other series, thus, at 12.2 A, exceeding my 10 A Maximum Series Fuse Rating value.

 

[LakeHouse]

My question is whether a short in one of the panels causes 6.10 A of short circuit current to pass through the cable at Point A, or if it includes the 6.10 A from the other series, thus, at 12.2 A, exceeding my 10 A Maximum Series Fuse Rating value.

6.1A max at point A in a short condition. Regardless of which of the four panels fails. The string with the failed panel doesn’t contribute to the short current, it all come from the intact string.

 

[chadmichael]

6.1A max at point A in a short condition. Regardless of which of the four panels fails. The string with the failed panel doesn’t contribute to the short current, it all come from the intact string.

This is because a series string with a short negates the current from all batteries in the string?

 

[timselectric]

With only two strings, no fuses needed.
A short in one string can only draw the current of one other string.

 

[timselectric]

When a string of panels develops a short.
It becomes a load instead of a source.
And since the load is a short, with very low resistance. It will draw all available current.
When one of two strings are shorted. There is only one available source left.