80VDC 10A fuse?

Whoops. I meant "4 panels in parallel" not series. Sorry, I was typing faster than I was thinking. Everything else still holds true despite I typed the wrong word.

OK, in this case, an unrated switch is not ideal. You'll get a slight arc any time you open up the switch. I believe every switch will arc to some extent. Arcing is not good. Arcing leads to contact corrosion which results in higher contact resistance and lower solar current. But in reality, you're probably not cycling the switch enough to increase contact resistance. Looking at the other extreme, most people need to replace the battery combiner solenoid because arcing destroys the contacts, resulting in very high contact resistance.

In your case, I don't believe the switch will maintain an arc for more than a fraction of a second.

Reminds me of work I did a long time ago to solve a gun design flaw. A machine gun firing mechanism is activated by a 20A relay. In burst mode, the relay contacts gets so hot from arcing that they melt together. All of a sudden the gun goes from burst mode to continuous fire. The fix was to add a capacitor, diode and resistor network across the relay contacts. As the contacts open, current now goes through the capacitor and there's no arcing.

Sal

smkettner wrote:
If panels are putting out max 100v & 8a at the time I flip the switch the arc may not extinguish as it has no real DC rating.

Almot wrote:
Fire is bad. You go fishing, come back and your home is gone :) ... It's better to follow the rules, especially when it doesn't cost much.

Good news is - in my particular case, when I look into the "handbook" by Cooper-Bussmann that BFL13 linked on the 1st page of this thread, I see that rules - at least in the interpretation of Mr. Cooper and pals - don't require any fusing.
On the page 5 it says that if Isc of parallel array does not exceed the conductor/string Isc calculated as 1.56*(PV Isc), no string fusing is necessary. In my case the parallel array Isc does not exceed the string Isc, because there is no parallel array - only one string of 2 panels.

This.

Fire is bad. You go fishing, come back and your home is gone :) ... It's better to follow the rules, especially when it doesn't cost much.

Good news is - in my particular case, when I look into the "handbook" by Cooper-Bussmann that BFL13 linked on the 1st page of this thread, I see that rules - at least in the interpretation of Mr. Cooper and pals - don't require any fusing.
On the page 5 it says that if Isc of parallel array does not exceed the conductor/string Isc calculated as 1.56*(PV Isc), no string fusing is necessary. In my case the parallel array Isc does not exceed the string Isc, because there is no parallel array - only one string of 2 panels.

KD4UPL wrote:

"When the fuse blows there will no longer be a short. The panel will return to Voc. If the gap left in the blown fuse isn't large enough the Voc may arc the gap."

In air at atmospheric pressure it is impossible for an arc to start at less than 327V.

KD4UPL wrote:

Let's say you have 4 panels in series at 8 amps each.

Amps in series don't add so you still have 8 amps total. A panel is designed to be short circuited (Isc). I can see a fuse for each parallel STRING but not panels in series. Besides the NEC says you don't need fuses until you have 3 or more parallel strings.

Salvo wrote:
Why?

Don't let the open circuit voltage fool you. If there's a short in the system, panel voltage drops to ZERO. A 12V fuse will do just fine.

Sal

smkettner wrote:
I think I will be updating my switch to Square D H221N with RK5 10 amp fuses.
Switch rated 250VDC, fuse rated 125VDC.

If panels are putting out max 100v & 8a at the time I flip the switch the arc may not extinguish as it has no real DC rating. I suspect usually the current will be much lower or I could disconnect the battery first or cover a panel if it became an issue.

The replacements are over twice the physical size with just empty space in the box so I am not excited about changing it out. Will be contemplating this for a bit.

No real concern about the 120vac 10a fuses.

Hi KD4UPL,

I always thought that volts in series add and amps in series do NOT add. Are solar panels some sort of "special case"?

KD4UPL wrote:
Let's say you have 4 panels in series at 8 amps each. If one panel shorts out the other 3 panels will try to push 24 amps thru it. This isn't good and will burn out the diodes, traces, cells, etc.
With only 1 string as I believe the OP has there is a much lesser risk.
It's possible the panel could short as well as the charge controller, this would sent battery current directly thru a shorted panel. Very far fetched I realize but it's happened. There have been fires caused by shorted solar panels on buildings. That's why article 690 of the NEC goes into great detail about where fuses should be placed. I realize the NEC doesn't really apply to DIY RV wiring but it's still a great resource and guide for proper system design.

Before we get any further, how on earth will a fuse connected to series string of panels blow? It's impossible!

Sal

KD4UPL wrote:

Salvo wrote:
Why?

Don't let the open circuit voltage fool you. If there's a short in the system, panel voltage drops to ZERO. A 12V fuse will do just fine.

Sal

When the fuse blows there will no longer be a short. The panel will return to Voc. If the gap left in the blown fuse isn't large enough the Voc may arc the gap.
Fuses have voltage ratings for a reason.

Salvo wrote:
Under what circumstances is the panel at risk? I see none.

Sal

KD4UPL wrote:
A fuse in line with your solar panels is to protect the panel, not the wire.

Let's say you have 4 panels in series at 8 amps each. If one panel shorts out the other 3 panels will try to push 24 amps thru it. This isn't good and will burn out the diodes, traces, cells, etc.
With only 1 string as I believe the OP has there is a much lesser risk.
It's possible the panel could short as well as the charge controller, this would sent battery current directly thru a shorted panel. Very far fetched I realize but it's happened. There have been fires caused by shorted solar panels on buildings. That's why article 690 of the NEC goes into great detail about where fuses should be placed. I realize the NEC doesn't really apply to DIY RV wiring but it's still a great resource and guide for proper system design.