# trying to understand WHY no fuses

Posts: 2,511 admin
In replacing a damaged panel, not having fuse on output of panel surfaced again. Why no fuse? I did a lot of searching and found of course that its not required but still don’t get WHY a fuse on output isnt necessary to protect panel from itself, ie a direct short in output wires before controller.
What exactly happens, how long could a panel sustain a direct short. I've seen pics of an install, the prewired MC cables on individual panels were plugged into each other making a loop. Why does this seem so contrary-going against everything I think I know about electricity- obviously I’m looking at a panel like a battery, which must be wrong.

## Comments

• Banned Posts: 17,615 ✭✭
Re: trying to understand WHY no fuses

Solar panels are a current source, not a Voltage source. Where there's only one panel or two panels in parallel, the most current they can output is their Isc rating (Impedance short circuit). There really is no safety hazard from shorted output, unlike with batteries which produce whatever current as a function of available Voltage right down to zero.

More than two panels in parallel, however, is another story: if one panel should become shorted the other two (or more) can produce 'X' times the Isc and cause fireworks in the shorted panel.

I'm not sure that's very clear.
• Super Moderators, Administrators Posts: 32,008 admin
Re: trying to understand WHY no fuses

Just to be clear what a current source is (most people not in electronics don't have a "feel" for what one is)...
• V=IR; R=V/I; R = delta V / delta I (delta meaning change in voltage or current)
• R = (12 volts no load - 12 volts full load) / (0 amps no load - 100 amps full load) = 0v/100a = zero Ohm resistance
In practical sense, a good deep cycle storage battery has less than 0.010 ohms or so (larger battery, lower resistance).

A voltage source, such as a battery or even your electrical outlet on the wall, will output a (more or less) constant voltage regardless of load (12 VDC, 120 VAC, etc.) until you draw so much current it pops a fuse or overwhelms the battery/transformer/generator. A true voltage source has near zero series resistance (hence, the "unlimited" current with fixed output voltage).

A current source, on the other hand, will output XX Amps from Zero Volts to Vmax and only XX Amps. And, believe it or not, a pure current source has infinite resistance. To see why that is:
• R = delta V / delta R = (12 volts - 0 volts) / (10 amps - 10 amps) = 12/0 -> infinity
In the perfect current source case, the output voltage varies depending on load, but the output current is fixed...

So, with a real current course like a solar panel, there is the maximum output Voltage (Voc open circuit) and the maximum output current Isc... Plus the Vmp/Imp (maximum power). So, there is a bit of delta V / delta I where the difference in current is non-zero--so there is some "real" resistance.

In the end, you can run a solar panel with zero current load, or 100% short circuit and not damage them.

One way to kill a solar panel is to hook them up backwards to a battery--The battery will push a whole bunch of current though the solar panel (forward biased diodes that are solar panel cells) and overheat/pop the internal wiring/cells.

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Solar Expert Posts: 157 ✭✭
Re: trying to understand WHY no fuses

Think of it this way: solar panels absorb photons, and convert them either to heat or to electricity. If the solar panel is disconnected, they all go to heat. If a solar panel is short-circuited, they all go to heat, but some of that heat is generated in the wiring instead of just the panel surface. But as long as the only source of energy if the incident sunlight, the solar panel won't be damaged: a short-circuited panel can't generate any more heat than a disconnected panel by the first law of thermodynamics.

If you hook them up to an external source of energy, like a battery or a bunch of other panels, there is the potential for damage if they are hooked up incorrectly or more power is forced through the panel than it can handle.
• Solar Expert Posts: 226 ✭✭✭
Re: trying to understand WHY no fuses
AnEv942 wrote: »
In replacing a damaged panel, not having fuse on output of panel surfaced again. Why no fuse? I did a lot of searching and found of course that its not required but still don’t get WHY a fuse on output isnt necessary to protect panel from itself, ie a direct short in output wires before controller.
What exactly happens, how long could a panel sustain a direct short. I've seen pics of an install, the prewired MC cables on individual panels were plugged into each other making a loop. Why does this seem so contrary-going against everything I think I know about electricity- obviously I’m looking at a panel like a battery, which must be wrong.

The short circuit current (Isc) is only a tiny bit more than the max power current(Imp).
The PV cells can handle a short circuit current indefinitely.

Fuses aren't really precision devices, typically they will blow at some current plus or minus 10%.
The difference between Isc and Imp is usually less than 10% so you can't really install a fuse that will blow at Isc and not blow at Imp.

So fuses are not needed and not practical (in this case) anyway.
• Posts: 6,290 admin
Re: trying to understand WHY no fuses

Appreciate the efforts to explain and simplfy. Kinda fuzzy but I think I got it, least makes sense enough to feel ok with resisting overwhelming urge to add a fuse. Which brought up second ? what size. As I’m gathering kinda of a circular thing, short or disconnected, so fuse even if added wouldn’t do anything (or blow) but not needed because basically panel wont generate more than it can handle.
I think key word was current. Interesting, not sure why this (no fuse) bugged me, I understand the external power source dangers( i.e. more than equal size panel or battery other input source).
This is single 135w panel system-might be adding another smaller footprint panel later. Which would make it a smaller wattage -so protecting it from this panel would be the task-but that’s later & maybe..
Cool so next time I have opportunity to open juntion box-I wont be scratching my head trying to figure out why? no fuse
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