Fusing a Solar Panel

[tkc100]

In a number of circuit designs I have seen a disconnect and fuse between the solar panels and the controller or batteries.
Perhaps I'm not seeing something here but I do not understand what good or of what use a fuse would be in such a configuration.
Let say for simplicity I have a solar panel capable of producing 5 amps. Well I intend to, or at least hope to get as much of that current to the batteries as possible so I install a 5 amp fuse. Now if the wiring, controller or something else shorted between the panel and the battery the fuse wouldn't blow. So what's the use?
What am I not seeing here? I can see the need for a disconnect but what use would the fuse be?

[BB.]

Remember, fuses/circuit breakers are there to protect the wiring (and attached devices) from getting too much energy and catching on fire.

So--The battery is the first "large current source". So, any positive cable coming off the battery should have a fuse/breaker, relatively close to the battery, to protect that protects down stream short curcuits (normally, no fuse/breaker is put in the negative/return lead).

So, for example, you may see an 80 am fuse/breaker near battery protecting the 60 amp solar charge controller/wiring from shorts.

Now... On the solar pane side--assuming the solar charge controller either does not pass current in a failure (MPPT type) (no additional fusing needed) or that it is a PWM which can--the PWM solar panel wiring will be the same rating as the wiring between the controller and the battery--so the battery fuse/breaker will protect the solar array wiring from shorts receiving energy from the battery side.

Then, we look at the solar array. Normally, we assume that if there is a short in the solar array (or its leads), only current from the array itself, or its neighbors can supply energy into the short.

One panel cannot "over current itself). And, it turns out, that two panels cannot over current a common short (exceed the series fuse rating). Also, it does not matter how may series panels there are in a string... The maximum output current is the same.

However, three or more panels/strings in parallel can over current a common short in the array. Therefore, each parallel connected string in the array needs a series protection fuse or circuit breaker to prevent the other panels in the array from causing a fire in the one panel's short.

Hence the reason that there are fused/breaker'ed combiner boxes for solar arrays.

Normally, NEC/NRTL (like UL)_ do not look/test for two simultaneous faults (shorted array and shorted PWM controller).

They look for one fault (a shorted panel OR a shorted controller), and all the things that can go wrong from that fault (over heated wiring, fire caused in an enclosure, etc.).

I hope it makes sense--lots of English to describe all of the configuration issues with safety.

-Bill

[audredger]

Another reason for multiple breakers or fuses is so you can isolate a component for maintenance or replacement. An example: I have a breaker on both the input and output side of my charge controller. I can shut off both breakers then do what ever with the controller.

You can't have too much circuit protection.

[BB.]

I agree that extra switches to turn off sections for debugging is nice.

However, it is possible to have too many CPD's (Circuit Protection Devices)...

Each typically adds a voltage drop and another set of connections--and frequently adds to wire run length. When working with low voltages (like 12 volts) and high currents--these can be real issues.

Also, fuses and breakers are "unreliable" by design. The more you have, the greater chance that one of them will false trip (or other electromechanical problems) someday.

I have also seen CPD's put in the return lines of equipment and even home wiring (old homes). When a "neutral/ground return" breaker/fuse pops--it still leaves a "hot line" for somebody to get zapped by.

Then there is the whole DC Ground Fault detection/trip circuit added to the NEC for solar arrays/systems... Violates rule number one by placing a circuit breaker right in the safety ground lead... Real smart. Line cross somewhere on the solar array causes ground breaker / fuse to trip--and array remains energized by the line cross... Sort of the exact opposite of "safety ground".

/rant off...

-Bill

[tkc100]

BB
Let me see if I understand what you are trying to tell me.
I have 6 - 12 volt panels wired in parallel and a PWM controller
1) I need a fuse/breaker between the battery bank and the Inverter.
2) I need a fuse/breaker between the battery bank and the controller.
3) I need a fuse/breaker between the controller and the solar panels.
Here is where I get a bit confused....
4) I need a fuse in the positive lead to each of the 6 panels. Each panel is rated at Isc (A) 3.74 so I would a fuse some where in the neighborhood of 4 amps. This is not to protect the individual panel from itself but to protect the array in the event that panel shorts out. In the event the panel shorted out current could be feed up from the battery and also from the other 5 panels if there is on protection.
Hope I am making sense here because I had not thought of it the way you presented it.
Presently all I have is a disconnect between the solar array and the controller.
The inverter is protected by its own internal circuit breaker.
Finally is there less voltage drop across a fuse or a breaker

[icarus]

Bill,

Question:

"However, three or more panels/strings in parallel can over current a common short in the array. Therefore, each parallel connected string in the array needs a series protection fuse or circuit breaker to prevent the other panels in the array from causing a fire in the one panel's short."

Why three panels in a parallel array, not two?

Tony

[BB.]

BB
Let me see if I understand what you are trying to tell me.
I have 6 - 12 volt panels wired in parallel and a PWM controller
1) I need a fuse/breaker between the battery bank and the Inverter.
2) I need a fuse/breaker between the battery bank and the controller.

Yep.

3) I need a fuse/breaker between the controller and the solar panels.

No--not quite... You need a fuse/breaker at the positive end of each string of solar panels (even if the string is one panel long). After the 6x fuses (or combiner box), all of the + fuse outputs are tied together and then run back to the solar charge controller's positive input.

All of the Negative solar panel leads are tied together, and ran back to the solar charger's negative input.

Remember, in this case, the solar panels are the "power source" and you are protecting each of the small power sources (if one gets shorted) from the energy of the other 5 flowing backwards into the string wiring/panel.

Here is where I get a bit confused....
4) I need a fuse in the positive lead to each of the 6 panels. Each panel is rated at Isc (A) 3.74 so I would a fuse some where in the neighborhood of 4 amps. This is not to protect the individual panel from itself but to protect the array in the event that panel shorts out. In the event the panel shorted out current could be feed up from the battery and also from the other 5 panels if there is on protection.
Hope I am making sense here because I had not thought of it the way you presented it.

Look up the spec. for your solar panels--there should be a Series Fuse rating... If not, from what I have seen take 2xIsc then round up to the next standard fuse breaker (CPD; circuit protection device):

• Fuse = 2 * Isc = 2 * 3.74 amps = 7.5 amps; round up to 8 or 10 amp CPD

Each panel is "rated" to carry 10 amps maximum... If there is a short in the panel or its leads, the other 5 panels can feed 5x3.74a=18.7amps into the 10 amp max circuit. Leading to overheating and possible fire.

The wire from the combiner box to the solar charge controller needs to be rated to handle the total current from the arrays * 1.25 (NEC, I think does another *1.25 again for *1.56 total--may be overkill).

Presently all I have is a disconnect between the solar array and the controller.
The inverter is protected by its own internal circuit breaker.
Finally is there less voltage drop across a fuse or a breaker

I would not put in an extra breaker/fuse in front of the controller.

"However, three or more panels/strings in parallel can over current a common short in the array. Therefore, each parallel connected string in the array needs a series protection fuse or circuit breaker to prevent the other panels in the array from causing a fire in the one panel's short."

Why three panels in a parallel array, not two?

Interesting question (OK--really, all of this safety stuff is boring to 99.9% of normal people out there).

It has to do with the energy path of a short circuit. Think of a wire with 10amp supply on each end... Any place you pick energy off that wire, you have 20 amps available--but the wire only needs to be rated for 10 amps (really 10x1.25-12.5 amps maximum).

That is because, nowhere in that piece of wire will there ever be more than 10 amps, except at the point where that power is "tapped" off.

In the case of a fault (short circuit), the only place 20 amps will flow is through the fault--which is not rated for current flow anyway... Sort of make sense?

In England (and probably other places), house wiring is a "ring". As I understand, you have the "branch circuit going around the home/room in a "circle". Each end fed by a 13 amp fuse. (it has been years--so I may have the amperage wrong).

When you plug in an appliance/light/etc. the plug and wiring is only rated for 13 amps maximum--so each plug needs it own 13 amp fuse to protect the branch from 26 amp total source.

And you can, for one "ring" branch circuit with multiple 13 amp plugs that add up to ~26 amps of total load on wire that can only carry 13 amps safely because the double ended feed.

So why two vs three... Two is above, with three sources, then there is always one section of wire where 2xIsc can flow from "one way" and 1xIsc from the other into a short.

At least, that is how I misunderstand the reasons.

-Bill

[crewzer]

Why three panels in a parallel array, not two?

See: http://www.nmsu.edu/~tdi/pdf-resources/CC125.pdf

HTH,
Jim / crewzer