Has anyone seen a good illustration/diagram that explains why two strings in parallel typically cannot provide fault current that exceeds the module max series fuse size?
Seems to me like simple math, but I am having a hard time getting folks to wrap their heads around the reasoning behind the general practice of fusing only when there are three or more strings in parallel.
First thing to note is that the string fuses are not intended to protect anything but the strings, and that the strings are inherently current limited when acting as sources, no matter how many you put in series.
From that starting point, look at how much current can be supplied to a
failed string in a pair. The only current source is the other string, and its output will be roughly between 1/3 and 2/3 of the series fuse rating.
If the current source is backfeed from the inverter or charge controller which has also failed, it will be feeding both strings in parallel and so probably not delivering damaging current to either of them based on the maximum series fuse size. (The fuse is sized to protect a single string against otherwise unlimited back current from defective load equipment OR other parallel panels.)
The only case allowing damage is a double fault.
For three or more strings, the other working strings in parallel can provide more current to the failed string than the specified series fuse would allow, so a single fault could damage a string IF that string fails.
If you fuse pairs of strings instead, then a single fault can still provide more current to the failed string in a pair than the fuse itself allows.
From there you should be able to draw the pictures.
