Modules are built to withstand only a small amount of current—generally about 15 A—and can fail catastrophically if they are exposed to higher current (for example, via backfed amps from paralleled modules or strings of modules). Backfeeding from other strings is most likely to occur if one series string of modules stops producing power due to a damaged circuit or cell.
Because PV modules are current-limited, there are some cases where series fusing may not be needed. For a single-string array, there is nothing that can backfeed into it, and thus no series string fuse is needed. In the case of two series strings, if one string stops producing power and the other string backfeeds through it, still no fuse is needed—each module is designed to handle the current of the remaining string. Some PV systems can even use three strings or more with no series fuses, due to an exception in NEC Article 690.9(A), Exception b, for situations when the series fuse rating is substantially higher than the short-circuit current (Isc) of the modules. Every PV module has a series-fuse rating included in its specifications, per UL-listing requirements.
Other than the exceptions above, to avoid the catastrophic scenario of unwanted current flow, overcurrent protection devices (circuit breakers or fuses) are added to the PV source-circuit conductors.