It is also easier to safeguard a PV system against unduly high reverse current that could damage the modules and the string cables, by installing a DC-compatible fuse on one side of the string and a disconnect terminal on the other. Under certain weather conditions, brief cloud enhancements exceeding 1 kW/m2 can occur that subject the system fuses to additional stress. Hence, in order to ensure that the fuses last a reasonably long time, their rated current ISN should be a factor kSN higher than the string’s STC short-circuit current Isc stc, whereby kSN contains a hybrid safety factor for an elevated ambient fuse temperature (45 °C), for cloud enhancements up to about 1.3 kW/m2 and for reverse load. If one side of the PV generator string is grounded, the fuse should be installed on the opposite side. String fuses should be rated for >1.2 ■ VOC-STC DC and for the entire current range, which means that gPV or possibly gR fuses must be used. Fuses rated solely for short-circuit protection (type aR) are unsuitable and can cause a smouldering fire. Thus
The higher the ratio between actual system current and nominal current, the more likely a fuse is to cut off a circuit. According to IEC standards, fuses need to be able to conduct nominal current for an unlimited period, but once guaranteed fusing current If — kf ■ ISN > ISN is reached, they must shut down the circuit within one hour (typical value, If — 1.45 ■ ISN; Figure 4.29). Thus
Inasmuch as more finely graded fuses are available today (see Table 4.2), the recommended upper limits for kSN relative to the values in [Hab07] can be defined somewhat more narrowly.
In desert regions with elevated fuse ambient temperatures such as 55 °C, kSN should be at least 1.5, and for Alpine systems at least 1.6 owing to possible snow reflection, although in such cases the upper limit
Table 4.2 Examples of commercial PV fuses (here, Siba fuses)
Examples of commercially available PV fuses (gPV):
Size 10 ■ 38 mm, VDC < 1000V: 2A 3A 4A 5A 6A 8A 10A 12A 16A 20A for kSN may need to be slightly above 2 (e. g. 2.1), in order for commercial fuses to be usable. Needless to say, all string wiring should be rated for currents > If = kf ■ ISN.
In addition, a string can be segmented for servicing purposes using fuses and disconnect terminals, once the system has been shut down via the main DC switch.