Solar rooftop PV systems: Selection criteria for Array Junction Boxes
Many of the components discussed in the previous section are accommodated in a special junction box near the solar generator and referred to as the generator junction box. The string wiring for large multi- array PV generators is initially realized in array junction boxes that exhibit the same structure as generator junction boxes. The array wires that lead out of the array junction boxes are then wired, in the super ordinate generator junction boxes, to the PV generator as a whole . These boxes protect the solar generator/array components from the elements and for outdoor installations should exhibit a
UV-resistant housing that meets protection class IP54 and preferably IP65 requirements. The housing should be constructed of a non-combustible or barely combustible self-extinguishing material. The various solar generator strings, which are wired in parallel into the generator junction boxes, contain the elements necessary for such wiring, such as string connection terminals, string diodes, string fuses and disconnect terminals, as well as (in many cases) DC circuit isolating switches and
varistors for power surge protection. Each generator junction box should also be rendered grounding fault- and short- circuit-proof via double-insulated terminals and positive and negative conductors, which should exhibit adequate reciprocal clearance and may also need to be insulated from each other. If desired, the positive and negative conductors can be accommodated in separate generator junction boxes, although this costs more and precludes optimal lightning current protection. Metal housings are non-combustible, which is a major advantage in the event of a terminal fire inside the housing. But as these housings are usually grounded for reasons of safety, realization of grounding fault-proof wiring inside the housing is somewhat more difficult.
Once the strings have been wired, each string is in turn wired to the string connection terminals (with the isolating switch open, the disconnect terminals open and all fuses removed) and the open-circuit voltage at each string is then measured.