PV Fuses for PV line with 450V MPPT RS

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hdp asked Apr 27, '21

PV Fuses for PV line with 450V MPPT RS

Hi All,

I'm excited about the RS Mppt range, more panels is series, less amps flowing. I used to implement combiner boxes with the 250v range (12 panels, 4 series, 3 groups parallel).

Now I want to string 2 parrallel strings of 6 panels (420Wp).

I want to run 2 cable pairs respectively (4-6mm cables) directly to the 2 independent trackers of the new Mppt.

Where do I fuse the strings:

Inline fuse on the roof closest to the PV panels? (maybe best to break the power closest to the generating source) or at the DB box close the Mppt in the house near the DB where the DC isolators/breakers and DC surge protection is?

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Answer 1 · 2021-04-28T11:18:20Z

Hi @HDP,

I am excited to hear you are excited about the new range.

PV string fusing is usually described in regional wiring codes and standards. So ALWAYS follow those first and foremost.

In my situation in Australia, for now at least, there needs to be load current rated DC isolation on the roof, and on the ground within 3M of the MPPT. Then if the short circuit current of a parallel string exceeds the maximum current rating of the PV panel (this is listed on the PV panel spec sheet, or back plate), then individual string fusing is required. This can be done as an inline MC4 type fuse on the roof, or as an appropriately rated DC breaker on the ground.

I can see the cost savings appeal of paralleling the strings on the roof with an inline DC fuse located on the PV side of the MC4 splitter/combiner.

HOWEVER I would suggest that there is some long term advantage to running the individual strings down to a panel in the control room, and then having individual rated DC breakers there, and then combining the strings in parallel AFTER that. This allows for an individual string isolation (of the paralleled pair), which you can use for diagnosis and fault finding without getting on the roof, if required.

You are also correct that, generally speaking, it makes sense from a safety perspective to have the fuse located as closely as possible to the generation source.

However, I personally think this is less important for fusing a PV panel. They are inherently limited current (unlike an uncontrolled battery fault short circuit current), so while it is possible that fault could occur (due to breakdown of the insulation in the PV wiring) on the panel side of the fuse, as long as the fuse is isolating the panel strings to prevent their combined current (when it exceeds the panels rated capacity) then the panel itself is unlikely to cause any issues to the array itself, or the wiring (as any 4/6mm cable is ~40A rated well above any possible PV string current capacity) making a wiring fault leading to fire much more unlikely.

Please note, with the reduced current comes increased voltage, and that has it's own dangers of electric shock. So make sure that the PV array is correctly configured so that the earth fault detection system is operating correctly, and that should go a LONG way to preventing wiring insulation damage from becoming an unidentified hazard.

As a disclaimer, none of the above is 'manufacturer advice' per se, just my individual opinion.

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