reverse current in the array: circuit breakers take care of it?

[Megunticook]

I'm installing a system at my house, new to solar, reading about reverse currents and noting what my module manufacturer says about this in the installation manual. QCells says I need string fuses if I have more than 2 strings in parallel. I have a Midnight Solar combiner box with their 20A breakers. Is this insufficient for preventing issues with reverse currents? My plan is 3 strings in parallel (a total of 6 strings), with 37A/120V per string.

I called QCells but haven't heard back. Here's what they say in the install manual:

Parallel Connection
Modules may be damaged by the occurrence of reverse currents
(caused by module defects, ground leaks, or defective insulation).

Ensure that the maximum reverse current load capacity indicated
in the data sheet is met.

In order to limit reverse currents that may occur, we recommend
using the following safety options:

1) Layout with a limited number of parallel connected strings :
Without undertaking further current blocking measures, a maximum
of two module strings may be operated in parallel on an inverter or
MPP tracker.

2) Layout with string fuses :
place fuses for each string of modules at the plus and minus ends.
Observe the maximum permitted number of strings as indicated
in the specifications provided by the respective string fuse manufacturer
and the technical guidelines.

[sensij]

I have a Midnight Solar combiner box with their 20A breakers. Is this insufficient for preventing issues with reverse currents? My plan is 3 strings in parallel (a total of 6 strings), with 37A/120V per string.

I don't really understand your plan. With 6 strings connected in parallel, each string needs to be individually fused. If you are using a combiner that fuses each string, that is sufficient.

[Megunticook]

So fuses, not circuit breakers? And both + and - conductors must be fused on each string?

The plan is a total of 6 strings of 3 panels each. But not all six strings are in parallel--one set of three goes to one charge controller, the other set of three goes to a second charge controller.

[sensij]

So fuses, not circuit breakers? And both + and - conductors must be fused on each string?

The plan is a total of 6 strings of 3 panels each. But not all six strings are in parallel--one set of three goes to one charge controller, the other set of three goes to a second charge controller.

Fuses or circuit breakers, doesn't matter. They both satisfy the requirement for an Over-Current Protection Device. Once you exceed two strings going to a single charge controller, each string needs to be protected. Since you have three strings going to each controller, all three strings need protection.

If the DC- is grounded, the OCPD only needs to be on DC+. If the system is floating, with no connection to ground, both DC+ and DC- need OCPD.

Are you sure three panels in series is OK for your controller? What panels, and what controller, are you using?

[Mike90250]

nighttime reverse current is prevented by the charge controller. the Midnight series controllers allows only a couple mA through, no issue at all.

combiner box helps prevent a daytime fire if a panel goes bad

[Megunticook]

Are you sure three panels in series is OK for your controller? What panels, and what controller, are you using?

Panels are QCell 305 watt, controllers are Outback FM80. Unless I'm missing something, 3 panels in series should be fine with those controllers in terms of wattage/current/voltage.

So my plan is to have each string's positive conductor run through a 20A breaker in the Midnight Solar combiner box. Does that protect me from reverse currents just as well as fuses?

[sensij]

QCells has a 60 cell version and a 72 cell version that are rated for 305 W. The 60 cell version should be ok, but the 72 cell voltage might be too high if you live someplace that experiences cold in winter.

What is the 37 A you mentioned in the first post? Also, why the FM80 instead of the FM60 for that configuration? Room to add a 4th string?

Yes, the combiner with breakers provides the required protection.

[Megunticook]

It's the 60 cell version. I ran through a bunch of calculations, factoring in cold, etc., and seemed like it would work. 37 amps is the current from three combined strings. I chose the FM80 because, yes, I might add a fourth string. So one controller would handle 3 strings and one controller would handle 4. I'll have to check my figures but as I recall the fourth string was starting to approach the FM80's max. input watts.

Glad to hear I don't have to change out my breakers for fuses. So if breakers are fine for OCPD, including reverse currents, why do some people use fuses? What's the advantage?

[bcroe]

So if breakers are fine for OCPD, including reverse currents,
why do some people use fuses? What's the advantage?

When running a 400VDC array, breakers of this rating are big and expensive. Fuses are still
cheap and compact. Bruce Roe