Best solar day of the year tripped my 40A PV DC breaker

[ricardocello]

Well, best solar day of the year so far.

Unfortunately, I found out that I was too conservative designing the main PV breaker at 40A (4s3p configuration).
It tripped today. Guess I’ll bump it to 50A. Cable size is adequate, no issue there.

Good news: The inexpensive amazon Langir DC breaker actually worked!
Bad news: The Victron MPPT during its scans really spikes the current.
Good news: Wife knows where all the disconnects are now.

 

[wpns]

What’s the Isc on those panels?

 

[ricardocello]

What’s the Isc on those panels?

12.17A if you count some bifacial current, 11.12 otherwise.
I know, 3 * 12.17 = 36.51A. Producing way way more than expected.

 

[740GLE]

Gotta love cooler temps.

 

[ricardocello]

Gotta love cooler temps.

about 70 deg F here today, very clear skies
Also, no HVAC loads to suck up all that power, so battery was filling quickly.

And then, everything stopped.
But 40 minutes later the breaker was reset.

 

[G00SE]

Pv breakers are supposed to be sized 1.56x string no?

 

[Hedges]

And thermal breakers age. I've had some trip well below rating.
Consider magnetic-hydraulic (e.g. Midnight/CBI). They are also guaranteed not to trip below 105%, so I think sizing 1.25x rather than 1.56x should be fine.

But before you mess with anything, take IR temperature readings. Loose terminal/wire connection could be the culprit (if it is a thermal breaker now).

 

[ricardocello]

Pv breakers are supposed to be sized 1.56x string no?

Now you tell me!

I'm definitely not the expert on that part of the code, I've mostly just read what people post here.
I've also seen comments that 1.25x is sufficient depending on the configuration.

I'm putting in 50A breakers, which will cover the 1.25x case, and clearly cover those spikes that took me out.
I think it is silly to have to bump up to 60A just to cover a 1.56x rule, what is it protecting anyway?
I'll wait for the experts to chime in.

The individual strings have 16A breakers in the combiner box that feed the 40A that tripped (soon to be 50A).

I also have a Midnite GFPD that is rated 50A in the path (MNDC-GFP50-300).
I only consider the Langir breakers as disconnects because I doubt their capabilities.

 

[ricardocello]

But before you mess with anything, take IR temperature readings. Loose terminal/wire connection could be the culprit (if it is a thermal breaker now).

That's a really good point, I will check that in the morning.

 

[Hedges]

NEC says breakers on normal circuits are to be 1.25x continuous loads.
For PV circuits, 1.56x = 1.25 x 1.25, the extra one for cloud edge effects.

But breakers that are continuous rated, you can get away with less. Some CBI are guaranteed not to trip below 105%, Carling not to trip below 100%.

It isn't protecting anything (except maybe backfeed from inverter/battery). It is a disconnect.
If you have 3 strings in parallel, then they're supposed to each have a fuse or breaker (protecting each from backfeed by the others, if it shorts.)
However the required/allowed OCP isn't likely to trip at 2x Isc. If more than 3 strings then it makes sense.
That breaker must be non-polarized. (or, I say gang the handles together.)

 

[ricardocello]

But breakers that are continuous rated, you can get away with less. Some CBI are guaranteed not to trip below 105%, Carling not to trip below 100%.

So I believe the Midnite MNDC-GFP50-300 in the path satisfies the 1.25 rule and it is a good quality breaker. Looks like CBI.

My mistake was undersizing the inexpensive Langirs (used for disconnects) at only 40A.
Everything is two-pole ganged, the Langirs are non-polarized.

 

[Hedges]

You have 3 strings? Do they have individual breakers per string? Are they polarized or unpolarized?
If they are polarized, I'd like all 6 breaker poles ganged. Or maybe two 3-pole breakers, one used for all PV+, the other for all PV-.
My theory is, if one is backfed and trips (but can't interrupt the arc due current flowing in wrong direction), it turns off the other two which source the current.

Or, fuse per string would be fine. I think that needs to be on both PV+ and PV-.

 

[ricardocello]

You have 3 strings? Do they have individual breakers per string? Are they polarized or unpolarized?
If they are polarized, I'd like all 6 breaker poles ganged. Or maybe two 3-pole breakers, one used for all PV+, the other for all PV-.
My theory is, if one is backfed and trips (but can't interrupt the arc due current flowing in wrong direction), it turns off the other two which source the current.

Or, fuse per string would be fine. I think that needs to be on both PV+ and PV-.

3 Strings, each string has its own 16A two-pole non-polarized Langir breaker.
They have never tripped. Maybe they work, maybe not.
All are in a combiner box at the arrays.

I do not have all 6 breaker poles ganged. They are din rail, and I could do that!
I see what you are getting at, I like the idea of disconnecting them all.
Though I enjoyed flipping them on one at a time when building the arrays.

All three feed into the 40A 2-pole disconnect that tripped today, soon to be replaced with a 50A non-polarized Langir.

The Midnite MNDC-GFP50-300 polarized breaker/GFPD is where the PV enters the house.
I trust that one to be a real breaker if the inverter dead shorts to battery or something.
And another 40A two-pole Langir for a disconnect because the GFPD is not allowed to act as a disconnect.

 

[Hedges]

Only reason for ganging the breakers would be to make-do with polarized type. Because they can't protect against backfeed if tripped.
Since yours are non-polarized, they are fine independent (assuming they do work.)

If you have spares (or take one off), you could test them. At least for trip current with AC. Then for ability to interrupt an arc in either direction by using it to short a PV string, then to short 2 or 3 parallel strings. Three would be a good test, gives margin.

So far my testing has been of AC breakers, either at full voltage overloaded with multiple heaters, or current stepped up and voltage down with transformer.

 

[ricardocello]

But before you mess with anything, take IR temperature readings. Loose terminal/wire connection could be the culprit (if it is a thermal breaker now)

FLIR did not show any hotspots with 32A, everything was still tight.
I’ve replaced the 40A breaker with a 50A.

Fortunately, the wiring from the ground mount to the house has oversized conductors.

From the combiner box, it is 220 feet of URD Aluminum 2 AWG (with no-alox in the terminal blocks).
Then about 50 feet of 6 AWG THWN2 in EMT through the crawl space.
Then a short run of 8 AWG Silicone 200C between junction boxes.