ARC fault

[Zee]

Well it finally happened. I have to trace an elusive arc fault on an unreachable steep and tall roof that we had scaffolding for when we installed it.
Not easy physically. It shuts down the SMA SB3800TL-US-22 inverter. 16 panel array. 2 strings of 8.

The arc fault seems to appear in wet weather, that is during our winter rainy season only.
Then I reset the inverter, and the arc fault error message disappears.....for a while. A week or a year. (!)

This summer, I disconnected string A then string B for over 1 month each...... and could not reproduce the arc.
I had hoped I could at least determine if the nuisance arc was in string A or B - cutting down my work by 50%.

Any methods of locating the arc diagnostically that ease the pain? I.e. not requiring removal of all panels.?

 

[letgomywago]

I don't know much about solar arc faults but could they have the same issues with radio interference?

 

[ramsy]

Any methods of locating the arc diagnostically that ease the pain? I.e. not requiring removal of all panels.?

Does your periodic fault coincide with anyone accessing enclosures, or touching the wiring?

 

[BackCountry]

Well it finally happened. I have to trace an elusive arc fault on an unreachable steep and tall roof that we had scaffolding for when we installed it.
Not easy physically. It shuts down the SMA SB3800TL-US-22 inverter. 16 panel array. 2 strings of 8.

The arc fault seems to appear in wet weather, that is during our winter rainy season only.
Then I reset the inverter, and the arc fault error message disappears.....for a while. A week or a year. (!)

This summer, I disconnected string A then string B for over 1 month each...... and could not reproduce the arc.
I had hoped I could at least determine if the nuisance arc was in string A or B - cutting down my work by 50%.

Any methods of locating the arc diagnostically that ease the pain? I.e. not requiring removal of all panels.?

I’ve had to search for an arc fault myself. We use a fluke insulation resistance meter, put 1000v on it and find the fault that way. Isolate the PV and disconnect the MC4’s. If you don’t find the fault in your circuit going to the array, then you can start checking the modules themselves.

In my case, we had a knick in the insulation on some #10 THHN, and when it got wet (this was a ground mount in PVC conduit below grade) it would arc. Pretty easy to find with the meter.

 

[ramsy]

put 1000v on it and find the fault that way.

1000v thru wet raceway most likely destroys wet-wiring insulation, solving Arc Fault issues by re-installing everything.

 

[BackCountry]

1000v thru wet raceway most likely destroys wet-wiring insulation, solving Arc Fault issues by re-installing everything.

I’ve had great success with underground runs in PVC with an insulation resistance tester, especially when you’ve got multiple DC runs in one raceway. We isolate each pair and test with a Fluke 1507.

 

[ggunn]

1000v thru wet raceway most likely destroys wet-wiring insulation, solving Arc Fault issues by re-installing everything.

I don't think that high voltage meggering can supply enough current to do that.

 

[Zee]

I don't know about radio interference.

Also, nobody is accessing enclosures or touching the wiring.

BackCountry, using an insulation resistance meter is new to me. I appreciate it.

 

[Zee]

Basic question: how does it work?
I mean, if the arc is not currently detected by the inverter (maybe due to dry conditions), how does the tester find it?
(I assume the purpose of applying 1000V is to re-create the arc, and then to detect its signal?)
I appreciate it.

 

[jaggedben]

If it's a parallel arc (say to ground from nicked insulation in the conduit going to the roof, or between splices going bad in a j-box) then the megger will give a fail. By disconnecting the array and testing the home runs by themselves you can find out if the problem is on the array or in the home run conduit. Using the higher voltage setting just gives you a higher chance of finding a small fault.

If it's a series arc - say a pair of MC4 connectors between panels is loose - then I'm not sure the megger will actually help.

 

[Zee]

Thanks! Interesting distinction between parallel vs series arcs.

The one time I found an arc, it was at a PV panel-to-panel series connection. Not too evident by looking at it though. Slightly deformed possibly.
The MC connectors had melted themselves together. When pulling it apart they would not budge or crumbled in my hands, the metal ripping out of the plastic housing. They were burnt to a crisp.

That is what I will start by looking for: MC conns that don't pull apart.

 

[1983Rich]

Hi, any help over in this threaded would also be appreciated. Thanks

DC arc fault detection instrument/tool

Good Day, Most inverters are running DC arc fault detection and shut the inverter off when the system declares a DC arc fault. With long strings the DC arc fault could be in any of the approx. 40 MC4 connectors. Is there a DC arc fault detection instrument/tool that I can insert in series...

forums.mikeholt.com

 

[ramsy]

The one time I found an arc, it was at a PV panel-to-panel series connection.

interesting research from Norway

A 2021 study based on statistics from Germany, Italy, Australia and the United States showed that these countries had 29 fires a year for every GW of installed solar power. The fires occurred mainly due to faults in solar panel modules, DC switches, inverters and connectors, with connectors being the largest contributor.

An extra 3 centimeters can prevent fires in rooftop solar panel systems

Solar panel systems are now being installed at a rapid pace. In 2022 alone, the Norwegian Water Resources and Energy Directorate (NVE) registered a doubling of installed solar power systems connected to the power grid.

techxplore.com

 

[jaggedben]

There is a whole process for racking systems amd panels receiving a Class A fire rating in a rooftop installation. Instructions should be followed.

 

[ramsy]

Instructions should be followed

That study did identify installation errors.

According to Stølen, the risk of fire is greatest when new systems have just been installed. This is due to errors and weaknesses in the installation.
.."I think that quite a few fires could have been avoided if the installation had undergone better checks before the system was put into operation."

 

[ramsy]

Instructions should be followed.

Incompetent & unqualified persons fail to solve arc fault protection issues with inside wiring errors as well.

Since unskilled laborers can't make arc fault protection work with installation errors, rather than require skills home builder's can lobby their chamber of commerce to amend out AFCI & GFCI requirements.

 

[ActionDave]

Incompetent & unqualified persons fail to solve arc fault protection issues with inside wiring errors as well.

Since unskilled laborers can't make arc fault protection work with installation errors, rather than require skills home builder's can lobby their chamber of commerce to amend out AFCI & GFCI requirements.

Because it's never a problem with the AFCI, they function perfectly, and there have been zero compatibility issues with GFCIs and listed equipment

 

[winnie]

I'm more inclined to believe the reliability of detecting arc signatures on a DC system supplying a well defined load (the inverter). I suspect that the OP has a real arc which they are trying to locate.

To my mind, the symptom of showing up in wet conditions suggests some sort of insulation fault that is conducting during water intrusion, rather than a loose connection causing a series arc.

Jon

 

[ramsy]

Because it's never a problem with the AFCI, they function perfectly, and there have been zero compatibility issues with GFCIs and listed equipment

Since when do products that pass UL testing for their category, always get along with each other?

GFCI's didn't like magnetic ballasts, and dimmers didn't like fluorescent lights or LED's, but real electricians kept the lights on.

 

[jaggedben]

For whoever needs to be reminded...

This thread was originally about DC arc fault detection on PV systems. Comparisons to AFCI and GFCI on alternating current breakers are not warranted.

Also the subject of the Norwegian study is only tangentially related. Lots of other things can set a roof on fire.