Change in 2023 NEC code around PV arc fault.

[rhino]

So why is it so hard to get a device that we can add that is rated to at least 500V DC and will stop the series arc fault? If Midnite is including this in their charge controller there surely is some way to have at least a separate device to do the equivalent functionality.

 

[cs1234]

Just keep your solar voltage below 80v.

 

[FilterGuy]

So why is it so hard to get a device that we can add that is rated to at least 500V DC and will stop the series arc fault? If Midnite is including this in their charge controller there surely is some way to have at least a separate device to do the equivalent functionality.

Good Question. I have been wondering about this for a long time.

Stand Alone Solar Arc-fault protection devices.

I have been studying the NEC requirements for wiring solar panels and one of the requirements is arc-fault protection if the PV array is at 80V or more. However, when I search for PV arc-fault protection product.... I find no stand-alone products. Some SSRs have it built in. Most grid-tie...

diysolarforum.com

My hypothesis is that there is not a large enough demand. In the US, the vast majority of the professionally installed systems that require NEC compliance are done with Micro-inverters that keep the PV voltage under 80V, so there is no need. The new professionally installed systems that don't use Micro-inverters use string inverters with it built-in or high-end battery systems with it built in. Meanwhile, some chunk of the DIY market is done with inverters that have it built in (Such as SolArc). My guess is the rest of the DIY market is done without compliance or inspection so it does not create a demand for an arc-fault product.

I don't know what the requirements are outside of the US, but my impression is the Arc Fault requirements are not as stringent outside the US.

This leaves the stand-alone arc fault device market as a tiny niche that no company has decided to go after.

Having said that, if one of the value-priced inverter companies would release a product that had Arc Fault and Ground Fault protection built in, I think it would be a big hit in the DIY community as long as it did not jack up the prices much.

 

[rhino]

So just looking at this in more detail this was very interesting research on PV arc faults: https://www.analog.com/en/technical-articles/arc-detection-analysis-for-solar-applications.html

Edit: TI even has block diagram with parts on how to create one https://www.ti.com/solution/solar-arc-protection

 

[400bird]

California has their own electrical code based on the NEC. They are currently on California electrical code 2022 which is based on the 2020 NEC. I would guess California probably won't have a code based on the 2023 NEC until 2025.

Minnesota generally adopts the new code on July 1st of the year of the code. For example, the 2017 NEC was adopted on July 1, 2017. The 2020 NEC was adopted on Nov 17, 2020 due to the pandemic. I assume the 2023 NEC will be adopted on July 1, 2023.

And people think California is strick with regulations ?
We're at least 2 years behind. Because after the state writes CEC 2025(or whatever) it also needs to be adopted by the local AHJ, there might be a specified timeline for that, but it still adds time.

 

[Hedges]

So just looking at this in more detail this was very interesting research on PV arc faults: https://www.analog.com/en/technical-articles/arc-detection-analysis-for-solar-applications.html

Edit: TI even has block diagram with parts on how to create one https://www.ti.com/solution/solar-arc-protection

What exactly is the definition of an arc-fault in relation to a PV array?

Code requires detection of an arc with 300W power or something like that.
PV wires have switching power supply fundamental and harmonic frequency peaks much higher than what the arc produces (spread over frequency), so difficult to detect with simple analog. And AFCI circuits can trip due to RF pickup.

It seems that DSP, FFT, or other sophisticated digital processing is required. Stand-alone AFCI would need to function with many brands of inverter/SCC MPPT. Some inverters (older Sunny Boy) I've read had false trips to the point of being unusable. Sensitivity is adjustable in some models.

I read that qualification testing was performed by connecting PV circuit in series with steel wool, but some people argued that didn't represent an arc in the middle of the array.

It may detect it but how does it address the issue if there is an arc-fault? If the DC circuits are arcing somewhere between the Midnite Classic and the array you'd still need something else to actual stop the arc.

Series arc, it can stop by interrupting current flow. Arc to ground it might stop if no other paths to ground. Multiple paths to ground, perhaps inverter shutdown would be ineffective.

For that reason I would like to trigger RSD when there is an arc fault. I don't know if inverters do that or not. The one I'm going to use (TriPower) has arc-fault built in but not RSD. I will see if its "error" status output relay can be used to shut of RSD keep-alive (and not trigger for benign conditions like "no PV voltage".)

 

[zanydroid]

And people think California is strick with regulations ?
We're at least 2 years behind. Because after the state writes CEC 2025(or whatever) it also needs to be adopted by the local AHJ, there might be a specified timeline for that, but it still adds time.

I think California code adoption is quite fast, it just might take an older version than other parts of the country...

Twice I've run into people doing a mad dash. Once was about 7 years ago when an architect asked me if they could delay my project with them so they could rush some projects in before some Title 24 revision was taking effect.

And in December I had plans revision fighting in the City review queue with projects that were trying to slip in before 2022 building code came into effect on January 1.

Luckily for me, they decided to try to talk to me to clarify the revision on the phone instead of sending back for another official pass (which would have taken even longer). Good thing I picked up that call instead of letting it through as spam.

 

[zanydroid]

Is Arc Fault Protection / Ground Fault Protection in DC strings supposed to eliminate the bad situation, or just notify you of the situation? (by flagging an alarm or shutting down the inverter so that you notice and pay attention).

 

[Hedges]

Arc fault is supposed to prevent a fire.
For house wiring, turning off power to circuit should accomplish that.
PV string with distributed generation, even RSD might not if +/- leads from panel arc to frame.

It may not be perfect, but it should protect against series arcs caused by incompatible connectors in the required RSD modules.

 

[FilterGuy]

Is Arc Fault Protection / Ground Fault Protection in DC strings supposed to eliminate the bad situation, or just notify you of the situation? (by flagging an alarm or shutting down the inverter so that you notice and pay attention).

They don't say series arc fault.... but the wording makes it clear that they are talking about series arc fault and not parallel arc fault. The wording also makes it clear they expect the arcing current to be interrupted.

 

[zanydroid]

OK, for a series fault I can see that it’s stoppable by inverter in most cases despite the source being up in the array (assuming no integration with MLPE)

Not sure about ground fault though.

 

[Hedges]

Ground fault, if detected by inverter, causes it to stop drawing current. Sometimes that is a fuse between PV- and ground.
A single fault to ground that starts arcing could not continue unless a second path to ground existed. That could be the case if DC path to battery (-). So opening both PV +/- could work.
If the arc burns through wire insulation in multiple places, all bets are off.

The arc fault and RSD rules were both about 80V and 300W. Now a single panel is above 300W. If that becomes an issue, maybe in the future RSD per diode-bypassed portion of PV panel. I just bought Tigo RSD for two panels. Previously there were 4-panel versions. These things could integrate into a single junction box, for panels configured that way. No dangling wires? In that case single panel RSD in the junction box might be sufficient.

 

[FilterGuy]

Not sure about ground fault though.

Stopping a ground fault is a tougher nut to crack.

As an example: Imagine one of the positive leads coming out of a panel rubs against the frame and wears off the insulation. You now have a ground fault but nothing short of going up and fixing the fault will totally stop it.

From examining how various ground fault devices work, it seems like it is sufficient to just disconnect the PV from the rest of the system.

 

[400bird]

Arc to ground it might stop if no other paths to ground.

Ground fault, if detected by inverter, causes it to stop drawing current. Sometimes that is a fuse between PV- and ground.
A single fault to ground that starts arcing could not continue unless a second path to ground existed.

That should set off the ground fault detection, right? The second path to ground is through the ground fault detection unit.

I think California code adoption is quite fast, it just might take an older version than other parts of the country...

Twice I've run into people doing a mad dash. Once was about 7 years ago when an architect asked me if they could delay my project with them so they could rush some projects in before some Title 24 revision was taking effect.

And in December I had plans revision fighting in the City review queue with projects that were trying to slip in before 2022 building code came into effect on January 1.

Luckily for me, they decided to try to talk to me to clarify the revision on the phone instead of sending back for another official pass (which would have taken even longer). Good thing I picked up that call instead of letting it through as spam.

Poor planning (by those others you mention) isn't unique to California ?
No matter how far out you publish an upcoming change of any sort, there will be people rushing to complete before the deadline.

 

[Hedges]

Yes, my transformer type inverters would detect the first short to ground and turn off. If fuse type, fuse blows. That could stop the fault before an arc occurs or continues. The fuse of course is rated to interrupt 1000VDC or something like that.

The tranformerless inverters test for ground fault by driving the isolated array high before connecting to grid.
If a ground fault occurred afterwards, that could have AC + DC feeding the arc, and I don't know that would shut off inverter (and disconnect galvanic connection to L1/L2) without arc-fault.

 

[rhino]

The Victron MPPT RS has this in the manual:

3.2. MPPT grounding, detection of PV array insulation faults & Earth fault alarm notification
The RS will test for sufficient resistive isolation between PV+ and GND, and PV- and GND.
In the event of a resistance below the threshold (indicating an earth fault), the unit will stop charging and display the error.

So this still would not do anything for series arc fault, correct?

 

[400bird]

The Victron MPPT RS has this in the manual:

3.2. MPPT grounding, detection of PV array insulation faults & Earth fault alarm notification
The RS will test for sufficient resistive isolation between PV+ and GND, and PV- and GND.
In the event of a resistance below the threshold (indicating an earth fault), the unit will stop charging and display the error.

So this still would not do anything for series arc fault, correct?

Nope, ground fault detection is designed to find ground faults, not arc faults. Two separate detection methods. Maybe if the arc happened to go to ground, but that's unlikely unless the arc happen very close to something grounded.

 

[SeaGal]

I don't know what the requirements are outside of the US, but my impression is the Arc Fault requirements are not as stringent outside the US.

Not a requirement in UK for Solar. In fact arc-fault detection is only just being mandated for new buildings on regular AC circuits for buildings of high risk... "arc fault detection is now mandatory on all socket circuits up to and including 32 Amps within higher risk residential buildings, such as buildings containing several floors, houses of multiple occupation, purpose-built student accommodation, and care homes"

Having said that, if one of the value-priced inverter companies would release a product that had Arc Fault and Ground Fault protection built in, I think it would be a big hit in the DIY community as long as it did not jack up the prices much.

The Solis Hybrid AIO has arc-fault detection - but does false trigger!

 

[zanydroid]

Hmm. I've been trying to game out whether Victron will ever go for UL9540 / other residential standards in the US (they did list the 48V MultiPlus 3000 recently). It feels like not having a compliant SCC will make this kind of hard. Unless only the inverter/charger portion of the system needs to be considered for UL9540 (feels kind of weird to me since the SCC is joined at the hip with the battery too).

 

[SeaGal]

But I read your interpretation as saying if not on buildings, only wire in metal raceway or underground avoided arc-fault requirement.
Therefore, if MC cables between panels, arc-fault still required.

I presented this to show such an interpretation would be absurd (not to say that is not what the words might mean.)

Good point and goes to show how even the clearer 2023 version isn't that clear. For 2020 guess it depends what is meant by 'output' in "PV output circuits".

We think not. If PV array on ground mount and DC wires in metal conduit from array to inverter either on outside wall or inside house, because DC circuits are "in or on a building", arc-fault is required per NEC 2023.

So I see it as saying conversion from DC to AC must occur on a non-building structure. Maybe rigid conduit pops up out of the ground one foot away from house, hybrid inverter is mounted on that conduit, AC (and possibly battery DC) enters house.

The thot plickens... glad I don't live in the USA!

FWIW, the intent of the 2020 code (and by my read the 2023 code, but equally poorly written) is to requrire the PV wiring *within the occupied structure* to be in metal conduit or raceways...

I agree.