Disco Needed when PV and Inverter on Separate Buildings?

[PWDickerson]

When I started as a PV designer 12 years ago, I remember that there was a code requirement that, in the case where the inverter and the solar array were located in/on separate structures, a DC disconnect was required at the building with the solar array. I remember that code section being not so easy to find, and I suspect that it disappeared when rapid shutdown was invented. But I would sure like to confirm that. I have looked back through my 2008 and 2011 code books, and I can't find that older requirement. Does anybody else remember that? Can you point me to the code section?

 

[jaggedben]

I don't recall anything being in 690 but I think a strong argument can be made that the relevant requirements in Article 225 apply. Perhaps a strong counter argument could also be made. It would come down to whether or not the DC circuit is a feeder: essentially, it meets the definition, unless maybe 690 definitions turn it into something that isn't a feeder.

In most cases I would probably install the disconnect. Its safer if you need to put the array wiring not-under-load for service, especially if the distance to the inverter is far.

 

[electrofelon]

I don't recall anything being in 690 but I think a strong argument can be made that the relevant requirements in Article 225 apply. Perhaps a strong counter argument could also be made. It would come down to whether or not the DC circuit is a feeder: essentially, it meets the definition, unless maybe 690 definitions turn it into something that isn't a feeder.

In most cases I would probably install the disconnect. Its safer if you need to put the array wiring not-under-load for service, especially if the distance to the inverter is far.

If the PV was a ground mount, would you think that would be different or would you be less inclined to have a Disco at the array?

 

[jaggedben]

If the PV was a ground mount, would you think that would be different or would you be less inclined to have a Disco at the array?

Actually I assumed it was a ground mount.

If it's a building then you need a rapid shutdown initiator at the building. A disconnect would probably do that, although there might be other options.

If it's a ground mount or other structure that doesn't require rapid shutdown, then code aside for my personal taste it would depend if the inverter is a short walk and within sight or not, along with the economics as well. Short walk and within sight then a disconnect is just if AHJ requires it. Otherwise I'd think about it. I mean in some troubleshooting situations $500 worth of disconnect could save that much worth in time, especially if you're a company owner who also does service but is not making money when you're doing service.

BTW OP said DC. If the feed to the structure is AC then I think 225 kicks in clearly.

 

[PWDickerson]

The situation we sometimes run across is when we are installing a solar array on a detached garage (not a dwelling, so no need for an exterior RS switch), and the inverter is located at the home, which also has a solar array. I am almost certain that there used to be an explicit requirement for a DC disconnect at the garage for the garage array. Like I said, I think it was removed at some point, but I can't seem to find it in my old code books. The one we wrapped up last week only had a 10' separation between buildings, and we didn't install a DC disconnect at the detached garage.

 

[jaggedben]

690.12 for Rapid Shutdown applies to 'buildings' not 'dwellings' so good luck continuing to avoid RS for those detached garages.

I'm positive the intention was for it to apply equally to commercial and residential spaces, hence not only dwellings. But perhaps an exception could be pushed through for structures with no habitable or work space. In my mind a solar carport should absolutely be exempt but the code case is not quite bulletproof under the NEC definition of a building. A detached garage is even harder to make that case for.

 

[PWDickerson]

I didn't mean to suggest that RS was not required for PV on a garage, just that the location of the RS switch in a non-dwelling can be either inside or outside.

 

[jaggedben]

Interesting point.

 

[ggunn]

Actually I assumed it was a ground mount.

If it's a building then you need a rapid shutdown initiator at the building. A disconnect would probably do that...

A DC disconnect might not do it even if there are RSD boxes on the modules. Remember, when RS first came in, SolarEdge thought they were compliant, but the DC voltage did not fall quickly enough when their optimizers shut down. This was due to trapped potential on the conductors; until they modified their inverters we had to install bleed resistors between the + and - conductors for the system to be RSD compliant.

 

[PWDickerson]

I remember that too. I think they addressed that pretty quickly though.

 

[ggunn]

I remember that too. I think they addressed that pretty quickly though.

They did, but it showed that simply opening a DC disco won't necessarily initiate a code compliant rapid shutdown.

 

[jaggedben]

They did, but it showed that simply opening a DC disco won't necessarily initiate a code compliant rapid shutdown.

With SolarEdge it always would have complied for the building with the optimizers, although proving it might have required showing the AHJ directly. It doesn't work for the building with the inverter. But that's not where we're talking about having a disco.

With other DC RSD hardware like AP smart that uses a PLC signal to a box that's not at the building, I believe it would also work.

 

[ggunn]

With SolarEdge it always would have complied for the building with the optimizers, although proving it might have required showing the AHJ directly. It doesn't work for the building with the inverter. But that's not where we're talking about having a disco.

With other DC RSD hardware like AP smart that uses a PLC signal to a box that's not at the building, I believe it would also work.

It wasn't a matter of the optimizers not responding when the AC power went off, it was that the DC conductors had no path to ground when the inverter disconnected from the DC lines. Do AP Smart and Tigo boxes provide one when they go into RS mode?

 

[jaggedben]

It wasn't a matter of the optimizers not responding when the AC power went off, it was that the DC conductors had no path to ground when the inverter disconnected from the DC lines. Do AP Smart and Tigo boxes provide one when they go into RS mode?

It has nothing to do with a path to ground per se and the SolarEdge compliance solution on older inverters didn't provide a path to ground. It provided a resistor between the DC conductors to discharge energy from capacitors, presumably as heat.

That said, I looked up some pictures and realized I was probably wrong. The resistor is on the array side of the inverter DC disconnect, so the problematic capacitors are apparently in the optimizers, not the inverter. Unless SolarEdge supports an additional disco with resistor or something, a DC disco at a remote building likely isn't compliant.

I'm pretty sure that the AP smart devices consist merely of a power transistor or something similar (along with some communications electronics) that choke off the DC voltage from the panels when they no longer get their 'heartbeat' communication from the control unit. They don't do optimization, and I have no reason to believe they contain any problematic capacitors. So a DC disco would work for them, I believe.

I don't know about Tigo.

 

[ggunn]

It has nothing to do with a path to ground per se and the SolarEdge compliance solution on older inverters didn't provide a path to ground. It provided a resistor between the DC conductors to discharge energy from capacitors, presumably as heat.

Yes, you are correct; I misspoke. The path is + to - , not a path to ground for either conductor. For the record I didn't say that it would definitely be a problem, only that it might be. I don't know of any testing done on systems using AP or Tigo RS devices, just the problem with SolarEdge systems before they modified their inverters to provide the bleed path.