Section 690.47: Grounding

Grouch1980

Senior Member
Location
New York, NY
Hi all,
I have a few questions on section 690.47.. a, b, and c. the 2008 NEC. Electrofelon asked these questions on a post from a few years ago, however I wasn't able to follow all the answers. Either this section is not clear, or my brain is too smooth with no wrinkles.

for part a: does this have to do with AC modules / microinverters? if so, how are microinverters grounded? is there a diagram somewhere?
for part b: what exactly is a DC system? so there's no AC output? how does this work?
for part c: i'm assuming this has to do with regular inverters with integral transformers? DC input, with an AC output?

Thanks!
 

romex jockey

Senior Member
Location
Vermont
Occupation
electrician
yes iirc..... 'solidly grounded' Vs 'functionally grounded' was bantered about, along with a CMP waffling over PV's are a SDS or not an SDS

fwiw, I've had this same conversation with my ahj

~RJ~
 

Grouch1980

Senior Member
Location
New York, NY
I think I got it, after reading all the responses on that particular post. It just irks me that i have to read things 1000 times until i understand it.
part a: yes, it has to do with the micro inverters.
part b: it's purely a DC system... so from what I gathered, there are no AC loads. everything is DC. so yes, there is no AC output.
part c: this includes all types of inverters. whether transformer or transformerless based. DC in, AC out.
 

jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
It doesn't have to do with micro-inverters per se.

Try this...

If there's an inverter then there's a need for AC grounding.

If it's AC modules (i.e. no DC field wiring) then it's an 'AC system' only, no DC grounding requirement. If you buy microinverters and modules separately you aren't buying AC modules.

If there's a DC grounded conductor, either solidly grounded or 'functionality grounded' according to later codes, you have a DC grounding requirement. (Or if it's a stand alone system with DC field wiring, you'll need to bond equipment to premises grounding.)

You're really going to have a problem applying the 2008 code logically to present day inverters which are pretty much all ungrounded (or 'functionally grounded' through the premises AC grounding). However, mostly what it amounted to back then was treating all the ground wires like GECs, minimum #8 with bonding on both ends.
 

Grouch1980

Senior Member
Location
New York, NY
Thanks for the clarification... i see what you mean about the AC modules vs buying the microinverters separately.

Once I get through this section, 690.47, i'll delve into later codes to read up on the functionally grounded and other terminology.

Is my part b interpretation correct however? when it's purely a DC system, meaning it only feeds DC loads?
 
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jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
If I recall correctly (I don't have the 2008 book), section (b) applies whether or not the system only feeds DC loads or also has an inverter. Unless you have AC modules then you have a DC grounding requirement. Section (c) tells you how you can combine AC and DC requirements. At least, that's how it is in the 2011.

Back when the 2008 was in widely in force it was common to ground one of the DC conductors in the inverter using a fuse, typically 1A for small systems. The fuse was supposed to blow if there was a DC ground fault, and the system would sense this and either stop operation or open the DC circuit. This met the ground-fault-detection-and-interruption requirement. The conductor grounded this way was considered a grounded conductor and required a grounding electrode conductor on the ground side of the fuse. The inverter usually had a designated grounding electrode conductor terminal, and you would run that GEC according to the (b) or (c) section you're studying, if I recall correctly. Nowadays this is considered one way to 'functionally ground' a system, and such inverters afaik are no longer being made.

For small off-grid systems under 50V it is still common to solidly ground the DC negative, especially if there is no inverter.
 

Grouch1980

Senior Member
Location
New York, NY
If I recall correctly (I don't have the 2008 book), section (b) applies whether or not the system only feeds DC loads or also has an inverter. Unless you have AC modules then you have a DC grounding requirement. Section (c) tells you how you can combine AC and DC requirements. At least, that's how it is in the 2011.
this is the 2008 NEC section b).... "if installing a dc system, a grounding electrode system shall be provided in accordance with 250.166 for grounded systems or 250.169 for ungrounded systems. The grounding electrode conductor shall be installed in accordance with 250.64."

The inverter paragraph i believe you're mentioning was added in 2011. so it looks like in 2008 section b) was only for DC loads. Would that be right?

I feel so behind still being on the 2008 NEC lol.
 

Grouch1980

Senior Member
Location
New York, NY
If there's a DC grounded conductor, either solidly grounded or 'functionality grounded' according to later codes, you have a DC grounding requirement. (Or if it's a stand alone system with DC field wiring, you'll need to bond equipment to premises grounding.)

You're really going to have a problem applying the 2008 code logically to present day inverters which are pretty much all ungrounded (or 'functionally grounded' through the premises AC grounding). However, mostly what it amounted to back then was treating all the ground wires like GECs, minimum #8 with bonding on both ends.
Now that I'm going through the grounding sections, am I interpreting the 2008 code correctly?.... so it seems like it calls for a solidly grounded PV system, while also calling for ground fault protection via a fuse... which wouldn't make the system 'solidly' grounded. Is that what pushed for the later codes to define 'functional' grounded systems?... so that it would make more sense since the GF fuse is there?
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
Now that I'm going through the grounding sections, am I interpreting the 2008 code correctly?.... so it seems like it calls for a solidly grounded PV system, while also calling for ground fault protection via a fuse... which wouldn't make the system 'solidly' grounded. Is that what pushed for the later codes to define 'functional' grounded systems?... so that it would make more sense since the GF fuse is there?
When I was working under the 2008 NEC we used to drive a ground rod at the inverter(s) bonded to the EGC back to the service..
 

jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
Now that I'm going through the grounding sections, am I interpreting the 2008 code correctly?.... so it seems like it calls for a solidly grounded PV system, while also calling for ground fault protection via a fuse... which wouldn't make the system 'solidly' grounded. Is that what pushed for the later codes to define 'functional' grounded systems?... so that it would make more sense since the GF fuse is there?
I don't think the 2008 or 2011 code uses thr phrase 'solidly grounded'. That distinction came later. The vagueness back in '08 allowed precisely what you descibe, i.e. 'grounding' a DC conductor through a fuse that would leave the system ungrounded when it blew. UL was fine with it. The code was fine with it. There's even a code proscribed warning label that if a ground fault is indicated, normally grounded conductors may be ungrounded and energized.

These days they don't make inverters thay way any more, at least not that I know of under 30kW or so. Typically neither DC conductor is grounded but the system is 'functionality grounded' because there's a voltage reference to the AC grounded conductor when operating.

Everything I'm about here is about whether or how there's DC system grounding, not grounding electrode stuff.
 

Grouch1980

Senior Member
Location
New York, NY
I don't think the 2008 or 2011 code uses thr phrase 'solidly grounded'. That distinction came later.
I can definitely say the 2008 code did use the phrase solidly grounded... i'm looking at it now actually... 690.41: For a PV power source, one conductor of a 2-wire system with a PV system voltage over 50 volts and the reference conductor of a bipolar system shall be solidly grounded.
 

Grouch1980

Senior Member
Location
New York, NY
These days they don't make inverters thay way any more, at least not that I know of under 30kW or so. Typically neither DC conductor is grounded but the system is 'functionality grounded' because there's a voltage reference to the AC grounded conductor when operating.
Now i'm lost again. i thought functional grounded meant through the use of a GF fuse. This is going to take a while for me to get this all. I need diagrams lol.
 

jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
I can definitely say the 2008 code did use the phrase solidly grounded... i'm looking at it now actually... 690.41: For a PV power source, one conductor of a 2-wire system with a PV system voltage over 50 volts and the reference conductor of a bipolar system shall be solidly grounded.

Quote the rest of the sentence. "... or shall use other methods that accomplish equivalent system protection..." and it goes on to cite 250.4(A) and 690.35 in an exception.

Also look at 690.5.
 

Grouch1980

Senior Member
Location
New York, NY
What's functional grounded? Is this correct? from an article: "First, the advent and ultimate requirement of DC ground fault protection in PV systems led to overcurrent protection (OCP) becoming a part of the system ground connection. A system with OCP on the ground conductor is no longer in accordance with the coded definition of a solidly grounded electrical system. In essence these systems are functionally rather than solidly grounded."

So does functional grounded mean your system is grounded via the ground fault protection? or something else?
 

jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
Now i'm lost again. i thought functional grounded meant through the use of a GF fuse. This is going to take a while for me to get this all. I need diagrams lol.
Functionally grounded can take multiple forms. The fuse was one. So yes, " if your system is grounded via the ground fault protection?" then it is functionally grounded. But the converse is not true, meaning if it it is not grounded through the GFDI it may still be functionally grounded. The other common form of it is a non isolated inverter (i.e. no transformer between DC input and AC output). On these inverters if you measure the DC terminals to ground when itnis operating you'll get an AC voltage that is predictable if you know the DC voltage. Thus they are still functionally grounded according to the code's definition at the beginning of 690, even if 690.41 isn't very explicit about that.

Since you are most likely going to be using ungrounded inverters, in the 2008 code you need to show inspectors 690.35. Ungrounded Systems. All the systems you'll find that are being produced today should meet those requirements. Hopefully you can convince inspectors that you don't need a grounding electrode conductor from you inverter to a ground rod or something, bit worst case you'll do that like ggunn described.
 

Grouch1980

Senior Member
Location
New York, NY
Since you are most likely going to be using ungrounded inverters, in the 2008 code you need to show inspectors 690.35. Ungrounded Systems. All the systems you'll find that are being produced today should meet those requirements. Hopefully you can convince inspectors that you don't need a grounding electrode conductor from you inverter to a ground rod or something, bit worst case you'll do that like ggunn described.
Just curious, if the inverter is the ungrounded type, how come an inspector would say a GEC is needed?
 

jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
Just curious, if the inverter is the ungrounded type, how come an inspector would say a GEC is needed?
Because you're on the 2008, which was kind of written as if they all needed one. You'll need to appeal to later codes to show an inspector language that supports just having a EGC for a grid tied system.
 

Grouch1980

Senior Member
Location
New York, NY
Because you're on the 2008, which was kind of written as if they all needed one. You'll need to appeal to later codes to show an inspector language that supports just having a EGC for a grid tied system.
Hmmm not following. in the 2008, section 690.35, it doesn't call for a GEC... I'm sure i'm missing something.
 

Carultch

Senior Member
Location
Massachusetts
What's functional grounded? Is this correct? from an article: "First, the advent and ultimate requirement of DC ground fault protection in PV systems led to overcurrent protection (OCP) becoming a part of the system ground connection. A system with OCP on the ground conductor is no longer in accordance with the coded definition of a solidly grounded electrical system. In essence these systems are functionally rather than solidly grounded."

So does functional grounded mean your system is grounded via the ground fault protection? or something else?

"Functionally grounded" is a term that originated in the 2017 NEC, that essentially made most systems fit this term. Even systems that were previously distinguished as grounded and ungrounded, both get considered functionally grounded from 2017 and later. It ultimately means that there is an indirect reference of the DC side of the system to ground.

The kinds of systems that are not functionally grounded would be
1. Systems that are solidly grounded
2. Systems that use non-isolated inverters, and are connected to ungrounded AC grids.

Since most grounded systems use a GFCI method to bond the grounded polarity and EGC, rather than a solid bond, these classify as functionally grounded. It is rare that a grounded system has a solid connection to ground, since GFCI has been a requirement for decades, and this was the way it used to always be done. These become ungrounded and turn off the inverter, when there is a ground fault. Under ordinary conditions, these have one polarity grounded, and the other polarity live, and generate a waveform with one of its peaks at zero, rather than a waveform that is symmetric. To remove the DC offset, they integrate a transformer in the inverter, so that the output can be symmetric about ground.

Systems that use non-isolated inverters were previously known as ungrounded, and are now called "functionally grounded" (I don't agree with this term in this meaning, but it is what we have) when connected to a grounded AC grid. The functional grounding happens means of not isolating the DC side from the AC side, such that the DC polarities are equal and opposite about ground, in order to generate the positive and negative halves of the waveform. The inverter doesn't need an isolation transformer, because the waveform doesn't start with a DC offset.
 
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