Ungrounded Vs Grounded Inverters

[jaggedben]

Transformerless, isolationless , galvanically conducting inverters rely on fast, current limiting fuses to kill direct faults and GFCI to kill high resistance faults.

Sounds like pretty good safeguards to me.

This discussion ensued because you said isolated inverters are more hazardous. (post #5) You've offered nothing substantive to support that assertion. In particular, you've pointed to nothing that is nearly as hazardous as the GFDI 'blind spot' in isolated inverters with a fuse-type GFDI. Not to mention the greater hazards associated with installing and servicing grounded PV systems.

 

[ggunn]

Sounds like pretty good safeguards to me.

This discussion ensued because you said isolated inverters are more hazardous. (post #5) You've offered nothing substantive to support that assertion. In particular, you've pointed to nothing that is nearly as hazardous as the GFDI 'blind spot' in isolated inverters with a fuse-type GFDI. Not to mention the greater hazards associated with installing and servicing grounded PV systems.

You mean non-isolated, don't you?

 

[electrofelon]

With respect to a PV inverter, you've still got to consider the fact that the device that is closing the circuit is a device that only closes the circuit if all the voltage and frequency parameters are checking out every handful of cycles. So yes, the current will subtract, but that physically entails the voltage going out of whack, and pretty much as soon as that happens the circuit will be opened and the fault cleared. And the GFDI and anti-islanding shutdown requirements for the devices are really fast, faster than max trip times for thermal magnetic breakers, as far as I can tell.

If you have a fault that subtracts less than the inverter output current, it will get caught by the GFDI; if the fault occured while the inverter wasn't operating (at night), then the inverter won't even turn on and there will never be an AC fault. If you have an AC fault that is larger than the inverter is rated for, it would pretty much have to be from some freak occurrence causing sudden major damage to the wiring while it was operating. (Say, a tree falling on the array). A blown inverter is likely to be the least concern.

Finally: Even if Electric-Light is completely right about the possiblity of a non-isolated inverter getting destroyed by utility AC flowing through it, that is a safer, less costly outcome than an isolated PV system setting a building on fire because the first fault in the grounded conductor was never detected. The latter has been documented publicly. The former, to my knowledge, has not.

I don't disagree with all that, nor do I think TL inverters are unsafe/less safe, nor that this utility fault through the inverter thing is an issue. I was just agreeing with EL on the principle that a utility fault could flow through a non isolated doohickey regardless of current flow and direction from other systems - its a path back to the source and all that matters is the impedance through the supply (ignoring any controls to sense and shut down such an event).

 

[jaggedben]

You mean non-isolated, don't you?

Yeah. :slaphead:

 

[SolarSavage]

at work

at work

Just wanted to let you guys know that I have been sharing this information at work and we are all enjoying the knowledge/discussion.


Thanks to all of you awesome folks that helped us understand all this wonderful information. It helps us do our work correctly and keep everyone safe!


Im like almost half way through the responses ahaha, its a rather long 9 page read! :jawdrop:

Im sure i will have follow up questions like why did the united states use Transformers in the first place? What does a transformer have to do with grounding?

 

[electrofelon]

What does a transformer have to do with grounding?

A transformer transfers energy through a magnetic field so there is no physical electrical connection between the primary and secondary sides. What this means is that any system grounding you had on the primary side, you will lose on the secondary side. For example take a typical residential service: the neutral conductor is grounded to earth and all the metal stuff so now that system is referenced and has potential to the earth and the metal stuff. Now run that system through a transformer, and the new system will no longer have any reference to earth and metal stuff. The confusion in the NEC that I was complaining about is that usually when the NEC says "grounded/ungrounded SYSTEM" it is referring to system grounding - whether or not a conductor of the system is connected to earth, not which conductors you happen to be using for a circuit. Apparently it is different in article 690.

 

[ggunn]

A transformer transfers energy through a magnetic field so there is no physical electrical connection between the primary and secondary sides. What this means is that any system grounding you had on the primary side, you will lose on the secondary side. For example take a typical residential service: the neutral conductor is grounded to earth and all the metal stuff so now that system is referenced and has potential to the earth and the metal stuff. Now run that system through a transformer, and the new system will no longer have any reference to earth and metal stuff. The confusion in the NEC that I was complaining about is that usually when the NEC says "grounded/ungrounded SYSTEM" it is referring to system grounding - whether or not a conductor of the system is connected to earth, not which conductors you happen to be using for a circuit. Apparently it is different in article 690.

It's the same in 690. On the DC side of an ungrounded PV system, neither the positive nor the negative conductor is connected to ground.

 

[electrofelon]

It's the same in 690. On the DC side of an ungrounded PV system, neither the positive nor the negative conductor is connected to ground.

Again, system grounding isnt about the combination of grounded and ungrounded conductors that you happen to be using, its about whether the system is referenced to earth. Take the secondary conductors of a transformer and ground a conductor to make it a grounded system. Just because you only utilize the ungrounded conductors to a load does not make that system ungrounded. Without galvanic isolation, you dont even have a new system, so its still a grounded system ( assuming ac is grounded)

 

[jaggedben]

Again, system grounding isnt about the combination of grounded and ungrounded conductors that you happen to be using, its about whether the system is referenced to earth. Take the secondary conductors of a transformer and ground a conductor to make it a grounded system. Just because you only utilize the ungrounded conductors to a load does not make that system ungrounded. Without galvanic isolation, you dont even have a new system, so its still a grounded system ( assuming ac is grounded)

I get what your saying. But with respect to NEC language, I think you would need to come up with suitable shorthand for "system with/without a DC grounded conductor". Because the rules (e.g. whether a GEC is required and/or what size) hinge on that, and not whether the system is isolated or not.

As for saying 'you don't even have a new system', well, that's problematic, too. I would agree that with a non-isolated inverter you don't have a 'separately derived system', which is something that is defined in the NEC, but you do have a 'Photovoltaic System', which is also defined in the NEC. Parsing the meaning of 'system' is just hopeless, in my opinion, if you don't have some modifiers and definitions.

Would you be satisfied if the NEC referred consistently 'ungrounded PV systems' vs. 'grounded PV systems' ?

BTW I just realized that the definition for "Photovoltaic System" doesn't appear in my addition of the 2014 NEC. :blink: But that appears to be an editing error, not a CMP decision.

 

[SolarPro]

Again, system grounding isn't about the combination of grounded and ungrounded conductors that you happen to be using, its about whether the system is referenced to earth.

Exactly. This is why the system grounding content has been subject to extensive re-writes as part of NEC 2017. Ungrounded PV systems are referenced to earth via the ac GEC when the system is operating. So these systems are effectively grounded. They just don't have a hard connection to ground on the dc side. However, grounded PV systems don't technically have a hard connection to ground either. The connection to ground in grounded system is accomplished via a fusible link or similar—not via a bolted connection.

When NEC 2017 get released, it should do a better job of describing the range of system grounding used in fielded system. To oversimplify things to make a point, the terminology "grounded" vs. "ungrounded" suggests that there are these black and white system grounding distinctions. But black and white don't really exist in practice. What you have instead are a range of shades of gray.

 

[electrofelon]

I get what your saying. But with respect to NEC language, I think you would need to come up with suitable shorthand for "system with/without a DC grounded conductor". Because the rules (e.g. whether a GEC is required and/or what size) hinge on that, and not whether the system is isolated or not.

I think they should mention isolated/non isolated. I would like to see something like this (this likely isnt perfect I havent really thought a lot about this just throwing it out off the top of my head)

690.41 System Grounding for isolating inverters

informational note: transformerless inverters do not provide isolation between the AC and DC sides, thus the DC side retains the grounding topology of the AC side.

1. The DC system side of an isolating inverter shall be permitted to be ungrounded and shall comply with 690.35
2........

Or something roughly like that. I like the big fat informational note in there to try to get this in the open and cleared up as many people dont understand the isolating concept.

As for saying 'you don't even have a new system', well, that's problematic, too. I would agree that with a non-isolated inverter you don't have a 'separately derived system', which is something that is defined in the NEC, but you do have a 'Photovoltaic System', which is also defined in the NEC. Parsing the meaning of 'system' is just hopeless, in my opinion, if you don't have some modifiers and definitions.

I just think systems being grounded or not, and separately derived systems are a fundamental aspect of electrical systems and used throughout the NEC. I think consistancy would be a good thing here.


Another idea would be to just let the inverter take care of it, (and maybe in the process get rid of the silly GEC and just let the EGC take care of it for all grounding arrangements. Perhaps something like:

690.41 System grounding. A listed inverter or charge controller shall be permitted to establish the system grounding arrangement.

We would need some wording for what to do if there is no inverter or charge controller though

I dont know I have a full time job...and a cat.....its not my job to fix 690

 

[jaggedben]

I think they should mention isolated/non isolated. I would like to see something like this (this likely isnt perfect I havent really thought a lot about this just throwing it out off the top of my head)

Or something roughly like that. I like the big fat informational note in there to try to get this in the open and cleared up as many people dont understand the isolating concept.

Again, not sure why it needs to be in the NEC if it doesn't affect the rules and how you go about following them. The CMP might say "the language is more appropriate for a product standard or instruction manual".

Another idea would be to just let the inverter take care of it, (and maybe in the process get rid of the silly GEC and just let the EGC take care of it for all grounding arrangements. Perhaps something like:

690.41 System grounding. A listed inverter or charge controller shall be permitted to establish the system grounding arrangement.

We would need some wording for what to do if there is no inverter or charge controller though

This stuff is sort of already in there in 690.41, but I agree it could be clearer. In fact it might even make sense to have the NEC require the inverter to establish the grounding, and prohibit field grounding except for non-inverter systems. That's basically the way it works now anyway, and could clear up confusion.

I dont know I have a full time job...and a cat.....its not my job to fix 690

Hey it only needs to take a couple of hours of your time every three years. :lol:
Seriously though, I had some ideas for the 2017 code which I didn't get around to submitting, so yeah.

 

[jaggedben]

I think they should mention isolated/non isolated.... [etc.]

Well it looks like you'll likely get your wish. I just looked up the 'First Draft' for 2017, and WOW is it going to change!

[New Definition]
Reference Grounded PV System.A PV system that has an electrical reference to ground that is not solidly grounded.

Informational Note: The reference to ground is often a fuse, circuit breaker, resistance device, non-isolated grounded ac circuit, or electronic means that is part of a listed ground-fault protection system. Conductors in these systems that are normally at ground potential may have voltage to ground during fault conditions.

690.41 System Grounding.

(A) PV System Grounding Configurations.
One or more of the following system grounding configurations shall be employed:

1. Reference grounded 2-wire PV systems with one conductor referenced to ground
2. Reference grounded bipolar PV systems with the reference (center tap) conductor referenced to ground and meets the requirements of 690.7(C)
3. PV systems not isolated from the inverter output circuit
4. Ungrounded PV systems
5. Solidly grounded PV systems as permitted in 690.41(B) Exception
6. PV systems that use other methods that accomplish equivalent system protection in accordance with 250.4(A) with equipment listed and identified for the use

Ground fault protection has been moved from 690.5 to the section right below that.
Also 690.35 is gone and PV wire is now required by default for all systems, with an exception for solidly grounded systems that can use USE-2.

And...

690.47 Grounding Electrode System.

(A) Buildings or Structures Supporting a PV System.
A building or structure supporting a PV system shall have a grounding electrode system installed in accordance with Part III of Article 250.
The PV system equipment grounding conductors shall be connected to the grounding electrode system of the building or structure supporting the PV system by means of the grounding electrode conductor, or the grounding bus of associated equipment connected to the grounding electrode conductor. This connection shall be in addition to any other equipment grounding conductor requirements in 690.43(C). The PV system equipment grounding conductors shall be sized in accordance with 690.45.
For solidly grounded PV systems, as permitted in 690.41(A)(5), the grounded conductor shall be connected to a grounding electrode system with a grounding electrode conductor sized in accordance with 250.166.

Informational Note: Previous versions of this Code treated all grounded PV systems as solidly grounded systems and therefore required a dc grounding electrode conductor to be connected to grounded PV system dc circuits. Since most PV systems installed in the past decade are not solidly grounded, this Code narrows the requirement for a dc grounding electrode to only those that are actually solidly grounded in accordance with 690.41(A)(5). All other PV system grounding configurations listed in 690.41(A) do not require a dc grounding electrode conductor.

(B) Additional Auxiliary Electrodes for Array Grounding.
Grounding electrodes shall be permitted to be installed in accordance with 250.52 and 250.54 at the location of ground- and roof-mounted PV arrays. The electrodes shall be permitted to be connected directly to the array frame(s) or structure. The grounding electrode conductor shall be sized according to 250.66. The structure of a ground-mounted PV array shall be permitted to be considered a grounding electrode if it meets the requirements of 250.52. Roof mounted PV arrays shall be permitted to use the metal frame of a building or structure if the requirements of 250.52(A)(2) are met.

That's the whole 690.47 in this draft.

 

[electrofelon]

Well it looks like you'll likely get your wish. I just looked up the 'First Draft' for 2017, and WOW is it going to change!




Ground fault protection has been moved from 690.5 to the section right below that.
Also 690.35 is gone and PV wire is now required by default for all systems, with an exception for solidly grounded systems that can use USE-2.

And...



That's the whole 690.47 in this draft.

Wow exciting, a new NEC term!

 

[Carultch]

Well it looks like you'll likely get your wish. I just looked up the 'First Draft' for 2017, and WOW is it going to change!

Ground fault protection has been moved from 690.5 to the section right below that.
Also 690.35 is gone and PV wire is now required by default for all systems, with an exception for solidly grounded systems that can use USE-2.

And...


That's the whole 690.47 in this draft.

Thank you for sharing this. It looks like they've cleaned up 690.47 very well.

Can you or anyone give an example of a "solidly grounded PV system"? To me it seems that all systems built since I was born, are either grounded through a ground-fault fuse/breaker, or ungrounded by design. And the standard ground fault protection device in grounded systems, makes it no longer solidly-grounded.

 

[Carultch]

Also 690.35 is gone and PV wire is now required by default for all systems, with an exception for solidly grounded systems that can use USE-2.

Except when you use a multiconductor non-metallic cable for this wiring, like TC, SER, SEU, or UF. 690.35(D)(1) allows this. Did this rule go somewhere else?

 

[jaggedben]

Thank you for sharing this. It looks like they've cleaned up 690.47 very well.

Can you or anyone give an example of a "solidly grounded PV system"? To me it seems that all systems built since I was born, are either grounded through a ground-fault fuse/breaker, or ungrounded by design. And the standard ground fault protection device in grounded systems, makes it no longer solidly-grounded.

I've never seen one, but I understand that some old-school off-grid battery systems were solidly grounded with no GFDI. Then the NEC required a GFDI and that was nipped in the bud.

The only solidly grounded systems that would be allowed under the 2017 First Draft would be those included in the current exception to 690.5, namely ground mounted and pole mounted arrays where the DC is isolated from buildings. I don't know why one would do this anymore, but it would be allowed.

Except when you use a multiconductor non-metallic cable for this wiring, like TC, SER, SEU, or UF. 690.35(D)(1) allows this. Did this rule go somewhere else?

Those specific wire types are not mentioned but multi-conductor cables 'listed for the purpose' are allowed. Hmmmm..

It's all part of 690.31 now.

 

[SolarAlternativesDesign]

One major difference is that ungrounded PV systems require that if DC fusing is necessary both the positive and negative conductors must be fused. In an ungrounded system the negative DC conductors are not grounding conductors.

Just to understand the principle, why would a fuse be needed on both the PV + and PV - conductor on a PV source circuit? If overcurrent were being fed into a module, wouldn't a fuse on the PV + conductor alone blow and open the circuit and protect it?

 

[ggunn]

Just to understand the principle, why would a fuse be needed on both the PV + and PV - conductor on a PV source circuit? If overcurrent were being fed into a module, wouldn't a fuse on the PV + conductor alone blow and open the circuit and protect it?

The main principle at work here is that the code says both positive and negative conductors need fusing.

 

[SolarPro]

Just to understand the principle, why would a fuse be needed on both the PV + and PV - conductor on a PV source circuit? If overcurrent were being fed into a module, wouldn't a fuse on the PV + conductor alone blow and open the circuit and protect it?

You are correct. NEC 2017 will only require fusing on one pole of an array, regardless of the inverter topology.

Check out this article by Bill Brooks:

NEC 2017 Updates for PV Systems

Another significant change is that NEC 2017 will require breaking both poles of the array at disconnecting means. Both of these changes relate to the new functional grounded PV system definition, which clarifies that the vast majority of the PV systems we install are neither solidly grounded or truly ungrounded. So we just need one set of design standards.