Ungrounded Vs Grounded Inverters

[SolarSavage]

Hello all,

My Co-worker and I are trying to figure out the difference between un-grounded and grounded inverters. I believe the difference is the Inverter itself, being transformer or transformer-less. But we want to know the exact reason why. I have done some reading and have figured out that it has something to do with the grounding conductors on the DC side. European systems tend to use transformer-less inverters and have no issues with grounding since it is not a requirement for them. So we see a natural evolution of transformer-less inverters that are being introduced to the American market currently.

Thank you to anybody willing to help. BTW this forum is awesome!

 

[Carultch]

Hello all,

My Co-worker and I are trying to figure out the difference between un-grounded and grounded inverters. I believe the difference is the Inverter itself, being transformer or transformer-less. But we want to know the exact reason why. I have done some reading and have figured out that it has something to do with the grounding conductors on the DC side. European systems tend to use transformer-less inverters and have no issues with grounding since it is not a requirement for them. So we see a natural evolution of transformer-less inverters that are being introduced to the American market currently.

Thank you to anybody willing to help. BTW this forum is awesome!

Three names for (usually) the same thing:
Transformerless Inverter, not to be confused with an external transformer inverter, like Solectria XTM
Ungrounded DC system
Non-isolated Inverter

And their opposites:
Inverter with isolation transformer
Grounded DC system
Isolated Inverter

The idea with a transformerless inverter, is that it uses power electronics and capacitors/inductors only, in order to generate the AC from the DC. There is electrical continuity throughout the circuit (except the dielectric gaps in capacitors). Unlike with a transformer, where the electric power is converted through magnetism to a different electric circuit.

Transformerless inverters have an ungrounded DC system, where both the positive and negative wires are energized at a significant voltage. Usually it floats close to an equal and opposite pair of voltages, on both sizes of the ground voltage. But there is no guarantee, as there is no connection to ground to define where the DC system has to be. Don't be confused by the name ungrounded, and think you don't need an equipment ground or module grounding system. You still do. It is just that there is no current-carrying wire that is fixed at ground voltage in the inverter. It might be more descriptive (though not an official term) to think of these as an "unneutralled" DC system, instead of ungrounded system, if you are confused with the fact that there is still an EGC/rack/module grounding system.

Inverters with an isolation transformer, fix one of the DC polarities at the same voltage as ground. Usually the negative, although occasionally positively grounded systems arise. It is more fancy than a direct connection, as it the grounded conductor ("neutral") is connected to the grounding conductor (EGC/GEC system) thorugh a ground fault fuse or breaker. This reference to ground is independent of the output AC circuit, because the transformer isolates the absolute voltages in each, only transmitting the time variation in the internal AC to the AC output.

For balance of systems components (combiners, disconnects, rapid shutdown, AFCI, etc), OCPD, disconnecting means and other circuit protection will be necessary on BOTH polarities in an ungrounded system. It is not necessary on a grounded system, and in fact is required to only occur on the ungrounded conductor(s).

 

[ggunn]

Hello all,

My Co-worker and I are trying to figure out the difference between un-grounded and grounded inverters. I believe the difference is the Inverter itself, being transformer or transformer-less. But we want to know the exact reason why. I have done some reading and have figured out that it has something to do with the grounding conductors on the DC side. European systems tend to use transformer-less inverters and have no issues with grounding since it is not a requirement for them. So we see a natural evolution of transformer-less inverters that are being introduced to the American market currently.

Thank you to anybody willing to help. BTW this forum is awesome!

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.

 

[Carultch]

Continued...

Unlike their grounded counterparts, transformerless inverters do not detect ground faults with a fuse or breaker, but instead use a set of CT's to make sure that the incoming and outgoing currents add up to zero. Upon a non-zero net current detected, it shuts off the inverter.


A few other system design considerations:
Ungrounded systems require specific labeling
Ungrounded systems require PV wire or multiconductor cables where outdoor and exposed. Grounded systems can use conventional USE-2, or any wire type permitted for ungrounded systems.
Ungrounded systems are prohibited from using white or gray as colors for wire or wire re-identification. These colors are reserved for the grounded conductor. Red(+) and black(-) is the recommendation for ungrounded conductors.
Ungrounded systems should not require a GEC, and a correct inverter wouldn't even have a terminal for it. The AC side EGC alone satisfies the purpose. The language in the NEC regarding this, is not as straightforward as we'd like it to be, so it can be difficult to defend this design decision to project stakeholders. Many TL inverters still have the GEC terminal, as either a grandfathered product, or a "better to have and not need, than need and not have" manufacturing decision.

 

[Electric-Light]

The idea with a transformerless inverter

In non-engineer speak...

Is cheaper to make/lower BOM, somewhat lower losses, but more hazardous. The same non-isolated type SMPS are often used for disposable CFL and LED ballasts.

It's a "hot chassis" design and there's no galvanic isolation between the line side and the panel side, so any contact of any part of wiring on the solar side with a grounded surface can fault, therefore, each side may require a fuse with appropriate interruption ratings.

Unlike their grounded counterparts, transformerless inverters do not detect ground faults with a fuse or breaker, but instead use a set of CT's to make sure that the incoming and outgoing currents add up to zero. Upon a non-zero net current detected, it shuts off the inverter.

Any contact with any raw surface of the panel or the wiring will cause a shock, so it uses a RCD/GFCI to minimize risk to personnel.

 

[jaggedben]

In non-engineer speak...

Is cheaper to make/lower BOM, somewhat lower losses, but more hazardous.

In my opinion, the difference in UL standards make ungrounded inverters much, much safer.

The same non-isolated type SMPS are often used for disposable CFL and LED ballasts.

Not exactly comparable in terms of the dangers because there aren't two power sources.

 

[jaggedben]

Unlike their grounded counterparts, transformerless inverters do not detect ground faults with a fuse or breaker, but instead use a set of CT's to make sure that the incoming and outgoing currents add up to zero. Upon a non-zero net current detected, it shuts off the inverter.

I believe they use Hall effect sensors, since this is DC and thus CTs wouldn't work.

 

[jaggedben]

Hello all,

My Co-worker and I are trying to figure out the difference between un-grounded and grounded inverters. I believe the difference is the Inverter itself, being transformer or transformer-less. But we want to know the exact reason why. I have done some reading and have figured out that it has something to do with the grounding conductors on the DC side. European systems tend to use transformer-less inverters and have no issues with grounding since it is not a requirement for them. So we see a natural evolution of transformer-less inverters that are being introduced to the American market currently.

Thank you to anybody willing to help. BTW this forum is awesome!

There are some serious safety differences with respect to installation and service, when the panels are connected to the inverter.
See this thread for a long discussion.

 

[SolarPro]

tl;dr = they cost less & work better

Benefits of Non-Isolated Inverters

As we will show, ungrounded PV systems have additional BOS requirements compared to conventional grounded PV systems. Ungrounded PV systems also require special inverters specifically designed and listed for use with ungrounded arrays. So why would anyone choose to go this route?

It turns out that many of the potential benefits of deploying ungrounded PV systems are specifically associated with the use of non-isolated inverters. The advantages most commonly attributed to non-isolated inverters include higher efficiency, improved economics and increased ground-fault sensitivity.

Higher efficiency. Advanced Energy has sold its bipolar transformerless inverters into commercial and utility-scale PV applications since August 2007. According to Tucker Ruberti, the company’s director of segment marketing, “The most obvious benefit of a transformerless architecture is higher inverter efficiency.” As an example, the weighted CEC efficiency of Advanced Energy’s transformerless Solaron 250 kW inverter is 97.5%, which is 1% higher than that of the company’s transformer-isolated PVP250kW inverter.

While modern transformers are exceptionally efficient, losses that can never be entirely eliminated occur in the core and in the windings. These losses are dissipated as waste heat, which is one of the reasons that 60-Hz transformer-based inverters often have relatively large heat sinks.

Since large transformers are generally more efficient than smaller ones, it is not uncommon to see a higher efficiency differential between isolated and non-isolated inverters at smaller inverter capacities. For example, SMA America’s 8 kW transformerless inverter (SB 8000TL-US) has a CEC efficiency of 98%, which is a full 2% higher than the efficiency of its 8 kW transformer-isolated inverter (SB 8000US). Generally speaking, non-isolated inverters are 1%–2% more efficient than equivalent isolated inverters.

Improved economics. Since non-isolated inverters are more efficient, they have the potential to increase a PV system’s specific yield and improve a customer’s return on investment as a result. In theory, non-isolated inverters should also cost less to purchase, ship and install than isolated inverters. As Wiles explains in Home Power magazine, “The transformer is usually heavy, costly and bulky—decreasing efficiency and increasing the inverter’s size and shipping costs.”

Eliminating the isolation transformer in a utility-interactive inverter may also enable additional savings. Verena Arps is the director of technical sales at REFUsol, an inverter manufacturer with a line of transformerless 3-phase string inverters ranging in capacity from 16 kW to 24 kW. Arps points out that REFUsol’s transformerless inverter topology does not require active cooling: “Because the inverters are more efficient, the internal heating losses are decreased, which allows for the elimination of active cooling components.”

Even when active cooling components are included, non-isolated inverters generally have a lower parts count than their isolated counterparts. A reasonable claim can be made that they have less embodied energy than transformer-isolated inverters since they are smaller and lighter. These same attributes could make them easier to install.

The extent to which the raw material reductions associated with non-isolated inverters translates to up-front cost savings still remains to be seen. In today’s market, a non-isolated inverter may cost about the same as an equivalent isolated inverter from the same manufacturer. However, non-isolated inverters have yet to achieve manufacturing efficiencies of scale. They are still a specialty or niche product compared to isolated inverters. Most industry experts agree that transitioning to non-isolated inverters will eventually drive inverter costs down in North America.

SolarEdge has developed a unique utility-interactive PV system that consists of module-level dc-to-dc power optimizers coupled with proprietary non-isolated inverters. According to John Berdner, the company’s general manager for North America: “Non-isolated inverters offer the best chances for future cost reductions since they do not include the large transformers found in low-frequency transformer-isolated inverter designs and have far fewer components than highfrequency transformer-isolated designs.”

Increased ground-fault sensitivity. When people refer to the safety benefits associated with ungrounded PV systems, they are almost certainly referring to the fact that non-isolated inverters are more sensitive to ground faults than isolated inverters. In a SolarPro magazine article (February/March 2011) identifying the limitations of GFDI systems used in listed isolated inverters, Brooks points out, “The only way to get ground-fault detection below 1 amp as part of the GFP scheme for large PV systems is to unground or resistively ground the array circuit, just as they do in Europe and Japan.” He continues: “Contemporary European inverters, for example, can detect changes in ground current as low as 300 mA, which is an order of magnitude lower than our solidly grounded systems.”

While the differential is less pronounced in residential applications, non-isolated string inverters are still three times as sensitive to ground faults as isolated string inverters. At present, transformer-isolated string inverters up to 15 kW in capacity typically use a 1 A GFDI fuse to provide ground-fault protection. UL 1741 allows higher-capacity inverters to use GFDI fuses with higher ratings. For example, inverters rated more than 250 kW in capacity are allowed to use a 5 A fuse. Since a fuse located between the grounded current-carrying conductor and the ground bond most commonly provides this protection, the time required to open this fuse is determined by the physical response time of the fuse itself, which varies depending on temperature and the amount of current flowing across it during the fault event.

The electronic GFP strategy employed by non-isolated inverters used on ungrounded PV arrays allows for much lower and more consistent current and trip-time settings. Non-isolated inverters 30 kW and below sold on the market today are tested to the current UL CRD requirements of 300 mA maximum fault current and 0.3 second maximum trip time. Additionally, there is a “sudden ground-fault current change and response time” requirement that causes the operation of this protection circuit at levels as low as 30 mA and as quickly as 0.04 seconds. Besides reducing the potential shock hazard in a PV array, this means that ground faults are identified and the fault current is stopped more quickly, before it turns into an arcing fault capable of starting a fire.

Furthermore, since non-isolated inverters test for ground-fault currents at the start of each day— before the inverter goes online—they can detect potential ground-fault conditions before a fault occurs. For example, compromised conductor insulation may first manifest as a high-resistance fault and only later as a low-resistance fault. The groundfault protection scheme used in transformer-isolated string inverters may respond to the low-resistance fault only, whereas the scheme used in non-isolated string inverters is more likely to identify the highresistance fault condition.

SMA America was the first manufacturer to certify non-isolated inverters to UL 1741, using UL as its Nationally Recognized Testing Laboratory (NRTL). Greg Smith, a technical training specialist with the SMA Solar Academy, notes, “Plan checkers and inspectors may mistakenly think that a non-isolated inverter is unsafe because it doesn’t have the isolation transformer in it.” The reality is just the opposite, Smith explains: “Because non-isolated inverters check for PV isolation resistance before connecting to the grid and producing power, current is never flowing in a potentially unsafe array with ground faults.”

Source: Ungrounded PV Systems in the NEC

 

[Carultch]

I believe they use Hall effect sensors, since this is DC and thus CTs wouldn't work.

I didn't think about that distinction. They look like CT's, if you look inside such an inverter.

I see gizmos in the AFCI combiners that look like CT's as well. Are they still CT's, because it is the noise in DC that arc fault detection measures, rather than the DC itself?


The idea is that the ground fault detection system in a TL inverter detects a difference in magnitudes of incoming and outgoing current on the two polarities, rather than tripping upon excessive current from the ground to the grounded polarity by means of an OCPD.

 

[Engineer#4827502]

Everyone calls them CTs (I think it's just familiarity with the term) but they do need to be something like a hall effect sensor to pick up DC current in steady state (a traditional CT would still read a change in DC current).

Inverters seem to have trouble with a common AC/DC ground without an isolation transformer. Grounded inverters with an external transformer are doing the same thing as an inverter with an isolation transformer, just moving the transformer outside of the unit. I have not seen a grounded DC transformerless inverter that allows you to ground the AC system at the inverter. Either the inverter has a neutral that is buried or it is configured as a delta system. They transformer can be grounded. This is, I believe, why most of these types of inverters require galvanic isolation from other units.

The exception to this is bi-polar architecture. These still behave similar to ungrounded DC arrays as the neutral will not carry current if the DC system is balanced.

 

[ggunn]

I have not seen a grounded DC transformerless inverter that allows you to ground the AC system at the inverter.

Can you provide an example of a transformerless inverter that has grounded DC?

 

[Carultch]

Can you provide an example of a transformerless inverter that has grounded DC?

I don't think there exists an example.

The closest thing you would have, which would be a misunderstanding of the term transformerless, would be an external transformer inverter with a grounded DC system, like Solectria XTM series. Built for utility scale applications, with the intent of "eliminating the middleman" of the 480V AC system, when you need a transformer to step up to medium voltage anyway. The transformer has to be built to specific requirements, and would have an exotic secondary voltage (like 380V delta) that is the natural output voltage of the inverter.

 

[Engineer#4827502]

The closest thing you would have, which would be a misunderstanding of the term transformerless, would be an external transformer inverter with a grounded DC system, like Solectria XTM series. Built for utility scale applications, with the intent of "eliminating the middleman" of the 480V AC system, when you need a transformer to step up to medium voltage anyway. The transformer has to be built to specific requirements, and would have an exotic secondary voltage (like 380V delta) that is the natural output voltage of the inverter.

That is what I meant. Most utility scale inverters are 'transformerless' in that they do not have an internal transformer but still require the external transformer to operate correctly.

 

[Carultch]

That is what I meant. Most utility scale inverters are 'transformerless' in that they do not have an internal transformer but still require the external transformer to operate correctly.

Correct. And in the unlikely event that you do have an external transformer type inverter, with a native voltage that happens to match the site utilization voltage, you still cannot directly connect to the grid without an isolation transformer built to the inverter MFR's requirements.

 

[jaggedben]

Inverters seem to have trouble with a common AC/DC ground without an isolation transformer.

Yes, if you ground one side of a (non-bipolar) DC circuit, and then you invert that conductor to an ungrounded AC conductor, what you're doing is creating a dead short every half cycle. If you're inverting to two or three hot conductors, as most inverters are, then it's a dead short all the time to one or another AC conductor.

It's a pretty simple and insurmountable problem. Hence...

I have not seen a grounded DC transformerless inverter...

Right.

The exception to this is bi-polar architecture. These still behave similar to ungrounded DC arrays as the neutral will not carry current if the DC system is balanced.

Yes, bi-polar works because you don't actually invert the DC neutral.

Can you provide an example of a transformerless inverter that has grounded DC?

I don't think there exists an example.

Unless we're talking bi-polar, it can't really exist. At least, not with AC systems that are considered safe in any country in the world, AFAIK.

 

[Electric-Light]

tl;dr = they cost less & work better
source: your own blog page. tl;dr

The component costs are less, cheaper to ship and the local efficiency is higher.
It's a non-isolated static power converter that makes every part of the PV system "hot".
Different wording used by the solar world is making things confusing.

It's cheaper, lighter and more hazardous.

We hear about electrocution involving electric guitars often used with a tube amplifier. The amplifier often uses the hot chassis design to get the B+ supply which is floating high above ground, energize the guitar and fault to ground through the performer.

http://www.dailymail.co.uk/news/art...ed-microphone-performing-stage-Argentina.html

Additionally,
My understanding is that it's a minimalist line commutated inverter with bare minimum filtration components and passes insane levels of Amps-THD like helping a steam roller turn forward with an impact wrench rather than a smooth and steady push.

Solar system owner may have been paid added kWh but maybe simply pushing filtration burden and losses into utility owned equipment rather than their own and there may not be ANY net efficiency gain. Someone like mivey can probably explain this.

https://books.google.com/books?id=qW_OBQAAQBAJ&pg=PA807&lpg=PA807

They have a useful diagrams here in the differences here:

http://www.solarworld-usa.com/~/media/www/files/technical-bulletins/grounded-vs-ungrounded.pdf
Note the emphasis on extra cable insulation requirements that apply to non-isolated static converter.

Connect a bridge rectifier to the outlet. The bridge juggles the output leads between hot and neutral. This means that either output leads can fault to ground.

Replace the bridge with SCRs and DC power can be juggled into the line... but if a DC side wire drops to ground it can form a line-to-ground fault, because the AC side and solar panel side are electrically connected.

If you want to see what it looks like, have a look at page 13
http://ecee.colorado.edu/copec/book/slides/Ch16slide.pdf

Not exactly comparable in terms of the dangers because there aren't two power sources.

It's the same hazard. If you touch the secondary side of uninsulated LED ballast, you can get shocked, because it has a potential relative to earth ground.

 

[Electric-Light]

Inside a power adapter, the utility side and load side are galvanically isolated. It's this isolation that prevents load side from conducting to ground and limiting fault current to the capability of the power supply, not the grid.

The PV panel side of a non-isolated inverter has the same fault current as the utility feeder, limited by the wire impedance and having proper current limiting fuses on each phase and capable of causing the same shock touching a phase conductor without a GFCI.

 

[SolarPro]

It's cheaper, lighter and more hazardous.

Actually, it's lighter, cheaper and safer.

Ground-Fault Protection in Non-Isolated Inverters

Since AHJs occasionally question the safety of ungrounded PV systems, it is helpful to understand how the ground-fault protection system works in a non-isolated inverter. UL developed the increased ground-fault protection requirements for non-isolated inverters in concert with the PV inverter industry. These requirements address the unique conditions that ground faults can present in an ungrounded PV system. The process is under way to formally add these requirements for the testing and listing of non-isolated inverters to the published UL 1741 standard.

The ground-fault protection system used in non-isolated inverters includes a regular test of PV array insulation resistance. This test is performed by an isolation monitor interrupter (IMI), which UL defines as “a device that monitors the insulation resistance of a PV array circuit to ground and prevents energization of the inverter ac output circuit or disconnects an energized output circuit when the PV array input resistance drops below a predetermined level.” The IMI performs the PV array insulation resistance test in the early morning hours, when the PV source-circuit voltage is high but there is not enough current for the inverter to begin operating.

The IMI measures any current leakage between all the conductors in the PV circuit to ground and identifies levels of leakage current above set values. This technique is very similar to the insulation tests that electricians perform on unenergized electrical conductors using portable megohm meters. If the tested source-circuit conductor insulation resistance is below a minimum level, the inverter will not interconnect with the utility. If the source-circuit conductor passes the test, the inverter will initiate its normal startup procedures.

It is common for ground-fault protection systems in nonisolated inverters to use what UL refers to as a functional ground, which is an intentional high-impedance connection between the ungrounded circuits that are being monitored and the equipment-grounding system. This connection exists for the sole purpose of fault detection. Since this intentional high-impedance path only exists when the inverter is operating, the PV system is not solidly grounded, according to the definition in Article 100 of the NEC. UL allows this strategy since it recognizes the role that these detection circuits play in reducing the potential for property damage due to stray ground-fault currents. An NRTL evaluates a non-isolated inverter’s ground-fault protection system, including any functional ground, as part of the product testing and listing.

As described previously, once the inverter is online, if it measures ground-fault current above the maximum level allowed or if it measures a sudden increase in fault current, even at very low levels, it will cease operating and indicate the presence of a fault.

Residual-current detector. Rather than using GFDI fuses to identify and interrupt ground-fault current as is typically done in isolated inverters, non-isolated inverters include a residual-current detector to continuously monitor the PV array. This detector circuit is similar to the ac ground-fault circuit interrupter (GFCI) devices with which most electrical professionals are familiar. Like a GFCI, the residual-current detector in a non-isolated PV inverter is an electronic monitoring circuit that identifies ground-fault current before it reaches destructive levels. Unlike a common household GFCI, this device functions to identify fault currents that could cause damage to property and is not specifically set to levels to protect people from electrical shock.

Residual-current detectors constantly monitor the current in an operating PV power circuit, and associated software looks for any imbalance. The outgoing and returning current in the dc circuit should be offsetting—equal in intensity but opposite in direction. In an ideal circuit, the sum of these currents would equal zero. If the residual-current detector indicates that the currents are imbalanced, then the control logic interprets this as a fault. The most likely fault path is to ground through the equipment-grounding system, but stray current in any other parallel circuit path would also be detected. If an imbalance is indicated, then the inverter ceases to operate and indicates that a ground fault has occurred.

In practice, all PV arrays have some small amount of residual leakage current due to a capacitance effect that is dependent on the specific module, the mounting system and the environmental conditions. This means that a residual-current detector system used in non-isolated inverters cannot actually be set at zero, as this would result in nuisance tripping, especially on very large arrays. However, since the residual-current detector is an electronic protection device, its trip points are much lower than the conventional GFDI fuse ratings commonly found in isolated PV inverters. The UL 1741 Standards Technical Panel, which includes manufacturer representatives, determined that the 300 mA ground-fault trip limit for nonisolated inverters up to 30 kW was adequate to prevent groundfault arcs that could ignite fires.

Note that residual-current detection does not provide overcurrent or short-circuit protection. On the ac side of the system, this protection is provided by the overcurrent-protection device required in NEC Section 705.12(D). Overcurrent protection for the PV source or output circuits may be required according to Section 690.9.

NEC Section 690.35 contains specific requirements for ungrounded PV systems. These requirements have implications for what products are used and how, from the inverter upstream to the PV array. It is important that electrical engineers and PV system designers understand these requirements so that they can specify the right components in their plans for ungrounded PV systems. Similarly, electricians and PV system installers need to understand these requirements well enough to verify that the correct components are called out in the plans and that suitable materials are available. The inventory requirements for ungrounded and grounded PV systems are meaningfully different. Many common mistakes can be avoided by ensuring that the components called for in ungrounded PV system designs meet the requirements outlined in NEC Section 690.35.

 

[jaggedben]

It's a non-isolated static power converter that makes every part of the PV system "hot".

Every part of a PV system is 'hot', regardless, when the sun is shining and the utility side breaker is on. Optimizer systems excepted.

My understanding is that it's a minimalist line commutated inverter with bare minimum filtration components and passes insane levels of Amps-THD like helping a steam roller turn forward with an impact wrench rather than a smooth and steady push.

Solar system owner may have been paid added kWh but maybe simply pushing filtration burden and losses into utility owned equipment rather than their own and there may not be ANY net efficiency gain. Someone like mivey can probably explain this.
https://books.google.com/books?id=qW_OBQAAQBAJ&pg=PA807&lpg=PA807

Interesting.
The page you linked to has a paragraph that begins "Line commutated inverters do no provide significant fault current." With that said, that page doesn't seem to be addressing the differences between grounded and ungrounded inverters.

I don't know where your understanding of 'bare minimum filtration components' comes from. I believe that both types of inverters would have to meet the same IEEE standards for their output. Some of those TL inverters are really heavy with capacitors, too.

They have a useful diagrams here in the differences here:
http://www.solarworld-usa.com/~/media/www/files/technical-bulletins/grounded-vs-ungrounded.pdf

Well, they don't have any diagrams of ungrounded inverters in that PDF. That's not a particularly useful document for this discussion.

It's the same hazard. If you touch the secondary side of uninsulated LED ballast, you can get shocked, because it has a potential relative to earth ground.

I didn't say they weren't both hazardous. I said that they weren't exactly the same dangers. For one thing, I would never work on the secondary side of an LED ballast without de-energizing the primary side. With PV panels that option is unavailable (or at least highly impractical). The fact that your dealing with a power source on the DC side involves some fundamental differences in hazards.

In other words, you're focusing on a barely existent increased fault current hazard from the utility side, when the real danger for anyone working on a PV system is from the DC side, where not having a grounded conductor increases safety tremendously, because a single fault to ground does not result in a shock or arc flash until the system is connected and turned on.

The PV panel side of a non-isolated inverter has the same fault current as the utility feeder, limited by the wire impedance and having proper current limiting fuses on each phase and capable of causing the same shock touching a phase conductor without a GFCI.

I don't believe that is true regarding the fault current, given (among other reasons) that the inverter when operating is pushing current in the opposite direction from the utility. Current can't flow the opposite direction of voltage potential. A fault to ground on the DC side results in a DC arc, an inverter shutdown, and the disconnection of the DC conductors from the utility.

Even if what you say were true, fault current would be no worse than on an AC circuit of the same rating as the inverter output. So...not more hazardous than other AC circuits.

Transformerless inverters are MUCH better than isolated inverters at detecting ground faults. I had to re-wire a system with a groundfault current of approximately 3mA. Isolated inverters utilize a 1A (or higher) fuse.

As far as electric shock, yes there is a DC hazard regardless of whether the utility is even connected or even exists.

In conclusion: I believe that the appropriate standards mean that transformerless inverters aren't substantially different, as a group, from the utility's point of view; differences between actual inverter products are probably more significant. They are definitely much safer for me and my crews installing and servicing the DC side, and for homeowners with solar panels on their roofs.