Confused by array grounding

[zanydroid]

Would separate grounding systems be acceptable for transformer isolated SCC or inverter? IE no bonding between EGC to arrays and EGC for equipment inside the residence.

IE the inverter or DC DC converter employs a transistor and consequently has no galvanic path between the 120/240 system and the DC+/- from the panels.

 

[timselectric]

One grounding system for everything.
Unless it's a separate dedicated system for separate loads. Then it creates it's own grounding with its own N/G bond.
But I would still connect to the existing grounding system. Because everything that you can touch, should be safe from all sources.

 

[zanydroid]

Doesn’t the transformer isolated power topology substantially cut the risk of the PV riding on AC that you get in transformerless? And the transformer provides an extra measure of isolation against some kinds of faults.

You would not be required to connect the EGC/GEC in a separately derived AC system (I think the most common way to form that is with an isolation transformer; I imagine if the separately derived system is in the same building it would be more cost effecting to reuse the same grounding system).

 

[timselectric]

You would not be required to connect the EGC/GEC in a separately derived AC system

You would, if it serves loads in the same building.
Separately derived only refers to the neutral. Whether it's common between the two sources or not.
But the grounding is always tied together.

 

[zanydroid]

In that case that implies even an isolated PV system will with high probability trigger a need to bond the EGCs with the AC system of the building if it ever gets close enough.

For instance suppose you have an isolated topology inverter, and both AC and DC terminate in the same built in wiring area. Would the EGCs be obligated to be tied together since there are splices being made in one junction box?

 

[timselectric]

Yes

 

[Shimmy]

Would separate grounding systems be acceptable for transformer isolated SCC or inverter? IE no bonding between EGC to arrays and EGC for equipment inside the residence.

IE the inverter or DC DC converter employs a transistor and consequently has no galvanic path between the 120/240 system and the DC+/- from the panels.

How would you isolate the SCC with a transformer? That is a big part of the problem. In the "olden days" you would have an input isolation transformer => Rectifier, DC Bus, Inverter => output isolation transformer; that provides a truly isolated DC bus. The input isolation transformer might have a high-impedance ground for sensing ground fault, but that wasn't very common. The problem with that scenario is that you have an additional 5-10% losses compared to a modern design.

So, yes it can be done... but it is expensive and non-constructive.

(I fought long and hard against losing the galvanic isolation back in the day because of how many esoteric problems it solved in general. But the reality is that the efficiency and reduced cost are worth a lot more. The NEC adapted as well, forcing center-point grounding as the default. The current approach really is the most practical solution though, which is why those of us who have been around understand and go with it... at least now.)

 

[zanydroid]

How would you isolate the SCC with a transformer? That is a big part of the problem. In the "olden days" you would have an input isolation transformer => Rectifier, DC Bus, Inverter => output isolation transformer; that provides a truly isolated DC bus. The input isolation transformer might have a high-impedance ground for sensing ground fault, but that wasn't very common. The problem with that scenario is that you have an additional 5-10% losses compared to a modern design.

So, yes it can be done... but it is expensive and non-constructive.

I was assuming use an isolated Buck/Boost (as appropriate) topology on the DC side. I don't think there's a benefit, I was mostly theorycrafting as to whether the old school way of doing things would allow different grounding approach vs the most common systems today (transformerless ungrounded PV) by reducing some of the risks. Not even clear that the one risk I thought is slightly improved (the large AC ripple on PV conductors, and I guess probably also higher PV voltage relative to ground) is worth the trouble relative to the hardware and efficiency cost.

 

[Shimmy]

I was assuming use an isolated Buck/Boost (as appropriate) topology on the DC side. I don't think there's a benefit, I was mostly theorycrafting as to whether the old school way of doing things would allow different grounding approach vs the most common systems today (transformerless ungrounded PV) by reducing some of the risks. Not even clear that the one risk I thought is slightly improved (the large AC ripple on PV conductors, and I guess probably also higher PV voltage relative to ground) is worth the trouble relative to the hardware and efficiency cost.

Ultimately you need to isolate the AC on input and output to make it work. Using power electronics you are always going to have capacitive or inductive coupling to some form of ground.

If you want an ungrounded system, the way to do it is frameless glass panels.

 

[Pilot Bob]

Another thing I hope we can all agree on is that it’s better to run the EGC back now from the array to the SCC or inverter location (even if there turns out to be some combination of system types that may or may not require bonding to the EGC for the 120/240 system). Because that would be future proofed against changing system types or misunderstanding how to do

Code allows you to install an auxiliary ground rod. But it's not recommended.
But if an auxiliary ground rod is installed, it must be connected to the grounding system.

You are mixing up GEC and EGC all over the place. I suggest you spend more time understanding 250 before you try to understand why you are incorrect.

Also, read the FPN in 690.47.

I think there is more to this story. The original question was about grounding his system that has a ground mount. So looking at the 2020 NEC, Article 250, Part VIII. Direct Current Systems applies. In 250.160 General it states that DC systems shall comply with part VIII and other sections of Article 250 not specifically intended for AC systems. So #1, whatever is intended only for an AC system does not apply to the ground mounted DC array. #2, 250.162 DC Circuits and Systems to be grounded. DC circuits and systems shall be grounded as provided for in 259.162(A) and (B). 250.164 Point of Connection for DC Systems is divided between off premises or on premises sources. An off premises source shall not be made at any point on the premises wiring. Is a ground mounted array considered off premises? I’m not questioning anyone’s expertise, experience or knowledge. I thought I would share this information. If I had my inspector hat on, I would say a ground mounted array is off premises and roof mounted would be on premises. Just my thoughts.

 

[timselectric]

Even if you consider the ground mounted array off premises (I would). The wiring usually enters the premises. (That's where the loads are)
Therefore an on premises fault can travel to the array.
This is why we ground things. To protect people from faults.
If the entire system was off premises, then it wouldn't need to be connected to the premises in any way.
An example would be a solar farm located on a private property. (Land rented by the solar farm owner)

 

[Pilot Bob]

It's not protecting the DC system. It's protecting people who may come into contact with the DC system.

There is no ground coming from the grid.
Only current carrying conductors.
The ground for your system is created in your service panel. (By the N/G bond)

It requires that it is to be connected to your grounding system. Which begins at your service panel.
It doesn't have to go directly to the service panel. It can run to your equipment, that is also connected to the grounding system.

The Neutral coming from the Grid is called the grounded conductor in the NEC (it is grounded from the transformer to a grounding electrode) and is the main part of the grounding system for the main service panel. This is the reason that neutrals and grounds are bonded in the main panel.

 

[Shimmy]

The Neutral coming from the Grid is called the grounded conductor in the NEC (it is grounded from the transformer to a grounding electrode)...

No, the neutral is not grounded at the utility transformer (in the US). It is not called a grounded conductor until after the neutral-ground bond point.

 

[FilterGuy]

No, the neutral is not grounded at the utility transformer (in the US). It is not called a grounded conductor until after the neutral-ground bond point.

Actually, Neutral is grounded at the utility transformer in the US. The bond is called the SYSTEM Bonding Jumper. However, in the US the utility does not provide a dedicated ground wire. Consequently, the bond has to be re-established on the customer premise in order to ensure a low-impedance fault path for clearing faults. This is done at the first means of disconnect and is called the MAIN bonding jumper.

 

[FilterGuy]

Actually, Neutral is grounded at the utility transformer in the US. The bond is called the SYSTEM Bonding Jumper. However, in the US the utility does not provide a dedicated ground wire. Consequently, the bond has to be re-established on the customer premise in order to ensure a low-impedance fault path for clearing faults. This is done at the first means of disconnect and is called the MAIN bonding jumper.

View attachment 164756

BTW: This *does* kind of violate the golden rule of one and only one NG bond. However, the path for the undesired current is through the earth, not through an Equipment Grounding Conductor.

 

[timselectric]

The Neutral coming from the Grid is called the grounded conductor in the NEC (it is grounded from the transformer to a grounding electrode)

Correct

and is the main part of the grounding system for the main service panel.

Incorrect.
It is part of the return path for ground faults. When the source is the grid.
But the beginning of the premises grounding system is created at the premises N/G bond.

This is the reason that neutrals and grounds are bonded in the main panel.

The reason that they are bonded at the first means of disconnect. Is to give the grounding system a low impedance path back to the source.

 

[Pilot Bob]

No, the neutral is not grounded at the utility transformer (in the US). It is not called a grounded conductor until after the neutral-ground bond point.

WTH, it’s grounded at the transformer before coming to your house (utility side). This is why it’s called a grounded conductor.

 

[Pilot Bob]

Correct

Incorrect.
It is part of the return path for ground faults. When the source is the grid.
But the beginning of the premises grounding system is created at the premises N/G bond.


The reason that they are bonded at the first means of disconnect. Is to give the grounding system a low impedance path back to the source.

Understood. Whether you have a pole mounted transformer or a ground transformer the neutral coming from that transformer is connected to ground at the transformer. This is the first point of grounding before it ever comes to your meter and service panel.

 

[FilterGuy]

Whether you have a pole mounted transformer or a ground transformer the neutral coming from that transformer is connected to ground at the transformer.

That is correct. This bond is called the System bonding jumper

The reason that they are bonded at the first means of disconnect. Is to give the grounding system a low impedance path back to the source.

That is also correct. This bond is called the Main bonding jumper.

The reason the bond at the utility transformer is not sufficient is that in the US the utility does not provide an independent ground wire. It only provides the two hots and the Neutral. This means the neutral-ground bond has to be re-established on the customer premise in order to have a low impedance path for ground faults.

The Neutral coming from the Grid is called the grounded conductor in the NEC

It is not called a grounded conductor until after the neutral-ground bond point.

What it is called can be an interesting conversation.... but how it is wired in the US is not debatable.

As I stated above, the neutral coming from the utility is tied to earth ground at the utility transformer and it is also tied to earth ground at the first means of disconnect on the customer premise. Having said that, I do not know for sure what it is called.

The System bonding Jumper does not provide a low-impedance path for ground faults so it is not providing one of the intended purposes of a grounded conductor. However, once the Main bonding jumper is in place, it certainly becomes a grounded conductor.

The other bit of trivia that throws in a question mark is that the NEC explicitly states that it does not cover the Utility. As such, does the NEC term "Grounded Conductor" apply to a wire that is part of the Utility?

 

[timselectric]

The other bit of trivia that throws in a question mark is that the NEC explicitly states that it does not cover the Utility. As such, does the NEC term "Grounded Conductor" apply to a wire that is part of the Utility?

It's whatever the utility wants to call it.
In my area, they call it a neutral or common. But, that can change tomorrow.