Switching the Neutral

DanS26

Hi All,

This is my first post but have been lurking around since I installed my grid tied solar system in November of 2011.

System size is 8.6 kwh. 36 235w panels using a Fronius 7.5 inverter.

I have a technical question about switching the neutral on the normally required AC disconnect for grid tied solar systems.

I realize that the solar system and inverter are not considered a "separately derived system" as defined by the NEC. Therefore it is required to have the neutral be a solid link to the distribution panel.

Now I also realize the NEC is always right and I also realize that the AHJ can also override the NEC in their local capacity. After thinking about the electrical theory involved, I think that a switched neutral is advantageous in a grid tied solar system. My thinking is that a serious fault in the inverter could energize the neutral and be dangerous to utility linemen. By switching the neutral, the inverter and any stray voltage is COMPLETELY separated from the grid.

On the other hand, a switched neutral is more dangerous to the owner/operator, since the single point of ground is breached and thus stray voltages may find a dangerous path to ground in the system on site.

I think it is much preferable to have stray voltages be confined to the generating location than to be floating around on the grid in the event of a serious fault in the inverter, say not shutting down when the grid goes down.

Comments?

nsaspook

Re: Switching the Neutral

My reading of the NEC "separately derived system" means galvanic isolation by transformer or complete electrical isolation. Transformer-less grid-tie systems don't have galvanic isolation and it's not a design requirement so the NEC is correct require hard connections and the protective systems built in the inverters depend on that bond being made to operate safely.

http://www.iaei.org/magazine/2009/11/separately-derived-systems/

Some examples of separately derived systems would include:

Transformers, shown in photo 1, where the supply side is isolated from the derived side except by magnetic coupling
Generators (motor, wind, or engine driven), one example shown in photo 2 and figure 1, where it is either a totally stand-alone system or is an alternate source of power and the grounded conductor (neutral) is not solidly connected in the transfer switch or transfer equipment
Battery/inverter systems where the output is not interconnected
Photovoltaic systems where there is no interconnection to the grid or another energy source (off grid system)

If you really want to neutral switch a grid-tie system a separate isolation transformer before the switch would be needed.

http://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=3&ved=0CE8QFjAC&url=http%3A%2F%2Fwww.kraftpower.com%2Fpdfs%2FKPC_Info_05_switching_neutral.pdf&ei=6JxqT5K9NYa6iQfo56WIBg&usg=AFQjCNEv-VUeTGLRoe8cKOZUNFkG5qTqVg&sig2=7VZkjsakENOfz1bF_inKcg
Each "separately derived system" (utility or solar with galvanic isolation) would have it's own neutral to ground bond at the point of origin of power (utility transformer or solar inverter) and each "separately derived system" ground wire from it's ground point would be bonded to the main panel (house) ground.

SolaRevolution

Re: Switching the Neutral

DanS26 wrote: »

I realize that the solar system and inverter are not considered a "separately derived system" as defined by the NEC. Therefore it is required to have the neutral be a solid link to the distribution panel.

The NEC recognizes the DC side of a grid tied inverter as a seperately derived system. This is why a batteryless inverter requires a Grounding Electrode Conductor. Each seperately derived system is required to remain solidly bonded to ground unless it is disconnected by a tripped GFCI device which has an visible indicator that shows when a fault has occured.

If you have 2 seperate non-battery grid-tie inverters you have a total of 3 seperately derived systems. (2 PV dc systems and one AC system.)

If your system is built using micro-inverters on each of 40 PV modules you have 41 seperately derived systems. When you connect a grounding conductor to an Enphase type microinverter it is actually a grounding electrode conductor and is required to be continuous and/or spliced with irreversible means. The AHJ may require the grounding electrode conductor to be a minimum of a # 6 copper. I believe this is why there may not be an equipment grounding conductor in the micro-inverter AC output cables.


Hmm...
I wonder about the XWMPPT80-600. I just checked and there is no continuity between the PV-N and the Battery-N when the charge controller is not energized. Also, the ground bond can be made in the GFCI connection in either the positive or the negative. I think that all of the other CCs I am familiar with have at least some continuity between the PV and battery negatives (even de-energized) and if they have GFCI protection they lift the Ground bond but do not isolate the PV-N from the Battery-N when there is a fault.


Alex

DanS26

Re: Switching the Neutral

SolaRevolution wrote: »

The NEC recognizes the DC side of a grid tied inverter as a seperately derived system. This is why a batteryless inverter requires a Grounding Electrode Conductor. Each seperately derived system is required to remain solidly bonded to ground unless it is disconnected by a tripped GFCI device which has an visible indicator that shows when a fault has occured.

If you have 2 seperate non-battery grid-tie inverters you have a total of 3 seperately derived systems. (2 PV dc systems and one AC system.)

If your system is built using micro-inverters on each of 40 PV modules you have 41 seperately derived systems. When you connect a grounding conductor to an Enphase type microinverter it is actually a grounding electrode conductor and is required to be continuous and/or spliced with irreversible means. The AHJ may require the grounding electrode conductor to be a minimum of a # 6 copper. I believe this is why there may not be an equipment grounding conductor in the micro-inverter AC output cables.


Hmm...
I wonder about the XWMPPT80-600. I just checked and there is no continuity between the PV-N and the Battery-N when the charge controller is not energized. Also, the ground bond can be made in the GFCI connection in either the positive or the negative. I think that all of the other CCs I am familiar with have at least some continuity between the PV and battery negatives (even de-energized) and if they have GFCI protection they lift the Ground bond but do not isolate the PV-N from the Battery-N when there is a fault.


Alex

Alex,

Totally agree with you on the DC side of things.

But on the AC side, my Fronius 7.5 grid tied inverter is not considered a separately derived system and thus the neutral is not switched. Maybe I'm misinterpreting you when you say:

"If you have 2 seperate non-battery grid-tie inverters you have a total of 3 seperately derived systems. (2 PV dc systems and one AC system.)"

Could you clarify?

nsaspook

Re: Switching the Neutral

The Fronius 7.5 grid tied inverter does have galvanic isolation (HF transformer) so it is technically possible to wire it as a "separately derived system (Ground to Neutral bond at the inverter)" but as it requires the grid (also with a ground to neutral connection) to operate and the two systems neutrals in this condition should not be connected I can't see a reason to do it. A fault energizing the neutral in respect to the utility lines with the disconnect switch open with galvanic isolation on the inverter is a very low possibility.

DanS26

Re: Switching the Neutral

Thanks everyone for responding. You have confirmed my thinking that the NEC chose the route of least risk. That is very logical.

ggunn

Re: Switching the Neutral

DanS26 wrote: »

Thanks everyone for responding. You have confirmed my thinking that the NEC chose the route of least risk. That is very logical.

Mitigating the risk of fire and injury is its job.