Definition for terms used in NEC 690.5 Re Ground-Fault Protection for PV systems

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Nick Toth

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I'm looking at Exception no. 1 for ground-fault protection: "Ground mounted or pole mounted photovoltaic arrays with not more than two paralleled source circuits and with all dc source and dc output circuits isolated from buildings shall be permitted without ground-fault protection."

My question is regarding the statement "isolated from buildings". We are installing an off-grid solar power system for a house, where the two solar panels (in series) are ground-mounted 85 feet away from the house. The DC output circuit runs underground from the panels to the house.... does that qualify as "isolated from buildings"? It is physically isolated, but electrically connected to the building and I'm not clear on the exact intent of the NEC regarding this. Do we need GFP for this installation, or not?

Thank you! Nick
 

GoldDigger

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I'm looking at Exception no. 1 for ground-fault protection: "Ground mounted or pole mounted photovoltaic arrays with not more than two paralleled source circuits and with all dc source and dc output circuits isolated from buildings shall be permitted without ground-fault protection."

My question is regarding the statement "isolated from buildings". We are installing an off-grid solar power system for a house, where the two solar panels (in series) are ground-mounted 85 feet away from the house. The DC output circuit runs underground from the panels to the house.... does that qualify as "isolated from buildings"? It is physically isolated, but electrically connected to the building and I'm not clear on the exact intent of the NEC regarding this. Do we need GFP for this installation, or not?

Thank you! Nick

I would say that if the inverters were mounted at the array or anywhere between the array and the house and only AC wiring came into the house, that would qualify as the DC circuits being isolated.
"all dc source and dc output circuits isolated from buildings" makes it pretty clear to me that they are not talking about just physical isolation of the panels and mounting equipment.
 

Nick Toth

Member
I would say that if the inverters were mounted at the array or anywhere between the array and the house and only AC wiring came into the house, that would qualify as the DC circuits being isolated.
"all dc source and dc output circuits isolated from buildings" makes it pretty clear to me that they are not talking about just physical isolation of the panels and mounting equipment.

I agree with your thought that they are not talking about physical isolation.

The single inverter is mounted inside the house. So, does that mean we DO need a GFP device at the house end of the PV line from the panels?

I have gotten opinions both ways on this question, from installers and manufacturers ....... some saying we qualify for the exception and don't need a GFP.... some saying that we DO need a GFP. Apparently there is much confusion over this issue! How do I get a definitive answer? Is there a way to get it directly from NEC???

Thanks for your help! Nick
 

jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
The intent is to prevent buildings from burning down from an undetected ground fault. (The first ground fault in a PV system usually doesn't cause any real danger. The second ground fault, on the other conductor, results in an uncontrollable arc that starts a fire. That's why the first ground-fault is required to be detected and to shut down the system so that someone will do something about it before the second one can occur.)

I think it's pretty clear that if the DC conductors are in or on the house then you can't invoke the exception. I'm not sure what's confusing about that.

I suppose it might be possible to mount the inverter on the outside of the house and have the DC conduit come directly out of the ground to the inverter with no support from the building, or very minimal contact. That's about as far as I think you can push this exception.

Of note if you are still on the 2011 code is the other exception to 690.5. But that was removed in the 2014.
 

Nick Toth

Member
The intent is to prevent buildings from burning down from an undetected ground fault. (The first ground fault in a PV system usually doesn't cause any real danger. The second ground fault, on the other conductor, results in an uncontrollable arc that starts a fire. That's why the first ground-fault is required to be detected and to shut down the system so that someone will do something about it before the second one can occur.)

I think it's pretty clear that if the DC conductors are in or on the house then you can't invoke the exception. I'm not sure what's confusing about that.

I suppose it might be possible to mount the inverter on the outside of the house and have the DC conduit come directly out of the ground to the inverter with no support from the building, or very minimal contact. That's about as far as I think you can push this exception.

Of note if you are still on the 2011 code is the other exception to 690.5. But that was removed in the 2014.


Thank you very much for your explanation! I see from that why this installation DOES require a GFP and why exception No. 1 does not apply. I don't know why there is confusion about this, but there appears to be, judging by the inconsistent answers I have gotten elsewhere.


Could you possibly explain more about the first paragraph of your response? Are you possibly referring to an ungrounded system? In a grounded system (as this one is) one conductor (negative in this case) is connected to earth ground at one point only. As previously stated, this is an off-grid, stand-alone installation. The PV conductors from the panels go to a charge controller inside the house, with an inverter running off a 24v battery bank that is kept charged by the charge controller. You explained about the first ground fault on one conductor and a possible second ground-fault on the second conductor?.. in this particular installation, wouldn?t a single ground-fault (on the positive conductor) be problematic, possibly causing arcing that could lead to a fire?

Thanks again for your input! Nick
 

jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
...
Could you possibly explain more about the first paragraph of your response? Are you possibly referring to an ungrounded system? In a grounded system (as this one is) one conductor (negative in this case) is connected to earth ground at one point only. As previously stated, this is an off-grid, stand-alone installation. The PV conductors from the panels go to a charge controller inside the house, with an inverter running off a 24v battery bank that is kept charged by the charge controller. You explained about the first ground fault on one conductor and a possible second ground-fault on the second conductor?.. in this particular installation, wouldn?t a single ground-fault (on the positive conductor) be problematic, possibly causing arcing that could lead to a fire?

Thanks again for your input! Nick

What I said about first and second faults applies equally to both grounded and ungrounded systems, as far as the intended purpose of GFDI goes. (Technically with solar it is Ground Fault Detection and Interruption, meaning interruption of the operation of the system. This is distinguished from other Ground Fault Protection devices which typically de-energize the faulted conductor(s). GFDI in solar does not de-energize the faulted PV conductor(s).)

To address your question in your final sentence, in a grounded system the GFDI device ungrounds the grounded conductor when it trips, thus interrupting the arcing that could lead to a fire if there is only one fault. Typically the grounded conductor is connected to ground through a fuse and if the fault current is enough to blow the fuse then the connection is broken.

Fused methods of GFDI on grounded systems are less sensitive than methods for ungrounded systems, and have led to at least one major diaster. A fault on an ungrounded conductor is usually enough to blow the GFDI fuse since the entire short circuit current can possibly take that path. One smaller systems the resistance of the fault only needs to be less than the inverter, and on systems with combiners a fault in a source circuit can blow the source circuit fuse. However, a fault on a grounded conductor needs to be roughly equal in resistance to the actual circuit conductor, which requires a stronger fault to blow the fuse. In either case really small ground faults can be missed by the typical 1A fuse for residential systems.

By contrast, the methods used by ungrounded systems are very sensitive, even too sensitive in my opinion. (The inverter tests the insulation of the conductors before startup, and monitors the current electronically during operation to make sure it remains balanced.) In one case I had to replace wiring on a system where the fault being detected would have been passing around a third of a milliamp.

Pretty much all grid-tied inverters being sold these days are ungrounded for better safety. Off-grid systems with batteries haven't really caught up yet.
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
I agree with your thought that they are not talking about physical isolation.

The single inverter is mounted inside the house. So, does that mean we DO need a GFP device at the house end of the PV line from the panels?

I have gotten opinions both ways on this question, from installers and manufacturers ....... some saying we qualify for the exception and don't need a GFP.... some saying that we DO need a GFP. Apparently there is much confusion over this issue! How do I get a definitive answer? Is there a way to get it directly from NEC???

Thanks for your help! Nick
I am not that familiar with off grid inverters, but every grid tied inverter I know of has ground fault protection built in.
 
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