Understanding ground fault detection/indication in PV systems

[jbowman88]

Im working on an off-grid battery based PV system and a question concerning PV ground fault detection has come up. The system has a 1600W pole-mounted array that is connected to a building ~120ft away by an above-ground IMC/EMT run. The charge controller has a built in PV GFDI function that uses a 0.5 Amp fuse between PV(-) and GND. According to the manufacturer this feature only functions properly if the internal GFDI is the only (-) to GND bonding point for the DC system, which makes sense. When the system was installed, however, the DC(-) (negative battery cable) to GND bond inside the inverter panel was not removed.

I mostly understand the purpose of the PV GFDI (I think). What I am struggling with is the idea of ungrounding the entire DC system including the battery. The GFDI doesn't even resolve a fault if one where to occur(?) it just lets you know that one has occurred and then potentially leaves everything energized including the array, conduit runs, metallic enclosures, etc. How this a worthwhile trade off?

What am I not getting about this?

 

[pv_n00b]

GFDI has a couple of purposes, none of them are to remove the fault or put the PV array in a safe condition. Funny eh?

Here are the functions:

1. Stop the flow of ground fault current (only works if there is a single point bond of the grounded conductor through the GFDI fuse and a ground fault in the ungrounded conductor with enough current flow to open the GFDI fuse)
2. Shut down the inverter so someone will be alerted to the fault (only works if someone notices the PV system is not producing power and has the money to get is fixed)

Having GFDI seemed to fix the problem of having to have all the EGC conductors sized the same as the largest EGC in the array, a real problem in commercial systems back in the day. It also supposedly would raise an alarm if there was a ground fault in the array. But if there is no one to see the red light on the inverter is there an alarm?

What it did not do was make the PV array safe after a ground fault. It changed what we were calling a grounded array into an ungrounded array but if there was a fault between the previously grounded conductor and ground somewhere out in the array the fault current would continue to flow after the GFDI fuse opened. The code writers at the time felt that the chance of an additional connection to ground by the grounded conductor in the PV array was low to non-existent, but they were wrong. Turns out that it's fairly common to have faults in the grounded conductor that are not noticed. GFDI also does nothing for a line to line fault.

So that's GFDI in a nutshell. PV arrays with GFDI are no longer considered grounded as they were never solidly grounded. The GFDI is now considered to be a ground fault sensor in a PV array which the current crop of code writers decided to call "functional grounded," because they won't accept reality and call it what it really is, ungrounded.

New inverters don't use GFDI because there can't be a connection between a DC conductor and ground. So they have an array of electronic fault sensors that can detect a fault between either conductor and ground and test the overall insulation condition in the array. I still don't think they can detect a line to line fault.

All this is compounded by not having a way to disconnect the source of power in a PV array. We have spent a lot of time holding on to the tiger's tail and coming up with different ways to try to keep the tiger from biting us.

As for your battery grounding question, that's been a problem before. If you have a second grounded conductor to ground bond it's going to short out the GFDI and make it worthless.

 

[jaggedben]

A couple practical comments:

1) If you can move the inverter out of the building it serves, then you can meet the exception to 690.41(B) and it is not required that the charge controller's GFDI should function.

2) If you can find a different charge controller that uses a different method of GFDI, such as the hall-effect method used on ungrounded grid-tie inverters these days, then I think that would be compatible with a grounded battery negative. I actually have no idea if such a product exists, I have only had the thought that it ought to exist.

BTW, one method of dealing with your situation is that the GFDI for the PV also lifts the battery ground. One manufacturer (Morningstar?) had a product with two circuit breakers ganged together that accomplished this. However I don't think that would be compatible with the charge controller you're using.

 

[jbowman88]

Thanks for the replies.

I definitely am stuck with the existing charge controller (which has the GFDI built in) and the inverter location...

It seems like the PV GFDI requirement was created mostly with grid-tied systems in mind, where the PV array is really the extent of the DC system. In this way, I can see why it would make sense. If you have a PV array mounted on a wooden roof in a battery-less grid-tied system, you really have no good way of protecting against fire from ground faults in the PV system unless you have GFDI. Then at least maybe someone will notice the problem quickly...

Most of my understanding of grounding and bonding comes from working with AC systems, so my gut reaction is that “ungrounding” the DC system is a terrible idea. If there is no negative-to-ground bond then there is no way for fault current to get back to the source and trip the OCPD, and everything will remain energized. I guess with a strictly PV based DC system this is a moot point anyway, since the overcurent protection devices for the PV array are sized larger than the maximum current output the array can generate. You also can’t “de-energize” a PV array. Am I on the right track here?

Again, where this becomes a concern for me is that the PV array is not the only source of power in the DC system I am working with. In fact, it is the least concerning part. You couldn’t get this PV array to catch fire with a can of gasoline and a blow-torch. It’s mounted to metal racking which is sitting on a steel pole mount 10 feet in the air in the middle of dirt/concrete clearing. The conductors run through metallic conduit strapped to concrete blocks.

What concerns me is the large battery bank which is sitting in a metallic rack connected to a metallic inverter by metallic conduit, all of which are readily accessible/touchable inside the building.

So, setting aside the code requirement and speaking purely theoretically, does it really make sense to unground the battery rack just so that the GFDI on the charge controller will work? Honsestly, no one besides me will ever notice the GFDI indicator. Or is the battery rack less of an issue then I am making it out to be?

 

[BillK-AZ]

The Morningstar DC Ground-Fault Protection Devices look like they will work for your situation.

These two GFPD devices from Morningstar Corp. have more advanced safety features than the traditional method of breaking the grounded conductor bond. It detects a current imbalance between the conductors and breaks both conductors in the faulted circuit, isolating the circuit. It does not break the grounded conductor bond to ground and allows unaffected controller circuits to continue to function normally. The battery and DC loads in the system will continue to function in a safe grounded manner. The trip threshold is 300 mA for extra safety and is more accurate than a breaker-type GFP. There is visual and audible trip notification, LED status, and a test button. It does require a small amount of power from the system battery bank at 12 VDC to 48 VDC. Both units are listed to UL 1741, additionally the GFPD-150V is listed to UL-489 and the GFPD-600V is listed to UL-1077.

Thanks for the replies.

I definitely am stuck with the existing charge controller (which has the GFDI built in) and the inverter location...

It seems like the PV GFDI requirement was created mostly with grid-tied systems in mind, where the PV array is really the extent of the DC system. In this way, I can see why it would make sense. If you have a PV array mounted on a wooden roof in a battery-less grid-tied system, you really have no good way of protecting against fire from ground faults in the PV system unless you have GFDI. Then at least maybe someone will notice the problem quickly...

Most of my understanding of grounding and bonding comes from working with AC systems, so my gut reaction is that “ungrounding” the DC system is a terrible idea. If there is no negative-to-ground bond then there is no way for fault current to get back to the source and trip the OCPD, and everything will remain energized. I guess with a strictly PV based DC system this is a moot point anyway, since the overcurent protection devices for the PV array are sized larger than the maximum current output the array can generate. You also can’t “de-energize” a PV array. Am I on the right track here?

Again, where this becomes a concern for me is that the PV array is not the only source of power in the DC system I am working with. In fact, it is the least concerning part. You couldn’t get this PV array to catch fire with a can of gasoline and a blow-torch. It’s mounted to metal racking which is sitting on a steel pole mount 10 feet in the air in the middle of dirt/concrete clearing. The conductors run through metallic conduit strapped to concrete blocks.

What concerns me is the large battery bank which is sitting in a metallic rack connected to a metallic inverter by metallic conduit, all of which are readily accessible/touchable inside the building.

So, setting aside the code requirement and speaking purely theoretically, does it really make sense to unground the battery rack just so that the GFDI on the charge controller will work? Honsestly, no one besides me will ever notice the GFDI indicator. Or is the battery rack less of an issue then I am making it out to be?

 

[jaggedben]

Thanks for the replies.

It seems like the PV GFDI requirement was created mostly with grid-tied systems in mind, where the PV array is really the extent of the DC system. In this way, I can see why it would make sense. If you have a PV array mounted on a wooden roof in a battery-less grid-tied system, you really have no good way of protecting against fire from ground faults in the PV system unless you have GFDI. Then at least maybe someone will notice the problem quickly...

I wouldn't agree with all of that - I think the code making panel's concern was simply to prevent PV conductors from shorting to each other and causing fires (see next comment). This is why there's an exception if the DC conductors don't enter a building, because then the fire isn't on a structure with people inside. However I would agree that the code has never very adequately addressed how to integrate PV GFDI with a directly paralleled battery system that was traditionally grounded, and that since most systems are grid-tied there probably hasn't been any real pressure on them to do so.

... I guess with a strictly PV based DC system this is a moot point anyway, since the overcurent protection devices for the PV array are sized larger than the maximum current output the array can generate. You also can’t “de-energize” a PV array. Am I on the right track here?

Yes, you are getting it. If the grounded PV conductor faults to bonded metal, and that is not detected and fixed for years, and then the ungrounded PV conductor also faults to bonded metal, then you will have uncontrolled arcing that no overcurrent protection will stop. i.e. fire.

Again, where this becomes a concern for me is that the PV array is not the only source of power in the DC system I am working with. In fact, it is the least concerning part. You couldn’t get this PV array to catch fire with a can of gasoline and a blow-torch. It’s mounted to metal racking which is sitting on a steel pole mount 10 feet in the air in the middle of dirt/concrete clearing. The conductors run through metallic conduit strapped to concrete blocks.

Again, there's an exception to the GFDI requirement for that sort of install if the DC conductors don't enter the building. But if you're DC conductors enter the building they could still short to each other (or via a double fault) inside the building and cause a fire there. The concern isn't the PV array catching on fire. The concern is the PV power source conductor ssetting something else on fire because there is no feasible way to set up overcurrent protection to stop that.

What concerns me is the large battery bank which is sitting in a metallic rack connected to a metallic inverter by metallic conduit, all of which are readily accessible/touchable inside the building.

So, setting aside the code requirement and speaking purely theoretically, does it really make sense to unground the battery rack just so that the GFDI on the charge controller will work? Honestly, no one besides me will ever notice the GFDI indicator. Or is the battery rack less of an issue then I am making it out to be?

No it doesn't make sense to unground the battery source. That's just robbing Peter (battery safety) to pay Paul (PV safety). I'm sure arguments could be made about which danger is more serious but the proper approach would be to address both.

I'm inclined to say that if you really care about the safety issues then you need to address your PV output equipment. You could, for example, install this device or a similar one on the PV circuit, and get a new charge controller without GFP that is compatible. Unless the charge controller you have has an option to not use the GFP device.

FWIW, I think the direction technology is going is towards ungrounded systems and fancier ground fault protection than the fuse method can provide. You've got a relatively old-school setup.

 

[pv_n00b]

The NEC requirements for GFDI that opens the system bond do not really work for battery systems unless the charge controller is isolated, and most are not. Most charge controllers just run the negative conductor straight through and switch the positive conductor. The problem is that the NEC requires all conductors in the faulted circuit to be opened if the system ground is opened by a GFDI. This works for the PV array, ground fault opens the GFDI and the charge controller shuts down which "opens" all the PV conductors. But a fault on the battery circuit will also open the GFDI but it won't open the battery conductors leaving that circuit ungrounded, still energized, and faulted. If the battery circuit was solidly grounded then a fault would open the battery OCPD and disconnect it from the faulted conductors. The only upside, the GFDI opening will shut down the charge controller and that will force someone to fix it if they want power.

Components and equipment need to be selected very carefully to create a system that provides safety for both the PV array circuit and the battery circuit. If someone is given a box of parts the chances that a compliant system can be built are pretty low.

Does anyone know who is building an isolated charge controller? I know the 600V chargers are isolated but I can't say I know of a 150V charger that is isolated.

 

[jbowman88]

This is all extremely helpful; thank you. I think I’m going to have to look into a better PV GFDI device, such as the Morningstar. That will satisfy the GFDI requirement without having to unground the battery. It does not appear to be prohibitively expensive either, at least not compared to the overall cost of the system.

just curious, does anyone know of any another brands or models that are comparable to the Morningstar?

 

[BillK-AZ]

Magnum Energy PT-100 features both electronic GFDI and AFCI

This is all extremely helpful; thank you. I think I’m going to have to look into a better PV GFDI device, such as the Morningstar. That will satisfy the GFDI requirement without having to unground the battery. It does not appear to be prohibitively expensive either, at least not compared to the overall cost of the system.

just curious, does anyone know of any another brands or models that are comparable to the Morningstar?

 

[jbowman88]

Magnum Energy PT-100 features both electronic GFDI and AFCI

Haha, the PT-100 is the charge controller we are already using. Overall I’m very happy with it (and other Magnum Energy products), but it has the fuse type internal GFDI mechanism which is problematic in this case. Fortunately, it can be be disabled with a dip switch so that you can use an external GFDI device, one that is compatible with a grounded battery bank.