Ground fault detection -690.6 (D)

[SparkyRules]

We are hooking up the utility interactive part of a PV system comprised of self contained AC inverter modules (output 240/120 AC volts each). The inverter is built into each module, so no separate inverter. So where is the ground fault detection? Is there a component missing? There is a combiner box which takes our 2- 240/120 inverter output circuits thru their own OC (breaker) then combines them into one inverter output circuit which is then backfeed thru the house panel per 705-(D) (5) and (7). but said box is just 2- 2pole breakers in a 3R box. Could we use a "hot tub" GFI breaker ( like an HOM220GFI) for this if we spliced it between the PV modules and the house panel, with the line looking at the house panel and the load looking at the modules? Would this not meet the intent of GF detection? How many MA are we trying to detect? per 690.6(D)? would this even work having current flowing backwards thru a GFI breaker? How many UL folks would it take to figure out if this was "listed" for this use? :happysad: Would this not also serve as the inverter disconnecting means (705.21)?

 

[jaggedben]

Note first of all that GFI protection on the AC side has nothing-whatsoever-at-all to do with GFDI requirements on the DC side. There is no NEC requirement that inverters have GFI protection on the AC side. I think you are confusing these two issues.

Thus there is no reason you have to install a GFI breaker on the inverter output circuit. It is not required by 690.6, and may violate 705(D)(3).

Whether or not your modules are considered AC modules under the NEC is up to your AHJ. Do not assume that your AHJ is going to agree with that they are AC modules just because the manufacturer might say so. Look at the definition in 690.2.

If they do not agree, then DC GFDI is required. In any case, the inverters probably have that feature and you should know about it. If you're using Enphase inverters (e.g. on Westinghouse modules), then DC GFDI is internal to the inverters and is reset using the Enphase Envoy. I do not know for sure how SolarBridge or any others might handle this, but I'm pretty sure it would be similar.

 

[SolarPro]

The installation manual should indicate whether or not the microinverter or AC module product you are using incorporates ground-fault protection. Presumably it does. I am not aware of any modern listed interactive inverters that do not incorporate this feature.

You're probably working with an Enphase or a SolarBridge solution. Enphase has a good technical resource library online. I've found manuals online for the SolarBridge products as well. The SolarBridge instructions are more likely to be incorporated into the installation manual for the ac module, in which case it may be easiest to find on the module manufacturer's website.

 

[BillK-AZ]

Sq-D has a document "Backfeeding Ground Fault Circuit Breakers"

Sq-D has a document "Backfeeding Ground Fault Circuit Breakers"

You can not use the HOMxxxGFI or HOMxxxEPD for this backfed application. (or the related QO breakers)

Sq-D has a document "Backfeeding Ground Fault Circuit Breakers", 02/2012, Author: Ed Larsen, Document Number: 0900DB1001 R02/12

http://static.schneider-electric.us/docs/Circuit Protection/0900DB1001.pdf

This document clearly states that these breakers can not be backfed and gives an explanation.

Can Circuit Breakers with Ground Fault Protection be Backfed?

The answer to this question depends on how the ground fault protection
function is powered. The electronic circuitry used to provide the ground fault
protection may be line powered or fault powered.
Miniature Circuit Breakers (MCBs) of the type typically used in residential
and ?lighting panelboard? applications may provide ground fault protection
for personnel, in which case the circuit breaker is called a Ground-Fault
Circuit Interrupter (GFCI, Figure 1). GFCIs have a nominal 5 mA ground
fault trip level.

MCBs are also available with Ground-Fault Protection of Equipment
(GFPE), often with a nominal 30 mA trip level, for protection of heating
cables and other applications. Because of the low ground fault trip level of
these circuit breakers, their electronic circuitry must be line-powered.
These circuit breakers are distinctive in that they have a white ?pigtail? wire
intended for connection to the neutral bar in the panelboard in which they
are installed. This pigtail wire not only completes the branch circuit (the
neutral wire must be connected to the circuit breaker rather than to the
neutral bar), but also completes the power supply circuit for the electronic
ground fault detection circuitry.

Backfeeding (reverse connecting) these circuit breakers will result in
damaging the trip solenoid, rendering the ground fault trip function
inoperative. For this reason the terminals on these circuit breakers are
marked ?line? and ?load?.

The wording of 690.6 (D), "single detection device" indicates that this is not a feature of the AC PV module.

The question now is "What GFDI device can be used with AC PV modules?".

 

[SolarPro]

The question now is "What GFDI device can be used with AC PV modules?".

There is text related to this in the "Expedited Permit Process for PV Systems," which Bill Brooks wrote for the Solar America Board for Codes and Standards:

"690.6(D) permits one device to provide ground-fault protection for all the ac modules a single circuit. Note that this is not a requirement, but rather an allowance. An installer might want to provide this capability with a typical 15- or 20-amp GFCI breaker. While this sounds like a good idea, and appears to comply with 690.6(D), there are no GFCI breakers on the market, as of the writing of this document, that are listed for bidirectional power flow. Laboratory tests have shown that some GFCI breakers can be damaged while backfeeding power. A message to plan checkers and inspectors on this item—do not require GFCI breakers on circuits connected to ac modules until it is a code requirement and specifically listed products are available to properly provide protection."

John Wiles has said the same thing in an IAEI article:

"An ac GFCI device should not be used to protect the dedicated circuit to the microinverter or ac PV module even though it is an outside circuit. None of the small GFCI devices (5 ma–30 ma) are designed for back feeding and will be damaged if backfed. In a similar manner, most ac AFCIs have not been evaluated for backfeeding and may be damaged if backfed with the output of a PV inverter."

He explains that:

"If there is a line-to-line or line-to-ground fault on this circuit, the OCPD responds in a normal manner to the fault currents generated by the utility. The inverter(s) can generate no more than its rated current per UL Standard 1741 and when the fault occurs, the drop in line voltage will normally cause the inverter to shut down. And when the branch circuit breaker opens in response to the fault, the inverter shuts down."

While the OCPD provides a failsafe, so to speak, an Enphase microinverter, as an example, has an internal GFI sensor per UL 1741 that is set to 1 A max. As a result, the microinverter can identify ground faults and take the inverter offline before the circuit OCPD trips. The SolarBridge installation manual is less transparent, but is built to the same standard and provides a similar level of protection in practice. However, an AC module doesn't have to meet the requirements in 690.5, which apply to "grounded dc arrays" and 690.6(D) is an allowance not a requirement.

 

[BillK-AZ]

....While the OCPD provides a failsafe, so to speak, an Enphase microinverter, as an example, has an internal GFI sensor per UL 1741 that is set to 1 A max. As a result, the microinverter can identify ground faults and take the inverter offline before the circuit OCPD trips. .....

Note that the 1A internal GFI sensor in Enphase is on the DC side, not the AC side.

 

[SolarPro]

Right. Because it's not an ac module, Enphase needs to meet 690.5.

That doesn't mean that the module-integrated microinverter in an ac module won't do the same thing. It will see faults in the module, disconnect and throw an error code. However, NEC690.6(A) indicates that 690 requirements pertaining to PV source circuits (dc circuits) do not apply to ac modules. So an ac module manufacturer doesn't need to call out its GFP provisions on the dc side of the inverter.

The Code treats ac modules and microinverters in rooftop systems preferentially precisely because they should inherently improve system safety. For example, the arc-fault requirements in 690.11 (NEC 2011) do not apply to a PV system with a maximim (dc) system voltage of 80 volts or greater. That means that modules with microinverters are exempt from these new dc arc-fault protection requirements. The 2014 Code may require module-level disconnects for PV modules on buildings. Microinverters are not the only devices that can accomplish this, but they are one of three ways to do so.

 

[SolarPro]

More to the original topic: The allowance in 690.6(D) to install a single ground-fault detection device on a branch circuit of ac modules cannot be met at this time. This is not a problem as there are no ac ground-fault detection requirements for ac modules or interactive inverters in the Code. In some cases, the dc GFP scheme will see ground-faults on the ac side of the microinverter and take the system offline (at least I've heard inverter training/technical support personnel make this claim and presumably they are familiar with all of the likely system failure modes); if not, the OCPD on the ac circuit provides the necessary circuit protection in the event of a fault to ground.

 

[jaggedben]

The Code treats ac modules and microinverters in rooftop systems preferentially ....

AC modules, yes, but I don't think the code treats micro-inverters preferentially. Technically they must meet all the same requirements as string inverters.

 

[SolarPro]

AC module systems more so, but even microinverter systems are exempt from the arc-fault requirements as currently written.

 

[SolarPro]

You're correct that the Code doesn't treat microinverters preferentially. (The Code doesn't even define that term.) It's just that microinverter systems will typically have a system voltage under 80 Vdc and that low system voltage get a pass from the arc-fault requirments. That is very different than the way Code treats AC modules, which are defined in 690.