Interconnection at Ungrounded Subpanel

[jaggedben]

It seems the OP has gotten the idea that it's best to re-install the feed to the garage with a proper EGC. That's for the best.

As for an open neutral... Well, that's bad news if it happens, for sure. But I still don't see why it's worse than a hot-to-ground fault that energizes metal parts and doesn't trip a breaker because earth resistance is high and there's no N-G bond. Both are bad.

 

[Smart $]

It seems the OP has gotten the idea that it's best to re-install the feed to the garage with a proper EGC. That's for the best.

...

What's best may exceed being compliant...

250.32 Buildings or Structures Supplied by a Feeder(s)
or Branch Circuit(s).

(A)... [omitted for brevity]
(B) Grounded Systems.

(1) Supplied by a Feeder or Branch Circuit. An equipment
grounding conductor, as described in 250.118, shall
be run with the supply conductors and be connected to the
building or structure disconnecting means and to the
grounding electrode(s). The equipment grounding conductor
shall be used for grounding or bonding of equipment,
structures, or frames required to be grounded or bonded.
The equipment grounding conductor shall be sized in accordance
with 250.122. Any installed grounded conductor
shall not be connected to the equipment grounding conductor
or to the grounding electrode(s).

Exception: For installations made in compliance with previous
editions of this Code that permitted such connection,
the grounded conductor run with the supply to the building
or structure shall be permitted to serve as the ground-fault
return path if all of the following requirements continue to
be met:

(1) An equipment grounding conductor is not run with the
supply to the building or structure.

(2) There are no continuous metallic paths bonded to the
grounding system in each building or structure involved.

(3) Ground-fault protection of equipment has not been installed
on the supply side of the feeder(s).

If the grounded conductor is used for grounding in accordance
with the provision of this exception, the size of the
grounded conductor shall not be smaller than the larger of
either of the following:

(1) That required by 220.61

(2) That required by 250.122

 

[ggunn]

In the situation the OP described, there would still be no imbalance in the current delivered by the inverters. They would not care what the neutral voltage was, and if there were no ground/neutral bond the inverter ground would be just fine. If the service tried to send primary current through the secondary neutral to ground, then there would be neutral current, but only if there were a bond, or if the insulation in the inverters broke down.

In your situation there will be a damaging voltage on the neutral because there are unbalanced loads in the house. Or in other houses on the same transformer.

I have dealt with a couple of three phase inverters that use the neutral as a reference (SMA TL and REFUsol/AE). In both cases the neutral must be bonded to ground, and voltage on the neutral will shut the inverter down.

 

[Smart $]

I have dealt with a couple of three phase inverters that use the neutral as a reference (SMA TL and REFUsol/AE). In both cases the neutral must be bonded to ground, and voltage on the neutral will shut the inverter down.

I don't recall the entire discussion's details, but my uderstanding is that the PV system will require a GES. If there is none at the buiding, one will have to be created. That will also require the GES to be bonded to the neutral and EGB at the subpanel. If the subpanel was compliant when installed there still should be a GES already in place, as it was required to be back when the feeder EGC was not required.

 

[RERoyA]

UL1741 inverters/neutral sensing

UL1741 inverters/neutral sensing

Sorry for late weigh in on this...

Under normal operation, all UL1741 inverters are voltage-controlled current sources and by definition required to provide a balanced current on L1 and L2 (single phase) and there is never any neutral current from/to the inverter. UL1741 inverters measure the grid voltage only; they cannot change the grid voltage, and inject current to the grid as long as grid voltage conditions are normal.

The "neutral" connection on the inverter for M215s (as well as the majority of other UL1741 inverters) is not a true "current carrying" neutral, but instead a high impedance voltage sensing line that terminates inside the inverter control and used by the inverter control to make voltage measurements L1-N and L2-N; using these voltage measurements, the inverter control compares the grid voltage condition to the IEEE1547 abnormal disconnect matrix. I haven't seen any inverter connected to split-phase 240V that didn't require a neutral connection; without the neutral sense connected the inverter will not turn-on initially and will cease operation if the neutral is lost.

• If the inverter is a transformer-isolated design, the grounded array conductor from the array (typically the NEG) is connected to earth through the internal ground fault detector fuse, and the wiring from the inverter ground must be sized as a GEC, and requires a GE if there is no GEC from from the utility feed.
• If the inverter used is a transformer-less design, neither of the array conductors is grounded ("floating array) and the wiring from the inverter ground bar is the system EGC. If there were an EGC available from the utility a grounding electrode would not be required, but for this situation, one would need to be provided

Bottom line, if the appropriate ground connection is provided at the inverter by way of a ground rod, then it is best not to reference the utility neutral to ground at the inverter.

 

[jaggedben]

The discussion above mostly had to do with article 250, and the safest way to ground a subpanel in a detached garage. The presence of the solar system was actually somewhat irrelevant, except for the fact that it requires (by code) a connection to a grounding electrode system. An inverter output circuit, like any other AC circuit, needs to have a proper EGC, connected to a proper GES, to trip a breaker. What type of grounding the solar system has on the DC side strikes me as completely unrelated.