disconnecting one leg of inverter output as RSD initiation

electrofelon

electrofelon

Senior Member
Location
Cherry Valley NY, Seattle, WA
Occupation
Electrician
This would be code compliant correct? Example: Ground mount array with microinverters. Inverter output(s) run inside building. Only one pole per circuit is opened by RSD initiating device on outside of building. No other device that can serve as RSD initiation such as outside service disconnect. 2017 NEC.
 
No. Your voltage from the other conductor to ground would violate the voltage limits.

The 2017 is a little vauge on applying RS to AC circuits, so possibly one can argue no RS is necessary at all, although that pretty clearly goes against the intent, as has been clarified in subsequent codes. But in no case where RS is required on a 240V circuit would a single pole disconnect suffice.
 
jaggedben said:
No. Your voltage from the other conductor to ground would violate the voltage limits.

The 2017 is a little vauge on applying RS to AC circuits, so possibly one can argue no RS is necessary at all, although that pretty clearly goes against the intent, as has been clarified in subsequent codes. But in no case where RS is required on a 240V circuit would a single pole disconnect suffice.
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But wouldnt it make the inverter shut down if it lost one leg? Assume enphase.
 
electrofelon said:
Am I misunderstanding RSD? You dont necessarily need to disconnect the conductors. You need to trigger the inverters to shut down.
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Yes, you are misunderstanding. It is voltage between conductors that must be limited, regardless of where it comes from. The 2020 NEC makes this explicit by controlling conductors for 'PV system DC circuits' and 'inverter output circuits originating from inverters located within the array boundary'. The 2017 is just badly written in this respect (it refers to 'PV system circuits', which is undefined), but you can look to the 2020 for clarification.

The intent of rapid shutdown is to reassure firefighters that they can fight a fire around or perhaps even within a solar array after initiating rapid shutdown, because voltage will be safe for them with proper PPE. That should be based on anything like inverters or ac vs. dc that firefighter's don't necessarily know anything about.
 
jaggedben said:
Yes, you are misunderstanding. It is voltage between conductors that must be limited, regardless of where it comes from.
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Right, but with a grid-tied single phase inverter, if you open one of the two AC conductors, doesn't the inverter sense loss of grid and stop putting out any voltage on either conductor?

Cheers, Wayne
 
electrofelon said:
But wouldnt it make the inverter shut down if it lost one leg? Assume enphase.
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It should. It would be islanded and shutdown within whatever the anti-islanding time period is for those micros.

Keep in mind, those micros don't do anything with neutral so the other line doesn't "see" any voltage once one leg has been disconnected.
 
If you disconnect L1, the inverter will shut down and won't be a voltage source, but the grid will remain a voltage source to the microinverters, and you will still have 120V from L2 to N.
 
Since 690.12(A) says that "Requirements for controlled conductors shall apply to PV circuits supplied by the PV system" my assumption is that RSD relates to controlling the PV system's supply to those conductors, and so the fact that a conductor may remain energized from the grid is immaterial to compliance with 690.12. Yes or no?

Cheers, Wayne
 
wwhitney said:
Since 690.12(A) says that "Requirements for controlled conductors shall apply to PV circuits supplied by the PV system" my assumption is that RSD relates to controlling the PV system's supply to those conductors, and so the fact that a conductor may remain energized from the grid is immaterial to compliance with 690.12. Yes or no?

Cheers, Wayne
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Seems that way to me.
 
There are two approaches here. One is what the code says, and another is what the code should be. For the former, it seems clear to Wayne and I that the single leg of the inverter output circuit that remains energized by the utility is not covered by the RSD requirement. For the latter, it doesnt seem to me logical to require RSD for inverter output circuits. Isnt the theory of RSD that you want a means to deenergize circuits that remain energized when the service disconnect is open?
 
wwhitney said:
Right, but with a grid-tied single phase inverter, if you open one of the two AC conductors, doesn't the inverter sense loss of grid and stop putting out any voltage on either conductor?

Cheers, Wayne
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Irrelevant. The voltage of the other conductor to ground will be 120V (energized by the grid) so the circuit will not comply.
 
wwhitney said:
Since 690.12(A) says that "Requirements for controlled conductors shall apply to PV circuits supplied by the PV system" my assumption is that RSD relates to controlling the PV system's supply to those conductors, and so the fact that a conductor may remain energized from the grid is immaterial to compliance with 690.12. Yes or no?

Cheers, Wayne
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No.

The test that the NEC imposes (for other than listed arrays) is quite simple: operate the rapid shutdown switch, and voltage must be limited within the timeframe. It's agnostic to methods and to the source of voltage.
 
electrofelon said:
In this case, I guess I am struggling to understand what exactly RSD is trying to accomplish.
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It is there to reassure fire fighters that arrays are safe enough to conduct rooftop firefighting operations.

electrofelon said:
There are two approaches here. One is what the code says, and another is what the code should be. For the former, it seems clear to Wayne and I that the single leg of the inverter output circuit that remains energized by the utility is not covered by the RSD requirement.
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No. It's covered by the RSD requirement because the RSD requirement is concerned with any source that might energize a solar array. Because fire fighters are not supposed to have to know what energizes the solar array.

electrofelon said:
For the latter, it doesnt seem to me logical to require RSD for inverter output circuits. Isnt the theory of RSD that you want a means to deenergize circuits that remain energized when the service disconnect is open?
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The theory of RSD is simply that firefighters have a labeled Rapid Shutdown switch to throw that limits voltage in solar arrays so that they will conduct firefighting operations in and around the array instead of letting the structure burn down. This is up to and including a firefighter putting his/her chainsaw through the solar array - no joke in the case of the Tesla solar roof. The purpose of RSD has nothing in particular to do with the service disconnect. If your service disconnect accomplishes RSD and complies with the location requirements for the RSD switch, slap an RSD switch label on the service disconnect. If it doesn't, provide another switch that will make the system comply.
 
jaggedben said:
It is there to reassure fire fighters that arrays are safe enough to conduct rooftop firefighting operations.


No. It's covered by the RSD requirement because the RSD requirement is concerned with any source that might energize a solar array. Because fire fighters are not supposed to have to know what energizes the solar array.


The theory of RSD is simply that firefighters have a labeled Rapid Shutdown switch to throw that limits voltage in solar arrays so that they will conduct firefighting operations in and around the array instead of letting the structure burn down. This is up to and including a firefighter putting his/her chainsaw through the solar array - no joke in the case of the Tesla solar roof. The purpose of RSD has nothing in particular to do with the service disconnect. If your service disconnect accomplishes RSD and complies with the location requirements for the RSD switch, slap an RSD switch label on the service disconnect. If it doesn't, provide another switch that will make the system comply.
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Ok, but please note the specific situation in this thread which a ground mount array with an inverter output circuit only in the building. The array itself has NO rapid shutdown requirement.
 
electrofelon said:
Ok, but please note the specific situation in this thread which a ground mount array with an inverter output circuit only in the building. The array itself has NO rapid shutdown requirement.
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Sorry I kind of missed that.
In my opinion there's no rapid shutdown requirement for your ground mount array whatsoever, because you don't have an array installed on a building. This gets into the stupid issue of the NEC's definition of a building. But still my opinion.
 
electrofelon said:
Ground mount array with microinverters.
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Do you have a PV system disconnecting means at the array?

If so, then the 2020 and 2023 NEC make it clear 690.12 doesn't extend beyond the PV system disconnecting means, so your feeder from the building to the PV system disconnecting means would not be subject to RSD. While you've specified the 2017 NEC, sounds like there is some ambiguity about whether 2017 NEC 690.12 applies to AC circuits at all, so it seems reasonable to take advantage of that to reach the same conclusion as for the 2020 and 2023 NEC.

Cheers, Wayne
 
jaggedben said:
No.

The test that the NEC imposes (for other than listed arrays) is quite simple: operate the rapid shutdown switch, and voltage must be limited within the timeframe. It's agnostic to methods and to the source of voltage.
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I'm not convinced that's true because the issue has been conductors which are energized EVEN WHEN something like the main disconnect is opened. That's what firefighters have dealt with, since I believe it's common practice to de-energize a building when they start firefighting.
 
jaggedben said:
No. It's covered by the RSD requirement because the RSD requirement is concerned with any source that might energize a solar array.
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I am not sure that is true:

(A) Controlled Conductors. Requirements for controlled conductors shall apply to PV circuits supplied by the PV system.

...logically, it makes sense to be concerned with the conductors that are NOT de-energized by turning off the service disconnect.
 
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