Utility Interactive inverter AC short Circuit/withstand rating.

[Zane Bailey]

Lately I have been asked to inspect a lot of Utility interactive Inverters for both Solar and Battery storage. In both cases the inverters are labeled with the Utility Interactive rating label (NEC requirement) and also have the UL 1741 labeling.

The inverters are listed and labeled with a DC short circuit rating; however, they have no labels that indicate an AC short circuit rating.
All of the projects have had Short Circuit Coordination Studies (SCCS) performed. The studies show that the AC available fault current at the inverter locations are roughly between 6195 ka to say 8100 ka phase to ground fault. The manufactures cannot provide any information that would determine that the inverters were tested to withstand fault current at these levels.

I find it hard to believe that a piece of equipment that interacts with the Utility grid and Is permissible by Code to be connected line side of the service equipment (upstream of main breaker), would not have a AC SC rating let alone a very high rating.

I am being asked to allow this equipment to be energized without this verification being made.

Has anyone else in countered this or can shed light into this?

Zane.

 

[Smart $]

....

Has anyone else in countered this or can shed light into this?

Well, for one, the GTI's are not connected directly. They are connected through a disconnecting means with OCPD, which has to comply with SCCR requirements.

Regarding the GTI's themselves...

110.10 Circuit Impedance, Short-Circuit Current Rat-
ings, and Other Characteristics. The overcurrent protective
devices, the total impedance, the equipment short-
circuit current ratings, and other characteristics of the
circuit to be protected shall be selected and coordinated to
permit the circuit protective devices used to clear a fault to
do so without extensive damage to the electrical equipment
of the circuit. This fault shall be assumed to be either between
two or more of the circuit conductors or between any
circuit conductor and the equipment grounding conductor(s)
permitted in 250.118. Listed equipment applied in
accordance with their listing shall be considered to meet the
requirements of this section.

 

[shortcircuit2]

Lately I have been asked to inspect a lot of Utility interactive Inverters for both Solar and Battery storage. In both cases the inverters are labeled with the Utility Interactive rating label (NEC requirement) and also have the UL 1741 labeling.

The inverters are listed and labeled with a DC short circuit rating; however, they have no labels that indicate an AC short circuit rating.
All of the projects have had Short Circuit Coordination Studies (SCCS) performed. The studies show that the AC available fault current at the inverter locations are roughly between 6195 ka to say 8100 ka phase to ground fault. The manufactures cannot provide any information that would determine that the inverters were tested to withstand fault current at these levels.

I find it hard to believe that a piece of equipment that interacts with the Utility grid and Is permissible by Code to be connected line side of the service equipment (upstream of main breaker), would not have a AC SC rating let alone a very high rating.

I am being asked to allow this equipment to be energized without this verification being made.

Has anyone else in countered this or can shed light into this?

Zane.

Good question. Equipment must be able to withstand the available fault current that it is exposed to. This must include Grid Tied Inverters. Although it is not directly connected to the service conductors as Smart$ pointed out...the GTI should at a minimum be protected at a SCCR by the PV Service Disconnect ahead of it.

 

[Zee]

Good question. Equipment must be able to withstand the available fault current that it is exposed to. This must include Grid Tied Inverters. Although it is not directly connected to the service conductors as Smart$ pointed out...the GTI should at a minimum be protected at a SCCR by the PV Service Disconnect ahead of it.

Yes, any inverter that is grid-tied should have it's own OCPD. Whether it is line-, or load- side connected.

Zane, I am curious, were these inverter output circuits not routed thru fused disconnects, or breakers?

 

[Zane Bailey]

Utility Interactive inverters

Utility Interactive inverters

Yes, any inverter that is grid-tied should have it's own OCPD. Whether it is line-, or load- side connected.

Zane, I am curious, were these inverter output circuits not routed thru fused disconnects, or breakers?

The inverters have a circuit breaker at the point of connection, they also have a fused disconnect which feeds a panel downstream, the panel has multiple sng ph 40 amp circuit breakers that feed the inverters. the SCCS (short circuit coordination study) shows that the available fault current phase to ground at the inverter is between 6 ka and 8 ka. the inverters are listed for Utility interactive UL 1741. they are not marked with a withstand rating.

the following equipment is required to be marked with a short circuit rating. the manufacture will not provide this information.
-Industrial controllers
-motor controllers
-all equipment listed UL 508a
-HVAC
-Meter disconnects
-Industrial Machine electrical panels

it seems strange that the code would not require Utility interactive inverters to be marked with a short circuit rating, especially if they are permitted to be connected line side of the service disconnect means, where they can be exposed to high fault current?

Zane

 

[SolarPro]

While PV systems are allowed to be connected on the supply side of the main service disconnecting means, per 705.12(A) & 230.82(6), doing so requires the installation of a fused PV system service disconnecting means. The OCP in the fused service disconnect provides the required short circuit protection for the interconnected electric power production source. The utility-interactive inverters themselves are current limited in the event of a fault.

Note that Section 705.31 was added to NEC 2014 to clarify that the OCP for a supply-side connected parallel power production source must be located within 10' of the point of interconnection. Where this is not possible, cable limiters must be installed at the point of connection to provide short circuit protection up to the fused service disconnecting means for the PV system. As Mike Holt himself has pointed out, even though a PV disconnecting means in this scenario does not meet the technical definition of a service disconnect, it should be installed under the same rules as it needs to provide the same level of protection.

 

[SolarPro]

FWIW: Here's the proposal and substantiation that was accepted and is now part of NEC 2014:

4-410a Log #CP402 NEC-P04 Final Action: Accept
(705.31 (New) )
________________________________________________________________
TCC Action: The Correlating Committee directs this proposal be clarified
by replacing ?per 705.12(A)? with ?in accordance with 705.12(A)? to
conform with 4.1 of the NEC Style Manual.
This action will be considered as a public comment.
Submitter: Code-Making Panel 4,

Recommendation: Add a new section to read as follows:

705.31 Location of Overcurrent Protection. Overcurrent protection for electric power production source conductors, connected to the supply side of the service disconnecting means per 705.12(A), shall be located within 3m (10 ft) of the point where the electric power production source conductors are connected to the service.

Informational note: This overcurrent protection protects against short-circuit current supplied from the primary source(s) of electricity.

Exception: Where the overcurrent protection for the power production source is located more than 3 m (10 ft) from the point of connection for the electric power production source to the service, cable limiters or current limited circuit breakers for each ungrounded conductor shall be installed at the point where the electric power production conductors are connected to the service.

Substantiation: Often, when connections are made to, or ahead of, existing service entrance equipment, space limitations do not allow for a disconnecting means with overcurrent protection to be adjacent to the service entrance equipment and still have proper working clearances. Consequently, many of the required disconnects with overcurrent protection are being installed remote
from the service entrance equipment. These unprotected conductors from the service entrance equipment to the remote disconnect with overcurrent protection are a safety hazard since they do not have adequate short-circuit current protection. These conductors are subject to the available fault current from the utility and are not really ?protected? by the utility transformer primary overcurrent protective device(s). A fault on these conductors will likely result in a violent, explosive conductor vaporization and potential equipment damage or complete burndown. This proposal would require short-circuit protection (cable limiters) whenever the length of unprotected cable exceeded 3m (10 ft).

Panel Meeting Action: Accept
Number Eligible to Vote: 13
Ballot Results: Affirmative: 13

Per the logic above, everything on the power production source side of the fused disconnecting means is protected from the high fault currents available from the utility.
________________________________________________________________

 

[Zane Bailey]

FWIW: Here's the proposal and substantiation that was accepted and is now part of NEC 2014:

4-410a Log #CP402 NEC-P04 Final Action: Accept
(705.31 (New) )
________________________________________________________________
TCC Action: The Correlating Committee directs this proposal be clarified
by replacing ?per 705.12(A)? with ?in accordance with 705.12(A)? to
conform with 4.1 of the NEC Style Manual.
This action will be considered as a public comment.
Submitter: Code-Making Panel 4,

Recommendation: Add a new section to read as follows:

705.31 Location of Overcurrent Protection. Overcurrent protection for electric power production source conductors, connected to the supply side of the service disconnecting means per 705.12(A), shall be located within 3m (10 ft) of the point where the electric power production source conductors are connected to the service.

Informational note: This overcurrent protection protects against short-circuit current supplied from the primary source(s) of electricity.

Exception: Where the overcurrent protection for the power production source is located more than 3 m (10 ft) from the point of connection for the electric power production source to the service, cable limiters or current limited circuit breakers for each ungrounded conductor shall be installed at the point where the electric power production conductors are connected to the service.

Substantiation: Often, when connections are made to, or ahead of, existing service entrance equipment, space limitations do not allow for a disconnecting means with overcurrent protection to be adjacent to the service entrance equipment and still have proper working clearances. Consequently, many of the required disconnects with overcurrent protection are being installed remote
from the service entrance equipment. These unprotected conductors from the service entrance equipment to the remote disconnect with overcurrent protection are a safety hazard since they do not have adequate short-circuit current protection. These conductors are subject to the available fault current from the utility and are not really ?protected? by the utility transformer primary overcurrent protective device(s). A fault on these conductors will likely result in a violent, explosive conductor vaporization and potential equipment damage or complete burndown. This proposal would require short-circuit protection (cable limiters) whenever the length of unprotected cable exceeded 3m (10 ft).

Panel Meeting Action: Accept
Number Eligible to Vote: 13
Ballot Results: Affirmative: 13

Per the logic above, everything on the power production source side of the fused disconnecting means is protected from the high fault currents available from the utility.
________________________________________________________________

Thanks all for your replies, all good information. My question though is. The SCCS shows that there is between 6 to 8 ka of fault current at the inverter lugs. Wouldn't the inverter have to have a SC rating above these values? As stated previously, HVAC, motor controllers, industrial control panels etc. are required to be marked with a SCR? Why hasn't the code required this for Utility Interactive Inverters? It seems that it is impossible to verify without the marking/listing.

 

[GoldDigger]

Devices that by design either have to interrupt fault current or safely pass fault current through to such a device will need an SCCR value. Loads and generators, by their nature do not.

Tapatalk!

 

[SolarPro]

Right. Remember the NEC isn't the only applicable safety standard. The main standard that applies is the product safety standard, in this case UL 1741.

While I've never paid the big bucks to read the actual UL standard, the Table of Contents for "Inverters, Converters, Controllers and Interconnection System Equipment for Use With Distributed Energy Resources" are publicly viewable. The performance tests include short circuit and withstand tests. Underwriter's Laboratories designs product safety tests such that they are appropriate for the specific application, and listed products are considered suitable for that specific application, per Article 100 definitions.

In other words, inverters are suitable for supply- and load-side utility interconnections because that is what they are designed to do. The adequacy of individual product design is verified by Nationally Recognized Testing Laboratories, which run the tests outlined in the product safety standard. Section 690.4 further clarifies that inverters used for PV power applications must be listed for the PV application. Basically, Smart $ answered your question in the first post when he referred to Section 110.10, "Listed equipment applied in accordance with their listing shall be considered to meet the [short-circuit current ratings] requirements of this section."

 

[ggunn]

Note that Section 705.31 was added to NEC 2014 to clarify that the OCP for a supply-side connected parallel power production source must be located within 10' of the point of interconnection.

I assume that this 10' limit is measured "as the wire flies", not simply physical separation. That should be made clear in the code, IMO, otherwise someone might think that running 30 foot conductors up to a ceiling and back down to a switch that is physically 10' away from an interconnect is compliant.

 

[SolarPro]

The call for public input for NEC 2017 is open now.