A Peek at the 2014 NEC - Part 1: Page 2 of 2

Intermediate
Three-phase string inverter
Three-phase string inverters ranging from 8 kW to 30 kW with a maximum input voltage of 1,000 VDC are becoming common in commercial applications.

Protection: Ground-Fault, Arc-Fault, Overcurrent & Rapid Shutdown

New requirements for ground-fault protection (GFP) mandate detecting faults on intentionally grounded conductors, which some types of GFP devices cannot detect (the so-called “blind spot” in traditional GFP)—expect changes in manufacturer’s inverter-integrated GFPs. New text also allows the GFP to open (disconnect) the grounded conductor for the purposes of measurement—a common startup procedure for some types of inverters and retrofit GFP systems. The grounded conductor is automatically ungrounded and then insulation resistance between positive and ground, and negative and ground, is verified each day before the inverter begins operation.

Section 690.9(A) clarifies that when properly sized circuit conductors are connected to current-limited sources, such as a PV module, PV source circuit, or a utility-interactive inverter, the potential for overcurrent comes from other sources of “significantly higher current.” The intention is that overcurrent protection devices (OCPDs) should be placed at the source of the potential fault current. Thus, series fusing is located where multiple PV source circuits are connected in parallel in a combiner box, and the output of a utility-interactive inverter is back-fed through a circuit breaker or fused disconnect where it connects to the utility grid. Note that 690.9(D) now requires “listed PV overcurrent devices in PV source and output circuits” and that 690.9(E) clarifies that overcurrent protection is required on both the positive and negative DC conductors in ungrounded PV systems.

The requirements in section 690.11 for DC circuit arc-fault protection have been expanded by removal of the phrase “on or penetrating a building.” This, in effect, extends the arc-fault protection requirement to all systems with any DC circuits that have a maximum system voltage greater than or equal to 80 VDC.

A major shift in rooftop system design comes with the new Section 690.12, which requires the ability for rapid shutdown of PV systems on or in buildings. Due to firefighter concerns about energized PV system conductors inside buildings or running across rooftops, this section now requires equipment that will limit voltage between any conductors (including ground) to no more than 30 V and 240 volt-amperes within 10 seconds of initiating shutdown. This section applies to conductors that extend more than 5 feet inside a building, or extend more than 10 feet past the perimeter of the PV array. Systems with microinverters, AC modules, or DC-to-DC converters already comply, as the AC interconnection breaker provides this functionality, disabling the module-level electronics (MLEs) and isolating energized conductors to those between the module and the MLEs.

While 30 V is less than the open-circuit voltage of many modules, isolating energized PV conductors to within 10 feet of the array—even if series connections are made prior to that point—provides a compliant method for string-inverter-based systems. This strategy will require additional rooftop switching (240 volt-amperes is the equivalent of 24 V at 10 A, which could be used for a contactor control circuit for rapid shutdown of rooftop combiner boxes). Section 690.56(C) specifies the labeling requirement for rapid shutdown systems; note that the equipment used must be listed and identified [690.12(5)], though the Code does not state that the equipment must be explicitly listed for the purpose of rapid shutdown of PV systems.

Disconnects

Section 690.15(C) now requires load-break-rated disconnects on the DC output of combiner boxes mounted on roofs. These disconnects can either be integrated into the combiner box or be an external switch not more than 6 feet away from the combiner box. The disconnecting means can be remote-controlled (such as a contactor, which could also help meet rapid shutdown requirements), but must also be manually operable at the combiner box location. Along with rapid shutdown requirements, this will make it less likely for installers to run multiple PV source circuits from a roof-mounted array to a string inverter at ground level. A combiner box, which will provide means to quickly isolate the array, along with disconnecting means, will become the standard for roof-mounted systems, most likely with a contactor that can be operated either from the ground or locally on the roof.

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Brian Mehalic is a NABCEP-certified PV professional and ISPQ-certified PV instructor. He has experience designing, installing, servicing, and inspecting all types and sizes of PV systems. He is a curriculum developer and instructor for Solar Energy International.

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