Rapid Shutdown for PV Systems: Page 4 of 7

Understanding NEC 690.12

NEC 690.12 clarifies the types of PV systems that must comply with rapid shutdown, the circuits that the shutdown process must control, the maximum allowable voltage on these circuits, the maximum allowable time frame to accomplish shutdown, the system labeling requirements and the product listing requirements. The language intentionally does not specify what type of device you should use to initiate rapid shutdown or where you should locate it. Since the Code language is an installation requirement and not an instruction manual, here I address some of the frequently asked questions regarding rapid shutdown.

Which systems must comply? As stated in the section title, rapid-shutdown requirements apply to PV systems on buildings. If you are installing a roof-mounted PV system subject to NEC 2014, the rapid-shutdown requirements clearly apply to your project. If you are installing a ground-mounted or similar system where none of the PV system components or circuits contacts a building, the rapid-shutdown requirements do not apply.

This does not mean that all ground-mounted PV systems are exempt from rapid-shutdown requirements. Where PV system circuits from a ground-mounted PV system are physically attached to or penetrate a building, you should apply NEC 690.12. However, in this case, the conductors on or entering the building are subject to rapid shutdown, but the conductors off the building are not. Note that underground conductors that travel under buildings are not considered to be “on or in buildings.” Where buried conductors come up into a building, you are allowed to run them a distance of 5 feet from the point of penetration through the floor before installing a disconnecting means.

Which circuits must the shutdown system control? The rapid-shutdown requirements apply to “PV system circuits,” which includes both dc and ac circuit conductors. In utility-interactive PV systems, the primary rapid-shutdown objective is to control the dc PV power circuits, as you control the ac circuits associated with the PV system by opening the connection to the utility-supplied service, which is a standard emergency response tactic. In a stand-alone system or an interactive system with battery backup, the rapid-shutdown system must also control any ac circuits that remain energized in the absence of utility power.

As stated in 690.12(A), you must control the PV circuit conductors within 5 feet of entering a building or 10 feet of the array. If you install PV source-circuit conductors in metal conduit that directly enters the attic of a building, you must control those conductors within 5 feet. If you route your conduit across the roof instead, the maximum uncontrolled conductor length is 10 feet. No scenario permits you to add these allowable conductor lengths together; the maximum uncontrolled conductor length is 10 feet.

What is the maximum allowable controlled circuit voltage? During rapid shutdown, you must limit voltage on controlled conductors to no more than 30 volts. Per the ROC substantiation, the stakeholders who developed NEC 690.12 chose a 30-volt limit for two reasons. First, this is the touch-safe voltage limit for wet locations, established by various national and international standards. Second, this voltage level allows for the use of 24-volt control circuits, such as those used to control contactors in combiner boxes.

What is the allowable time frame? You must achieve safe voltage levels in control circuits within 10 seconds of rapid-shutdown initiation. Per the ROC substantiation, the 10-second requirement is intended to allow dc-side capacitor banks to discharge by means other than contactors and shunt-trip breakers. Verifying compliance with this time limit requires some diligence on the part of the designer, plan checker or inspector. The good news is that some inverters—typically transformerless string inverters—can meet this 10-second limit without further mitigation. The bad news is that other inverters—typically transformer-isolated string inverters and larger central inverters—cannot meet this 10-second limit without some method of controlling voltage from the capacitor banks.

To complicate matters further, some vendors already have a capacitor-bank control method integrated into their inverters, while others do not. For example, Solectria’s central inverters have a contactor in series with each of the fused inputs in its inverter-integrated subcombiners; when these contactors open, the subarray inputs are isolated from the capacitor bank. Some other inverters have large capacitor banks that take several minutes to de-energize to a safe voltage level and do not have any internal contactors. If an inverter cannot meet the 30-volt and 10-second limits and does not have an integrated isolation device, system designers need to add a control method external to the inverter to comply with NEC 690.12. If you are not sure whether you need to disable a particular inverter’s capacitor bank, contact the manufacturer’s applications engineering department.

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