You're right, that part of 690.12 is confusing and causes a lot of debate. Let's break it down

What "other wiring systems" likely refers to:
AC Utility Grid

This is the most common and, arguably, the only "other wiring system" being considered in this context. Think of the connection point where the PV system interacts with the utility grid.

Other DC sources

While less common, this could also refer to other DC power sources within the building, such as a large battery system unrelated to the PV system.

The Intent Behind the Requirement

The intent of 690.12 is to ensure that, in the event of rapid shutdown, no voltage from the PV system can backfeed into another wiring system. This protects first responders and other personnel from potential shock hazards. By disconnecting all ungrounded conductors of the PV system from any other wiring system, you isolate the PV array during an emergency.

Why the Neutral is a Point of Confusion



Solidly Grounded Systems

The neutral, if present in an AC distribution system, is typically solidly grounded at the service entrance. Because of that single connection point, you have to make absolutely sure that you are not disconnecting it.

Referencing to other "wiring systems"

This is the problem with the wording of the code. When talking about "other wiring systems" in this context, it does not make sense to disconnect your PV system's grounded conductor from itself. So the intent of 690.12 must be with disconnecting it from other wiring systems like the grid.

So, Do You Switch the Neutral? Almost certainly not, unless specifically required by the inverter manufacturer or another section of the code. In most installations in North America, disconnecting the neutral is illegal.

Why not just make it explicitly clear? Different Philosophies

There are varying opinions on the safety benefits of switching the neutral in all scenarios, which is part of the reason for the ongoing debate and less-than-perfect code language. Some argue that it provides an extra layer of safety in certain fault conditions, especially in systems with complex grounding configurations. However, there's a general consensus that it's usually detrimental.

Complexity and Cost

Requiring neutral switching adds complexity and cost to PV systems. This is a factor that code-making panels consider when weighing the potential benefits.

In Summary

The part of 690.12 you highlighted primarily aims to isolate the ungrounded conductors of the PV system from the utility grid or other DC power sources during rapid shutdown. The language is vague, but it almost never requires breaking the neutral unless specifically instructed by the manufacturer of the equipment.

Important Considerations: Always consult with the local Authority Having Jurisdiction (AHJ) for clarification on their interpretation of this code requirement. Their interpretation is the one that matters for your project.* Double-check the manufacturer's instructions for all PV system components (inverters, disconnects, etc.). They may have specific requirements regarding grounding and disconnecting.This is a complex issue, and interpretations can vary. When in doubt, err on the side of caution and consult with a qualified electrical engineer or inspector. I hope this clarifies the situation! Flag for review