You're right, you're onto a key point of how Rapid Shutdown (RSD) works and how it interacts with the disconnect requirements of NEC 690.13(E). Let's break it down

Your Understanding of Rapid Shutdown:

Your impression of RSD being a "stop generating" command is fundamentally correct. RSD typically reduces the voltage on the array conductors outside the RSD boundary (usually 1 foot from the array in 2023 NEC) to a specified level within a short timeframe (e.g., under 30 volts within 30 seconds). It achieves this by instructing the module-level power electronics (MLPEs) – optimizers or microinverters – to cease generating power.

The Disconnect Requirement (NEC 2023 690.13(E))

This section of the NEC requires a readily accessible disconnect to isolate the PV system from the building's electrical system. The purpose of this disconnect is to physically interrupt the flow of current, allowing for safe maintenance or emergency response.

Why RSD Alone Doesn't Always Satisfy 690.13(E)

Here's the critical part:

RSD is NOT a Physical Disconnect

While RSD stops generation and reduces voltage, it doesn't physically break the circuit. The conductors are still connected, even if they're not actively carrying significant voltage or current.

690.13(E) Requires Disconnection

The code mandates a physical break in the circuit. You need a device that opens the circuit to provide a true isolation point.

How Systems Meet Both Requirements

To comply with both RSD requirements and the disconnect requirement, systems typically use a combination of elements:1.

RSD Devices (MLPEs)

These provide the rapid shutdown functionality, reducing voltage on the array.2.

Disconnect Switches (or Circuit Breakers)

These provide the physical disconnection required by 690.13(E). These disconnects might be located:

At the Inverter

A disconnect switch integral to or immediately adjacent to the inverter is very common. This disconnects the DC input and the AC output of the inverter.

Between the Array and the Inverter

A disconnect switch on the DC conductors is often required, especially if the distance between the array and the inverter is significant. This ensures a disconnection point close to the array.

Inverter Operation and the "Missing Something"

You're right to suspect something about the inverter's role. Here's what you might be missing:

Inverter Disconnect Functionality

Many modern inverters have built-in relays or contactors that automatically disconnect the DC conductors when RSD is initiated. This is in addition to the MLPEs stopping generation. The inverter effectively opens the circuit on the DC side.

This is key! UL 1741 Compliance

Inverters that are designed to comply with UL 1741 standards for grid-tied inverters often include this disconnect functionality as part of the RSD implementation. UL 1741 specifically tests and validates these built-in disconnects.

So, does RSD disconnect the conductors? Typically, No (on its own)

RSD alone, as implemented by MLPEs, generally does not physically disconnect the conductors. It reduces voltage but leaves the circuit intact.

Yes (with Inverter Integration)

The system as a whole, when designed and installed correctly, does disconnect the conductors when RSD is initiated. This is because the inverter often includes a disconnect mechanism that is triggered by the RSD signal.

Example Scenario

1. A firefighter initiates RSD (e.g., flips a switch).
2. The RSD signal is sent to the MLPEs, which stop producing power, reducing array voltage to safe levels.3. The RSD signal also triggers a relay/contactor inside the inverter, physically disconnecting the DC input conductors from the inverter.4. The inverter also shuts down its AC output, and the AC disconnect (breaker or switch) can be opened to fully isolate the system.

In conclusion: You are correct in identifying that RSD alone doesn't satisfy the disconnection requirement. It is the combined functionality of the RSD devices and the inverter (with its internal disconnect mechanism) along with a physical disconnect switch (or breaker) that provides both the rapid shutdown and the required disconnection for safety. Always consult the specific inverter and RSD device documentation to understand how they are designed to work together to meet code requirements. Flag for review