You're correct, generally, the neutral (N) and ground (G) should be bonded at only one location in a typical electrical system, which is usually at the service entrance (main panel) or the source of a separately derived system.Let's break down why having N-G bonds in multiple locations is generally problematic and then address your specific situation

Why Single Point Bonding is Important
Preventing Ground Loops

Multiple N-G bonds create parallel paths for neutral current to flow. Some current will flow through the grounding system instead of the intended neutral conductor. This is a "ground loop."

Increased Potential for Electrical Noise

Current flowing in the ground conductor can induce noise in sensitive electronic equipment.

Elevated Ground Potential

Multiple N-G bonds can elevate the ground potential at different points in the system, which is dangerous.

Safety Hazard

If the neutral conductor is interrupted, the grounding system becomes a path for neutral current, potentially energizing normally non-current-carrying metal parts.

Analysis of your Drawing

Without seeing your drawing, I have to make some assumptions. However, based on your description (PV disconnect and utility main switchboard), here's the likely situation: Utility Main Switchboard

This is where the main service entrance is located. This is typically where the N-G bond should be located for the entire premises wiring system. This is usually required by the NEC.

PV Disconnect

This is where the circuits from the PV inverter are isolated from the utility.

NEC Considerations for PV Systems

The NEC covers PV systems extensively, and specific requirements exist for grounding and bonding. Here are the key areas related to your question: Separately Derived Systems

A PV inverter can be considered a separately derived system, if it meets the definition in the NEC. A separately derived system generally requires its own N-G bond, located at the source (the inverter, in this case).

Grounding Electrode System

All grounding electrode systems must be connected together. This means that the utility service electrode, any building steel electrodes, and any other electrodes must be interconnected to form a single grounding electrode system.

Equipment Grounding Conductors

Equipment grounding conductors (EGCs) must be run with circuit conductors to ensure a low-impedance path for fault current.

So, is a N-G Bond at the PV Disconnect Required? Not Usually

In most residential grid-tied PV systems, the inverter is

not considered a separately derived system. The inverter is designed to synchronize with the grid and inject power into it. Therefore, no N-G bond is permitted at the PV disconnect or inverter. The N-G bond is only at the main service. When it Might Be

If the PV inverter is a transformerless inverter, and the manufacturer requires a functional ground (a connection between the neutral and ground) to operate safely, then a small N-G bond might be permitted at the inverter location. This is not common, but check the inverter's documentation.

Off-Grid Systems

In a true off-grid system that does not connect to the utility grid at all, the inverter is considered a separately derived system, and an N-G bond would be required at the inverter.

Recommendation1. Consult the Inverter Documentation

The most important step is to carefully read the inverter's installation manual. It will specify whether a functional ground is required and where it should be located. Follow the manufacturer's instructions.2.

Verify the System Type

Is this a standard grid-tied system, a system with battery backup, or a completely off-grid system? The answer will impact the grounding requirements.3.

Local Codes and AHJ

Your local codes and the Authority Having Jurisdiction (AHJ, the electrical inspector) have the final say. Consult with them to ensure compliance.4.

Eliminate Extra Bonds

If the inverter documentation doesn't require a bond at the PV disconnect, and if the system is grid-tied, then you should remove the N-G bond at the PV disconnect. The N-G bond should only be at the main service entrance.

In summary: While generally you only want one N-G bond, PV systems can be more complex. Refer to the inverter documentation and consult with a qualified electrician who is experienced with PV installations to ensure proper and safe grounding and bonding. Flag for review