Okay, let's break down this electrical setup and analyze the considerations for an additional disconnect switch. Your assessment of the NEC requirements is accurate. Since you have a service disconnect switch (the pad-mounted switch before the transformer), the ATS is technically not required to be service-rated, and another disconnect before it isn't strictly required.However, your instincts about safety and ease of operation are spot-on. Here's a more detailed analysis and my advice

Arguments For Adding a Disconnect Switch Between the Transformer Secondary and the ATS:
Enhanced Safety

This is the biggest driver. Working on 4800V equipment hundreds of feet away is far more dangerous than working on 120/240V equipment nearby. Having a readily accessible, low-voltage disconnect right next to the ATS and generator provides a much safer and more convenient way to isolate the ATS, generator, and downstream equipment during maintenance, troubleshooting, or emergencies. The primary disconnect requires trained personnel and potentially specialized equipment for safe operation.

Ease of Maintenance and Troubleshooting

Imagine you need to troubleshoot the ATS, generator, or anything downstream in the electrical building. Having to walk several hundred feet to operate a medium voltage disconnect, possibly in adverse weather, is inefficient and time-consuming. A local disconnect allows quick and easy isolation.

Operator Familiarity

You've already identified that operators might be more comfortable and familiar with low-voltage equipment. This reduces the risk of human error and promotes safer operations. Less reliance on specialized personnel is also a benefit.

Future Flexibility and Expansion

While not immediate, a disconnect offers flexibility for future modifications or additions to the system. It provides a clear point of isolation for any upstream or downstream work.

Reduced Arc Flash Hazard

While not completely eliminated, isolating the low-voltage side with a disconnect reduces the potential arc flash hazard when working on the ATS, generator, or electrical building. While proper PPE is always required, minimizing the potential hazard is always beneficial.

Code Compliance in Certain Circumstances

While your basic understanding of the code requirements is correct, there may be local or project-specific requirements that call for disconnects to be present, or located closer to specific equipment. Check your specific local code requirements to ensure all standards are being followed.

Arguments Against Adding a Disconnect Switch



Cost

An 800A 120/240V disconnect switch, enclosure, and associated labor will add to the overall project cost.

Space

Requires additional pad space near the transformer/generator/ATS location.

Increased Complexity

Adds another piece of equipment to the system, potentially increasing the probability of failure (though a well-maintained disconnect switch is very reliable).

My Recommendation: Include the Disconnect Switch.

Given your concerns about safety, ease of use, and operator experience, I strongly recommend including an 800A 120/240V disconnect switch between the transformer secondary and the ATS. The increased cost is a reasonable investment in the long-term safety and operational efficiency of the gas compressor station. The convenience and confidence it provides during maintenance and emergencies far outweigh the additional expense.

Key Considerations for Implementation



Disconnect Switch Type

Choose a robust, reliable disconnect switch suitable for outdoor use and the expected load conditions. A fusible disconnect offers an additional layer of protection.

Enclosure

Ensure the enclosure is weatherproof and properly sized to accommodate the disconnect switch and associated wiring.

Location

Place the disconnect switch in a readily accessible location, close to the ATS and generator, and away from potential hazards (e.g., vehicular traffic). Consider ergonomics – can operators easily reach the handle?

Clear Labeling

Clearly label the disconnect switch with its function and the equipment it isolates.

Grounding

Ensure proper grounding of the disconnect switch and enclosure.

Coordination

Properly coordinate the overcurrent protection devices (bayonet fuse, disconnect switch fuses (if applicable), generator circuit breaker, and MCC circuit breaker) to ensure selective tripping.

Documentation

Update all electrical drawings and documentation to reflect the addition of the disconnect switch.*

Training

Provide operators with thorough training on the operation and maintenance of the disconnect switch.

In Summary:

While not strictly required by code in your described configuration, adding a disconnect switch between the transformer secondary and the ATS is a prudent decision that significantly enhances safety, simplifies maintenance, and improves the overall operability of the gas compressor station. Prioritizing safety and ease of use, especially with operators who may not be highly experienced with high-voltage equipment, is a wise choice. Carefully consider the implementation details outlined above to ensure the disconnect switch is properly selected, installed, and maintained. Flag for review