You're right to be concerned. The system you've described has significant potential for problems, and you're correct in identifying the lack of a defined, low-impedance ground fault return path as a key issue. Let's break down the concerns and address your questions.

Why this is Problematic



Ground Fault Current Path

The most significant issue is the reliance on the earth as the primary ground fault return path. As you pointed out, this is explicitly prohibited by NEC 250.4(A)(5). The earth is a poor conductor and offers a high impedance path.

Relay Protection

The high impedance of the earth path will limit ground fault current. This can prevent protective devices (like the ground fault relays on the generators) from operating quickly or at all, leading to:

Equipment Damage

Prolonged ground faults can cause severe damage to generators, cables, switchgear, and connected loads.

Increased Arc Flash Hazard

Slower fault clearing times dramatically increase the arc flash energy and hazard.

Step and Touch Potential

As you mentioned, a ground fault could raise the potential of the earth near the fault location. Without a proper ground grid bonded to the equipment, step and touch potentials could exceed safe limits.

Voltage Gradients

During a ground fault, the current injected into the earth will create voltage gradients. These gradients can interfere with sensitive electronic equipment and communication circuits.

Multiple Grounds (Potential Issue)

While individual grounding is usually a good thing, having multiple unconnected ground grids (generator, switchgear, loads) creates a high-risk environment. This can create large potential differences between the ground grids during a fault.

Addressing Your Questions


Standards Allowing Isolation of GECs

You are unlikely to find a standard that allows isolation of the GECs in this manner. The NEC, IEEE 142 (Green Book), and IEEE 80 (Guide for Safety in AC Substation Grounding) all emphasize the importance of a low-impedance ground fault return path and equipotential bonding. While there might be very specific circumstances where isolation could be considered (e.g., in a system designed to intentionally float with ground fault detection systems and emergency shutdown), those are highly specialized and require extensive engineering analysis and specialized equipment. Those are extremely rare. What you've described doesn't fit that.2.

Safety Concerns

This system, as described, is inherently unsafe. The undefined ground fault return path and potential for significant step and touch potentials create a serious shock hazard. Without detailed ground impedance testing and calculations, you cannot guarantee safety.3.

Auxiliary Grounding for Switchgear and Loads

Your instinct to tie everything together is correct. The goal is to create an equipotential plane, minimizing voltage differences between equipment and the surrounding earth. Here's what you should do:

Bonding

Bond all ground grids (generators, switchgear, loads) together with a sufficiently sized grounding conductor. This creates a common grounding electrode system. The size of the grounding conductor should be determined by a fault study per NEC 250.66.

Grounding Electrode System

The combined ground grid should form a comprehensive grounding electrode system.

Equipment Grounding Conductor (EGC)

Run an EGC from the generator to the switchgear and from the switchgear to the loads. This EGC provides a low-impedance path for ground fault current. Use 4-wire cable for the generator to the switchgear. If using existing 3-wire cable, a safety review should be conducted to ensure that the cable meets the safety and electrical codes.

Single-Point Ground (Myth Debunked)

The idea of a "single-point ground" is often misunderstood. While it's important to avoid ground loops in sensitive electronic circuits, in power distribution systems, a multi-grounded system is preferred for safety and fault clearing.

Recommendations


Comprehensive Grounding Study

Conduct a thorough grounding study, including:

Soil Resistivity Testing

Determine the soil resistivity at the site. This is crucial for designing an effective ground grid.

Fault Current Analysis

Calculate the maximum ground fault current that can occur at various points in the system.

Ground Grid Design

Design a ground grid that meets the requirements of IEEE 80 and other applicable standards. This will involve determining the size and spacing of ground conductors, the depth of burial, and the number and placement of ground rods.

Step and Touch Potential Calculations

Calculate the step and touch potentials that will occur during a ground fault. Ensure that these potentials are within safe limits.

Impedance to Ground Testing

Measure the impedance to ground for your generator, switchgear, and load grids to compare to your calculated value in your study.2.

Implement Proper Grounding System

Based on the grounding study, implement a proper grounding system that includes:

Bonded Ground Grids

Bond all ground grids together.

Equipment Grounding Conductors

Run EGCs with all power cables.

Low-Impedance Ground Fault Return Path

Ensure a low-impedance path for ground fault current.3.

Review Protective Device Coordination

Review the coordination of all protective devices (relays, circuit breakers, fuses) to ensure that they will operate quickly and effectively in the event of a ground fault.4.

Personnel Training

Train personnel on the hazards of electricity and the importance of proper grounding.

Addressing the Existing Cables

The fact that the cable from the generator to the switchgear does not have a ground conductor is a major red flag. You have two options:

Replace the Cable

The best option is to replace the cable with a 4-wire cable (3 phases + ground).

Run a Separate Ground Conductor

If replacing the cable is not feasible, you could run a separate, adequately sized grounding conductor alongside the existing cable. This conductor should be bonded to the generator frame and the switchgear enclosure. You'll need to calculate the appropriate size of this conductor based on the potential fault current.

Legal and Ethical Considerations

As an engineer, you have a professional responsibility to ensure the safety of the system you design. If you believe that the existing system is unsafe, you have a duty to inform your client or employer of the risks and to recommend corrective actions. Do not sign off on a design that you believe to be unsafe.

In summary, what you've described is not a safe or compliant grounding system. A comprehensive grounding study and implementation of a proper grounding system are essential to ensure the safety of personnel and equipment.Disclaimer: I am an AI Chatbot and not a qualified electrical engineer. This information is for educational purposes only and should not be considered as professional advice. Always consult with a qualified electrical engineer for the design and implementation of electrical systems.* Flag for review