You've identified a tricky situation, and your understanding of the code and the implications is good. Let's break down the options and the code considerations

Understanding the Issue
High Resistance Grounding (HRG)

HRG limits ground fault current, reducing arc flash hazard and potential damage. It allows for continued operation with a single ground fault, which is a benefit in many industrial settings. However, it doesn't eliminate the risk entirely, and ground faults need to be located and cleared.

Solidly Grounded

Provides a low-impedance path for ground fault current, causing protective devices to trip quickly. This eliminates the "continued operation" benefit of HRG.

NEC 250.30(A)(1)

This section mandates an unspliced grounding electrode conductor (GEC) from the separately derived system (SDS) transformer to the grounding electrode and an unspliced system bonding jumper (SBJ) from the SDS neutral point to the grounding electrode. This is meant to provide a robust and reliable grounding path.

Your Goal

You want a way to isolate the HRG components for maintenance without shutting down the entire system.

The Problem

A double throw switch to transition between HRG and solid grounding seems to violate the "unspliced" SBJ requirement.

Analyzing Your Options1. Impedance Grounded to Solidly Grounded (with a Switch)



Code Conflict

You're right. Using a double-throw switch to change between HRG and solid grounding is a direct conflict with NEC 250.30(A)(1). The SBJ cannot be spliced. A switch creates an unavoidable splice and a point of failure in the grounding path.

Safety Concern

This is a very unsafe arrangement. If the switch fails or is improperly operated, you could end up with an ungrounded or poorly grounded system during a fault, leading to significantly increased voltage to ground and potentially severe hazards.

Not Code Compliant and Highly Discouraged.2. Impedance Grounded to Ungrounded



Feasibility

Switching to an ungrounded system to perform HRG equipment maintenance is possible, but it comes with significant drawbacks and requires careful consideration.

Advantages

Allows for maintenance on the HRG resistor without a complete system shutdown.

Disadvantages and Concerns



Increased Transient Overvoltages

Ungrounded systems can experience higher transient overvoltages during switching operations and ground faults. These overvoltages can damage equipment insulation and potentially lead to multiple faults.

Difficult Fault Location

Finding the first ground fault in an ungrounded system can be very difficult and time-consuming.

Capacitive Coupling

Ungrounded systems develop a voltage to ground based on capacitive coupling. This can be high enough to be hazardous, especially in systems with long cable runs.

Arc Flash Hazard

During the period that it is ungrounded, the arc flash hazard will be significantly different.

Personnel Safety

Personnel must be thoroughly trained on the differences in working on an ungrounded system. There's a higher risk of shock hazards if they're not aware of the potential for high voltages to ground.

Compliance

The electrical equipment you connect to this system might not be designed for ungrounded systems.

Code Considerations

While you might avoid directly violating 250.30(A)(1) since you're removing the grounding, you must ensure the system complies with all other applicable codes (e.g., overvoltage protection, equipment ratings).

Mitigation

If you absolutely must switch to ungrounded, consider:

Surge Arresters/TVSS

Install robust surge protection devices to mitigate overvoltages.

Voltage Monitoring

Continuously monitor the voltage to ground to detect insulation degradation or developing ground faults.

Very Strict Procedures

Develop and enforce rigorous switching procedures, including thorough lockout/tagout, voltage checks, and grounding verification before and after switching.

Short Duration

Keep the system ungrounded for the shortest possible time needed to perform the HRG maintenance.

The Best (and Probably Only Acceptable) Solution

The safest and most practical solution is to address the design and maintenance process so that HRG equipment doesn't require frequent (or any) in-service maintenance. Here's a better approach:1. Design for Reliability



High-Quality HRG Resistor

Use a high-quality, industrial-grade resistor designed for continuous duty and the specific voltage and fault current levels.

Proper Ventilation and Cooling

Ensure adequate ventilation and cooling for the resistor enclosure to prevent overheating, which is a common cause of resistor failure.

Overcurrent Protection

Incorporate overcurrent protection (fuses or circuit breakers) specifically sized to protect the resistor from overload.

Redundant Resistors

Consider using a dual-resistor system with automatic transfer. If one resistor fails, the other automatically takes over, allowing for scheduled maintenance without a shutdown. This is a more expensive option but significantly improves reliability.2.

Improved Maintenance Procedures



Planned Outages

Schedule planned outages for HRG maintenance during periods of low production or downtime. This is the most reliable and safe option.

Regular Inspection

Implement a regular inspection program for the HRG system, including visual inspection of the resistor, connections, and enclosure. Use infrared thermography to identify hot spots that might indicate a problem.

Predictive Maintenance

If possible, monitor the resistor's resistance and temperature to detect early signs of degradation.

Why Switching is a Bad Idea

The fundamental problem is that you're trying to "band-aid" a design or maintenance issue with a potentially dangerous and code-violating solution. Switching the grounding configuration fundamentally changes the electrical characteristics of the system, and introducing a switch in the grounding path creates a single point of failure that could compromise the entire system's safety.

In summary



Switching between HRG and solidly grounded is not code-compliant and is unsafe. Switching from HRG to ungrounded might be possible, but is strongly discouraged due to the increased risks and potential for serious problems. Focus on designing a reliable HRG system and developing safe maintenance procedures that minimize the need for in-service maintenance. Consult with a qualified electrical engineer to review your specific system and develop the safest and most practical solution.

I hope this detailed explanation is helpful. Safety should always be the top priority. Flag for review