Okay, Art, this is a tricky situation and requires careful consideration. Let's break down the problem and possible solutions.

Understanding the Problem



Floating Ground in a Delta System

A delta system by design has no direct connection to ground unless intentionally grounded (corner-grounded delta, center-tapped delta - which you don't have). This means the voltage to ground on any of the three legs is not defined unless grounded. However, a "floating ground" within a grounded receptacle is concerning.

L6-30R Receptacles

These are 30 amp, 240V, 3-phase twist-lock receptacles. They require a ground conductor for safety.

Good Continuity to Panel/Earth Ground

You've verified that the ground pin on the receptacle is physically connected to the panel enclosure and ultimately to the earth ground system. This is good, but it doesn't solve the "floating ground" indication.

Isolated Problem

The fact that only two receptacles on this specific panel are exhibiting the issue, while others in the plant are fine, points to a local problem, rather than a systemic grounding issue throughout your entire electrical system.

Delta Transformer with No Center Tap

This is a crucial piece of information. Your 240V system is derived from a delta transformer. Without a center tap or corner grounding, the system relies on the grounding conductor (green wire) for fault current return.
Why the "Floating Ground" Indication and Possible Causes

The "floating ground" indication from your tester, despite good continuity, suggests one or more of the following:1.

Faulty Tester or Misinterpretation

Ruling this out first: Double-check your testing methodology and the tester itself. Try a different tester. Make sure you understand what the tester is actually measuring (AC voltage, impedance, etc.). It's possible the tester is designed to look for a specific voltage relationship to ground that a delta system inherently lacks. Tester Impedance: Some testers are high impedance, and in a poorly grounded system, can give false readings.2.

High Impedance Ground Path

Loose Connections: The most likely culprit. Even though you have continuity, the connection might be loose or corroded somewhere along the ground path from the receptacle, back to the panel, and finally to the main ground electrode system. High impedance restricts fault current flow, making the grounding system ineffective.

This is the MOST likely cause. Grounding Conductor Size: Is the grounding conductor (green wire) adequately sized for the circuit? It should be sized according to the overcurrent protection device feeding the circuit. Long Ground Runs: Very long ground wire runs can create impedance.3. Current on the Ground Conductor

Neutral Current (Unlikely in a Delta system): In a Wye system, neutral current can sometimes find its way onto the ground, especially with unbalanced loads. In your delta system, this should not be the cause. Leakage Current: Equipment connected to other circuits on the same panel or even on other panels can have small amounts of leakage current that return to the source via the ground. This is normal to some degree, but excessive leakage can be problematic.4.

Wiring Error (Less Likely, but Check)

Crossed Neutral/Ground (Almost impossible in a L6-30R but should be looked at.): While extremely unlikely with an L6-30R, double-check that the wiring is correct at the receptacle and at the panel. Make absolutely sure the ground wire is connected to the ground bus in the panel. If this is a standard 120V receptacle, a reversed polarity can have an impact.

What to Do (Investigation and Solutions)


Systematic Investigation of Ground Path

Power Down: Turn off the breaker feeding the affected receptacles. Visually Inspect:

Carefully inspect the wiring at the receptacle. Look for loose connections, corrosion, or damaged wires. Tighten all connections. Continuity Testing

Use a low-impedance ohmmeter to measure the resistance from the receptacle ground pin to the panel enclosure, and then from the panel enclosure to the main grounding electrode system (earth ground). You should have very close to zero ohms (less than 1 ohm). Pay attention to the connections during the test. Jiggle the wiring to ensure a solid connection throughout.2.

Check Ground Wire Sizing

Verify that the ground wire is sized appropriately for the overcurrent protection (breaker) feeding the circuit. Use the NEC (National Electrical Code) tables to determine the minimum required ground wire size.3.

Check for Leakage Current

Use a clamp-on ammeter capable of measuring very low currents (milliamps) to measure the current flowing on the ground conductor while the equipment is running. A small amount of leakage is normal, but excessive leakage indicates a problem with the connected equipment. Disconnect equipment until leakage is acceptable.4.

Panel Grounding

Grounding Bus: Ensure the panel's ground bus is properly connected to the building's grounding electrode system (ground rods, water pipe ground, etc.). Cleanliness: The panel should be free of any contaminants that could cause electrical resistance.

Regarding Changing the Transformer (Wye or Single-Phase)



Wye Transformer

Benefit: A Wye transformer would provide a neutral conductor and establish a more predictable voltage relationship to ground. This could make troubleshooting easier and potentially improve safety, IF your equipment requires a Neutral. Cost and Complexity: Replacing the transformer is a significant undertaking. You'd need to ensure that the new transformer is properly sized for the load and that the entire system is reconfigured correctly. Is it Necessary? Since most of your equipment doesn't reference ground and is running on this system already, it is not the first option I would consider. Fixing the existing ground problem is the best first step.

Single-Phase Transformer

Purpose: A single-phase 240V to 240/120V transformer would allow you to create 120V circuits, typically for control power, lighting, or convenience outlets. It would not solve the grounding issue for the existing 240V 3-phase equipment.

Recommendation


Focus on the Local Problem

The fact that only two receptacles are affected points to a local wiring or connection issue. Start with the systematic investigation outlined above.2.

Fix the Ground Path

Ensure a low-impedance ground path from the receptacle back to the panel and the main grounding electrode system. Tighten connections, replace corroded wiring, and verify proper ground wire sizing.3.

Re-evaluate if Problems Persist

If you exhaust all the troubleshooting steps and still have a "floating ground" indication, then consider more drastic measures like replacing the transformer with a Wye-connected unit.4.

Consult a Qualified Electrician

Given the complexity of 3-phase systems and the potential safety hazards, I strongly recommend consulting with a qualified electrician who is experienced with industrial electrical systems. They can perform more in-depth testing and ensure that your system is safe and compliant with electrical codes.

Important Safety Note: Working with electrical systems can be dangerous. Always de-energize circuits before working on them and follow proper safety procedures. If you are not comfortable working with electrical systems, hire a qualified electrician. Flag for review