Let's break down the situation and the potential causes of your rooftop unit problems. You've got a complex scenario with multiple factors to consider

Understanding Your System
Delta 4-Wire High-Leg (Stinger Leg) System

This is a type of three-phase electrical service where one phase (the "high leg" or "stinger leg") has a higher voltage to ground than the other two. Typically, it's 240V phase-to-phase, with 208V from the high leg to neutral. This is crucial to understand because many devices are NOT designed to handle this unusual voltage.

Rooftop Units (RTUs)

RTUs are self-contained HVAC systems. They likely have a combination of single-phase and three-phase components (e.g., fans, compressors, controls).

VFDs (Variable Frequency Drives)

VFDs are used to control the speed of motors, usually for fans or compressors in your RTUs. They rectify the incoming AC power to DC, then invert it back to AC at a variable frequency.

Error Codes

These indicate problems with the operation of the RTUs, such as overvoltage, undervoltage, communication issues, or sensor failures.

Original Grounding

Conduit (if properly installed and maintained) can serve as an effective grounding path. However, rusted conduit is a definite problem.

New Ground Wire

Adding an open-air ground wire is a good step, provided it's properly sized, connected, and bonded to the main grounding electrode system.

Potential Issues and Root Causes

Here's a structured way to think about the problems:1. High-Leg Voltage Sensitivity of VFDs



The Core Problem

Many VFDs are designed for standard three-phase systems (e.g., 208V or 480V) where all phases have the same voltage to ground. They may not be able to tolerate the unbalanced voltages of a high-leg delta system.

Why it Causes Errors

The VFD's internal circuitry might be damaged or malfunction due to the high voltage on one phase. It can also lead to incorrect voltage sensing and protection circuits tripping.

Check VFD Specs

The very first thing to do is to

CAREFULLY CHECK THE MANUFACTURER'S SPECIFICATIONS FOR EACH VFD. See if they are rated for use on a high-leg delta system. If not, this is a PRIMARY suspect. Mitigation (If Possible and ONLY if the VFD is partially compatible)

Some VFDs have adjustments that can be made to accommodate slightly unbalanced voltages. However, do not attempt this unless you are absolutely certain you understand the VFD's manual and the implications of the settings. It could lead to further damage.2.

Grounding Issues



Inadequate Grounding

Poor grounding (especially with rusted conduit) can lead to voltage fluctuations, noise, and improper operation of sensitive electronic equipment.

Ground Loops

Adding a new ground wire without properly bonding it to the existing grounding system can sometimes create ground loops, which can introduce noise into the system.

Proper Grounding Procedure

Make absolutely certain the new ground wire is: Sized appropriately for the service (refer to the NEC). Connected to a properly sized grounding electrode conductor (GEC) that connects to a grounding electrode (e.g., ground rod, water pipe). Bonded to the metal enclosures of the RTUs and any other metallic equipment. Check voltage to ground on all three phases including the stinger leg.3.

Voltage Imbalance on the Three Phases Uneven Loads

If the loads on the three phases are not balanced (i.e., too much load on one phase), it can cause voltage imbalances. This is especially critical on a high-leg system because the imbalance is already present.

Consequences

Voltage imbalances can damage motors, VFDs, and other equipment.

Diagnosis

Use a multimeter to measure the voltage between each phase and neutral/ground, and between each phase and the other phases, under load. This will reveal any imbalances.

Correction

Redistribute loads as much as possible to balance the phases. Consult with an electrician to check the overall load distribution and service capacity.4.

Harmonic Distortion



Cause

VFDs and other non-linear loads can generate harmonic currents, which can distort the voltage waveform.

Consequences

Harmonics can cause overheating of transformers, nuisance tripping of circuit breakers, and interference with electronic equipment.

Mitigation

Harmonic filters or reactors can be installed to reduce harmonic distortion. This is generally a more advanced solution.5.

Wiring Errors



Incorrect Connections

Verify that the wiring to the VFDs and other components is correct. It's easy to accidentally swap phases or miswire control circuits.

Loose Connections

Check for loose connections at terminal blocks, breakers, and other points.

Damaged Wiring

Inspect the wiring for signs of damage, such as frayed insulation or corrosion.6.

RTU Control System Issues Low Voltage Control Circuits

Many RTUs use 24VAC control circuits for thermostats, sensors, and relays. These circuits can be susceptible to voltage drops and interference.

Sensor Failures

Malfunctioning sensors (temperature, pressure, etc.) can trigger error codes.

Communication Issues

RTUs often have communication networks (e.g., Modbus, BACnet) to connect to building automation systems. Communication failures can cause errors.

Troubleshooting Steps1. Prioritize Safety

Work with a qualified electrician who understands three-phase power and high-leg systems.2.

Verify VFD Compatibility

This is the critical first step. Check the VFD specifications to ensure they are compatible with your high-leg delta system. If they are not, replacing them with compatible VFDs is likely necessary.3.

Thorough Grounding Inspection

Verify the size and integrity of the ground wire. Ensure it's properly connected to the main grounding electrode system. Check for continuity of ground paths throughout the system.4.

Voltage Measurements

Measure phase-to-phase and phase-to-ground voltages at the service entrance, at the RTUs, and at the VFDs. Record the measurements under both no-load and full-load conditions. Check the voltage on the stinger leg very carefully.5.

Load Balancing

Measure the current on each phase at the service entrance and at each RTU. Redistribute loads as needed to balance the phases.6.

Wiring Inspection

Visually inspect all wiring for damage or loose connections. Use a multimeter to check continuity and insulation resistance.7.

VFD Diagnostics

Consult the VFD manuals for troubleshooting procedures. Check for error codes and monitor VFD parameters (voltage, current, frequency).8.

RTU Control System Checks

Check the 24VAC control voltage levels. Verify sensor readings and calibrate sensors as needed. Troubleshoot communication network issues.

Key Recommendations Consult with an Experienced Electrician

Given the complexity and potential dangers of working with three-phase power, it's essential to have a qualified electrician perform the troubleshooting.

Replace Incompatible VFDs

If the VFDs are not rated for high-leg delta systems, replacing them with compatible models is the most likely solution.

Ensure Proper Grounding

Invest in a properly designed and installed grounding system.

Consider a Step-Down Transformer

If you have many devices that are incompatible with the high-leg system, you might consider installing a step-down transformer to create a standard three-phase system (e.g., 208V or 480V). This is a major undertaking.

Document Everything: Keep detailed records of all measurements, observations, and repairs.By systematically addressing these potential issues, you should be able to identify the root cause of your RTU problems and implement the appropriate solutions. Good luck! Flag for review