You've hit on a couple of key concepts related to high-leg delta systems. Let's break it down

Understanding High-Leg Delta
The Basics

A high-leg delta system (also known as a red-leg, orange-leg, or wild-leg delta) is a three-phase power system where one of the phases is center-tapped. This center tap is grounded, creating a single-phase voltage between each of the other two phases and the grounded neutral, plus a higher voltage (approximately 1.732 times the phase-to-neutral voltage) between the high leg and the neutral.

Voltage Imbalance

The problem is that this configuration inherently creates a voltage imbalance. While the line-to-line voltages (A-B, B-C, C-A) are balanced, the line-to-neutral voltages are not. The high leg (typically B phase) will have a voltage significantly higher than the other two phases to neutral.

Why the High Leg is Avoided for Neutral Loads

You're correct that the high leg is generally not used for line-to-neutral loads for several reasons:1. Higher Voltage

The most obvious reason is the high voltage itself. Standard 120V or 208V equipment cannot be directly connected to the high leg without damage.2.

Code Restrictions

Electrical codes (like the NEC in the US) have specific requirements for identifying the high leg (typically with orange marking) and prohibit its use for certain types of loads to prevent accidental connections.3.

Potential for Imbalance (but not instability)

While not the primary reason for avoiding the high leg, load imbalances on the single-phase portion of the system can influence the high leg voltage and potentially make it fluctuate more than the other phases. This imbalance is not the source of the instability you observed, though.

Unstable High-Leg Voltage - Your Observation

Now, let's address your specific observation of unstable voltage readings between the B phase (high leg) and ground/neutral. Is unstable high-leg voltage the result of open delta transformer configurations, and would it be normal if 3 transformers were used? Open Delta Can Exacerbate Issues

An open delta configuration (using only two transformers instead of three) is inherently more susceptible to voltage instability and imbalance than a closed (three-transformer) delta. Open delta systems have less capacity to handle unbalanced loads and voltage regulation can be poorer. However, it's still not usually the primary cause of fluctuating voltage.

Three Transformers Should Be More Stable

Using three transformers in a closed delta configuration should provide a more stable voltage profile. The fact that you're seeing significant fluctuations even with three transformers suggests a problem elsewhere.

What is it that causes the unstable voltage? Loose Connections/High Resistance

This is the most likely culprit. A loose connection or high resistance somewhere in the B phase circuit (either in the panel, at the transformer, or in the wiring between them) can cause erratic voltage readings. When you probe the circuit, the meter itself can momentarily alter the connection, causing the voltage to jump around before settling.

Transformer Issues

While less common, a winding fault within the transformer connected to the B phase could cause voltage fluctuations. This is more likely to be a gradual degradation issue than a sudden failure.

Load Changes and Imbalances

Significant and rapidly changing loads on the single-phase portion of the delta system (connected to the center-tapped transformer) can influence the high-leg voltage. Large motors starting or stopping, or welders drawing intermittent high current, can cause temporary voltage dips. However, these are usually short-lived and not as erratic as you describe.



Neutral Problems

A poor or loose neutral connection anywhere in the system can cause voltage fluctuations. A floating neutral leads to unpredictable voltage variations. This is a common cause of seemingly random voltage issues.

Harmonics

While possible, harmonic distortion is less likely to cause the type of fluctuating voltage you're seeing. Harmonics are usually present constantly, not intermittently.

External Interference

Under very rare circumstances, strong electromagnetic interference (EMI) could conceivably affect voltage readings, but this is highly unlikely.

Troubleshooting Steps

Given your observations, here's how to troubleshoot the problem:1. Inspect Connections

Carefully inspect all connections in the B phase circuit, starting at the panel breaker, then the wiring to the transformer, and the transformer connections themselves. Look for loose connections, corrosion, or signs of overheating. Tighten all connections securely. Pay special attention to the neutral connection, as that is a common source of issues.2.

Check Grounding

Verify that the grounding system is properly installed and that all ground connections are solid. A poor ground can contribute to voltage instability.3.

Monitor Load

Observe the load on the system during the times you see the voltage fluctuations. Are there any large loads cycling on and off?4.

Transformer Testing

If the problem persists, have the transformer tested (insulation resistance, turns ratio, etc.) to rule out a winding fault. This is best done by a qualified electrician or transformer technician.5.

Neutral-Ground Bond

Ensure the neutral-to-ground bond is present only at the service entrance. Multiple neutral-ground bonds can cause circulating currents and voltage problems.6.

Use a Recording Voltmeter

If you can't pinpoint the problem, a recording voltmeter can capture the voltage fluctuations over time and provide valuable clues about the timing and magnitude of the variations.

In Summary:

While high-leg delta systems inherently have voltage imbalances, significant and unstable voltage on the high leg is not normal and usually indicates a connection problem, load imbalance, or a failing component. Focus on inspecting and tightening connections, especially those related to the neutral and the B phase circuit. Flag for review