Okay, this is a serious and frustrating situation. Let's break down the information and explore potential causes, focusing on explanations beyond a simple overload and suggesting diagnostic steps.

Understanding the Problem Recurring Failure

This is the most important clue. The same breaker (C-phase connection) failing in the same manner after a year suggests a systemic problem, not just a random event.

Melted Connection

The fact that the connection melted points to excessive heat, likely due to high resistance at that specific point. This doesn't happen with a simple overload – it requires a localized hot spot.

Utility Event/Generator Transfer

The utility surge and subsequent generator transfer are highly suspect. These events can introduce transients and other power quality issues.

ATS Involvement

The dead C-phase at the ATS (further downstream) confirms that the damage originated upstream at the main breaker connection point.

Balanced Loads (Normally)

The fact that loads are normally balanced suggests the issue isn't constant imbalance. However, the utility event could have triggered an imbalance or resonance.

Repeated Replacement

This suggests a lack of investigation and simply replacing a symptom, not the cause.

Possible Causes and Diagnostic Steps1. Connection Issues (Even With Proper Torque)



Material Compatibility/Corrosion

Are the busbars and breaker terminals made of compatible metals (e.g., copper to copper, aluminum to aluminum)? Dissimilar metals can corrode, leading to increased resistance. Even if the materials are compatible, check for signs of corrosion. Consider using joint compound.

Surface Condition

Was the mating surface of the busbar and breaker terminal perfectly clean and free of oxides before installation? A thin layer of oxidation can drastically increase resistance. Consider the condition of buss when new and the buss now and if it needs a new bus.

Spring Washers

Are spring washers used under the carriage bolts to maintain pressure over time? If not, thermal cycling can loosen the connection.

Torque Verification

While you mention proper torque, verify this with a calibrated torque wrench. Even slight over- or under-torquing can create issues. Re-torque after a few days of operation and again after a week.

Busbar Integrity

Is the busbar itself damaged, thinned, or cracked near the connection point? This can reduce its current-carrying capacity and cause localized heating.2.

Power Quality Issues



Neutral Current

Even with balanced three-phase loads, harmonic currents can create significant neutral current. This neutral current can circulate through the grounding system and potentially find its way onto one phase conductor, exacerbating heating. Measure neutral current at the main panel. If it's high, investigate harmonic sources (VFDs, electronic ballasts, etc.).

Harmonics

The utility surge and generator transfer could have introduced or amplified harmonic distortion. Use a power quality analyzer to measure Total Harmonic Distortion (THD) of both voltage and current on all three phases. Pay particular attention to the current on the C phase. Harmonics can cause overheating of conductors, even if the fundamental current is within limits.

Voltage Transients/Surges

The initial utility event is highly suspicious. Even with surge protection, some transients may get through. Consider installing a high-quality surge protection device (SPD) specifically designed for industrial applications and monitor it for activity. Consider a surge counter to track events. The existing SPD may be inadequate, improperly installed, or damaged.

Resonance

Harmonics can interact with system inductance and capacitance to create resonance conditions. This can lead to amplified currents at specific frequencies. A power quality study can identify potential resonance points.

Voltage Imbalance

Even if loads appear balanced, a slight voltage imbalance from the utility can cause disproportionately larger current imbalances in the loads. Monitor voltage balance over time. If you see consistent imbalance, contact the utility.3.

Generator Issues



Generator Output Quality

The generator itself could be producing poor power quality (high harmonics, voltage imbalance). Have the generator's output tested under load.

Transfer Switch Problems

The ATS could be introducing problems during the transfer process (e.g., incorrect timing, momentary paralleling). Have the ATS inspected and tested.4.

Load Issues (Less Likely, But Still Possible)



Hidden Single-Phasing

While you say the loads are balanced, a subtle issue within one of the connected machines could be causing it to draw excessive current on one phase under certain conditions. Use an thermal imaging device to test the motors.

Motor Starting

Large motor starting can draw significant inrush current. If a motor connected to the C-phase is started frequently or has a high starting current, this could contribute to the problem. Consider a reduced-voltage starter for large motors.

Immediate Action Steps1. Thorough Inspection

Before installing the new breaker, perform a meticulous inspection of the following:

Busbars

Look for damage, corrosion, discoloration, and proper alignment. Clean the mating surfaces thoroughly.

Breaker Terminals

Inspect for damage, proper threading, and cleanliness.

Bolts/Washers

Ensure the correct size and type are used. Replace any that show signs of wear or corrosion.

Enclosure

Check for proper grounding and any signs of water ingress.2.

Power Quality Monitoring

Install a power quality analyzer to record voltage, current, harmonics, and transients before and after the breaker replacement. This will provide a baseline for comparison and help identify any ongoing issues. Record data for at least a week.3.

Infrared Thermography

Once the new breaker is installed and energized, use an infrared camera to monitor the temperature of the connections and busbars. This will help identify any hotspots early on.4.

Contact Experts

Engage a qualified electrical engineer or power quality specialist to conduct a thorough system analysis. They can perform more advanced testing and help identify the root cause of the problem.

Why Surge Protection Alone May Not Be Enough

Standard surge protection only addresses voltage spikes. It does not address harmonics, voltage imbalance, or other power quality issues that can contribute to overheating. A comprehensive power quality solution may be necessary.

Important Considerations



Safety

Work on electrical equipment should only be performed by qualified and experienced personnel following proper safety procedures.

Documentation: Keep detailed records of all inspections, tests, and repairs. This will help track the problem and identify trends.By systematically investigating these possibilities, you should be able to pinpoint the cause of the recurring failure and implement a long-term solution. Good luck! Flag for review