Diagnosis and troubleshooting of electrical panel overheating: thermographic inspection, loose connections, harmonic distortions and load balancing

1. Problem Description and Area of Application

Overheating of distribution boards and control cabinets is one of the most common causes of emergency shutdowns at industrial facilities. Excessive heat dissipation leads to premature insulation aging of conductors, false tripping of protective equipment, failure of frequency converters and PLCs, and in critical cases, to phase-to-phase short circuits and fires.

This manual covers power distribution boards (LV) with nominal up to 4000 A, motor control centers (MCC) and automation cabinets in accordance with the requirements DSTU EN 61439-1 (Low-voltage switchgear and controlgear assemblies). Problem severity level classification:

• Critical (Class A): Component temperatures exceed +85°C or the temperature difference ($\Delta T$) is more than 50°C compared to the ambient environment. Requires immediate intervention and reduction of load.
• Significant (Class B): Temperature in the range of +65°C ... +85°C ($\Delta T$ from 30°C to 50°C). Repair window must be planned within the next 48 hours.
• Minor (Class C): Temperature within normal tolerance limits, but a local anomaly is observed ($\Delta T$ from 10°C to 30°C). Schedule inspection during planned maintenance.

2. Safety Measures

DANGER ELECTRICAL SHOCK AND ARC FLASH!

Працювати під напругою заборонено, за винятком випадків проведення вимірювань, що вимагають увімкненого обладнання, згідно з правилами безпеки (DSTU EN 50110-1). Використовуйте засоби індивідуального захисту (ЗІЗ):

• Arc flash protection suit (minimum 8 cal/cm²).
• Dielectric gloves of class 0 or 1 (tested for breakdown according to EN 60903).
• Face shield with arc rating (EN 166 / EN 170).
• Dielectric tool with insulation up to 1000 V (EN 60900).

Before performing mechanical work (tightening connections, replacing equipment), carry out the lockout/tagout (LOTO) procedure: fully turn off the main circuit breaker, lock it with a padlock, attach a warning tag, and ensure the absence of voltage using a verified two-pole voltage indicator (EN 61243-3).

3. Required diagnostic tools

4. Initial Check-List for Station Assessment

5. Systematic Diagnostic Block Diagram

• Symptom: Local or general increase in temperature of the electrical panel walls or internal components above the norm.
  • Step 1: Perform an infrared inspection under load (not less than 80% of the nominal mode).
    • *If a local hot spot is detected on a terminal, busbar or apparatus contact ($\Delta T > 30^\circ C$):*
      • Go to Resistance diagnostics (Weak connection).
    • *If uniform overheating of the entire machine or group without local spots is observed:*
    • Check the total load with clamps. *If the current exceeds the nominal:* go to Overcurrent Protection.
    • If the current is within normal limits, but overheating occurs: check the ventilation condition or presence of higher harmonics.
    • *If the neutral conductor (N) or neutral cables are heated:*
    • Measure the current in the neutral. *If the neutral current exceeds the phase current:* go to Harmonic Analysis and Load Imbalance.

6. Fault Matrix and Causes

7. Root Cause Analysis of Each Fault

7.1. Weak Contact Connections

Why it occurs: Cyclic thermal deformations (heating under load, cooling upon equipment shutdown) cause gradual loosening of threaded connections due to "metal flow" (copper or aluminum). Vibration from adjacent transformers, motors or contactors accelerates this process.

Як підтвердити: За допомогою тепловізора виявляється локальне підвищення температури саме в зоні клемної колодки або болтового з'єднання. Під час планового знеструмлення перевірка моментом затяжки за допомогою динамометричного ключа показує кут зміщення гайки/болта.

Наслідки: Збільшення перехідного опору призводить до закону Джоуля-Ленца ($P = I^2 \cdot R$): зростання тепла випалює контакт, призводить до окиснення металу, збільшення дугоутворення, оплавлення ізоляції та повного виходу з ладу обладнання або пожежі.

7.2. Harmonic Distortions and Neutral Overheating

Why it occurs: Modern industrial equipment with thyristor converters, switched-mode power supplies and frequency drives generates non-sinusoidal currents. Odd harmonics, multiples of three (3rd, 9th, 15th — so-called triplet harmonics), do not cancel out in the three-phase neutral, but add up in arithmetic progression. As a result, the current in the neutral can reach 173% of the phase current, even with ideally symmetrical loading at the base frequency of 50 Hz.

Як підтвердити: Підключення аналізатора якості електроенергії (IEC 61000-4-30) до вводу шафи показує коефіцієнт гармонійних спотворень струму ($THDi$) понад 15-20% та перевищення струму в нейтральному проводі над фазними струмами.

Наслідки: Перегрів ізоляції нейтрального кабелю, оплавлення клем на нульовій шині, хибні спрацьовування диференціальних захистів (ПЗВ/дифавтоматів).

7.3. Internal Faults of Automatic Circuit Breaker Contacts

Why it occurs: Repeated switching of short-circuit currents or load currents causes erosion of the contact surfaces made of metal-ceramic (based on silver), formation of carbonization and disturbance of the spring force of the drive mechanism.

Як підтвердити: Вимірювання падіння напруги мікроомметром на кожному полюсі примусово замкнутого автомата. Значення, що перевищують заводські допуски більш ніж на 50%, свідчать про руйнування контактів.

Наслідки: Локальний нагрів всередині корпусу автомата деформує біметалеві розчіплювачі, що призводить до хибного спрацьовування теплового захисту при нормальних навантаженнях або, навпаки, до втрати здатності отключення аварійного струму.

8. Step-by-step troubleshooting procedures

Procedure 1: Restoration and wire drawing of contact connections

1. Perform full de-energization of the electrical panel and carry out the procedure LOTO (see section 2).
2. Remove the protective covers and partition panels to ensure free access to the busbars and terminals.
3. Visually inspect the contact surfaces for signs of whitening (for aluminum), blackening, melting, or corrosion.
4. Using sandpaper with fine grit or a special brush, clean the oxidized surfaces to a metallic shine. When working with aluminum, apply a thin layer of neutral conductive paste to prevent re-oxidation.
5. Using a calibrated torque wrench, tighten the bolted connections according to the specified tightening torques (for example, for copper busbars: M8 — 15-20 Nm, M10 — 30-35 Nm, M12 — 50-60 Nm, according to the equipment manufacturer's instructions).
6. Install spring washers (grovers) or disc washers type DIN 6796 for compensation of thermal fluctuations.
7. Post-repair verification: Perform a measurement of transition resistance using a micro-ohmmeter (resistance must be less than 20 µΩ) and carry out a thermal imaging inspection after 2 hours of equipment operation under load.

Procedure 2: Reduction of Harmonic Influence and Neutral Loading

1. Perform measurements using a harmonic analyzer to determine dominant orders (usually 3rd and 5th harmonics).
2. If the cross-section of the neutral conductor is smaller than the phase conductor, modernize the cable line, increasing the cross-section of the neutral conductor to 150-200% of the phase conductor (according to the recommendations IEC 60364-5-52 for nonlinear loads).
3. Install harmonic filters (passive or active harmonic filters - AHF) on the input power supply of the enclosure or group of harmonic generating consumers.
4. Install harmonic filters-reactors on the power supply line of frequency converters.
5. Verification after repair: Re-measurement of currents in the neutral and determination of the new value $THDi$ (target value $THDi < 8\%$ for industrial networks).

Procedure 3: Replacement of Faulty Automatic Circuit Breaker

1. De-energize the distribution board section and install the lockout LOTO.
2. Disconnect the input and output cables/ bars from the switch terminals, securing the markings with labeling tags.
3. Remove the defective automatic switch from the mounting board or DIN rail.
4. Install a new automatic circuit breaker of the appropriate nominal rating, tripping characteristics (type B, C or D) and breaking capacity (for example, 35 kA or 50 kA according to IEC 60947-2).
5. Connect the bars and cables, tightening the terminal screws with a torque wrench using the recommended torque value indicated on the device housing.
6. Verification after repair: Remove LOTO lockout, apply voltage, check mechanism functionality with manual on/off, perform voltage drop measurements under load and carry out a control thermography after 1 hour of operation.

9. Preventive Measures

Primary Cause	Preventive strategy	Monitoring method	Recommended interval
Weak contacts	Planned wire terminal crimping using torque tool	Thermal imaging inspection and selective torque inspection	Every 6 months (for critical cabinets — quarterly)
Harmonic distortions	Installation of harmonic filters and control of current spectral composition	Continuous power quality monitoring (Power Meter) or analysis audit by analyzer	Annually or when connecting new equipment
Cooling system contamination	Cleaning and replacement of air filters in control cabinets, inspection of fan functionality	Visual inspection of air flow indicators and temperature sensors in the cabinet	Monthly

10. Spare Parts and Components

Description of the part	Specification	When to replace	Category UNITEC
Power circuit breakers in cast housing (MCCB)	Rating 100A - 630A, 3P/4P, breaking capacity 36-50 kA (IEC 60947-2)	When a high voltage drop, melting or after tripping on earth fault	Switchgear Equipment
Cooling Fan Units	230V AC / 24V DC, productivity from 100 to 850 m³/h, with IP54 filter	When performance decreases, foreign noise appears, or bearings fail	Climate control systems for cabinets
Contact greases and lubricants	Conductive pastes for copper and aluminum connections, oxide protection	At each disassembly and cleaning of power contacts and busbars	Mounting materials and chemistry
Diagnostic equipment (Thermal imagers, Clamp meters)	True RMS, CAT IV, calibrated measuring instruments	According to the calibration schedule (usually 1 time per year)	Measuring instruments

For selection of the full range of components for repair and modernization of electrical panels, visit our electronic catalog at UNITEC-D E-Catalog.

11. References

• DSTU EN 61439-1: Low-voltage switchgear and controlgear. Part 1: General requirements.
• DSTU EN 50110-1: Operation of Electrical Installations.
• IEC 60947-2: Low-voltage switching and control equipment. Part 2: Automatic switches.
• IEC 61000-4-30: Electromagnetic Compatibility. Methods of Measurement of Power Quality.
• Maintenance manuals for power automation systems of leading European manufacturers (Schneider Electric, ABB, Siemens).
• Technical materials and instructions UNITEC-D for the operation of industrial electrical equipment.