Guide to the Diagnosis and Resolution of Thermal Overload in Industrial Electrical Panels

1. Description of the Problem and Purpose of the Guide

Abnormal overheating within industrial electrical panels is a critical indicator of underlying malfunctions, which can seriously compromise the reliability, safety and operational continuity of production facilities. This diagnostic guide was developed for maintenance technicians, reliability engineers, and plant managers working in the machine tool manufacturing industry, with the goal of providing a systematic, evidence-based approach to identifying, analyzing, and resolving the root causes of overheating.

The problem manifests itself with an increase in temperature beyond the established operating limits, detectable through thermographic inspection or, in the most serious cases, through burning smells, untimely tripping of protections or component failures. The electrical panels affected include, but are not limited to: Motor Control Centers (MCC), primary and secondary distribution panels, PLC/CNC automation panels and panels for powering complex machine tools.

The consequences of overheating can range from minor to catastrophic:

• Critical: Risk of fire, total interruption of production with prolonged downtime, irreversible damage to expensive components, danger to personnel.
• Major: Intermittent component failures, reduction in the useful life of equipment (capacitors, cable insulation, switches), degradation of machine performance, increase in energy consumption.
• Minor: Increase in internal panel temperature within tolerance limits but with a reduced safety margin, indicating potential progression of the problem.

An accurate diagnosis is essential to avoid unplanned machine downtime and to ensure compliance with safety regulations, such as CEI EN 61439-1 for low voltage switchboards and CEI EN 60364 for user electrical systems.

2. Safety Precautions

CRITICAL WARNING: Intervention on live electrical panels involves serious risks of electrocution and burns. Always carry out the following precautions.

• BLOCK/CUT (LOCKOUT/TAGOUT): Before any physical intervention inside an electrical panel, scrupulously apply the lock/cut procedure compliant with the CEI regulation EN 50110-1 and company directives. Check the absence of voltage on all conductors (phase and neutral) with suitable and tested instruments.
• PERSONAL PROTECTIVE EQUIPMENT (PPE):
  • Insulating gloves: Class appropriate to the nominal voltage of the system (e.g. CEI EN 60903).
  • Protective glasses or visor: Against electric arcs and splinters (UNI EN 166).
  • Fire-retardant clothing: Of a category suitable for the risk of electric arc (UNI EN ISO 11612).
  • Safety footwear: With protective toe cap and antistatic/insulating sole (UNI EN ISO 20345).
• STORED ENERGY: Pay attention to power factor correction or filter capacitors, which can maintain a dangerous charge even after disconnection from the mains. Wait for the discharge time specified by the manufacturer and check for the absence of voltage.
• THERMAL HAZARD: Hot areas can cause severe burns on contact. Always use appropriate PPE and insulated tools. During thermographic inspection, maintain an adequate safety distance from live parts.
• PROCEDURES: Always respect the safe working procedures for live and de-energized electrical work, as defined by the legislation CEI 11-27.

3. Necessary Diagnostic Tools

For an effective diagnosis of overheating, it is essential to have specific and calibrated instrumentation.

4. Initial Assessment Checklist

Before starting any invasive diagnostic procedure, it is essential to gather preliminary information and observe the operating conditions of the panel. This step directs the investigation and reduces diagnosis times.

5. Systematic Diagnostic Flow Chart

This flowchart guides the technician through a logical path for diagnosing overheating.

1. Initial Symptom: Overheating of the Electrical Panel or Anomalous Signaling
   1. Perform the Thermographic Inspection (without opening the panel):
      • Use a thermal imager with adequate resolution, setting the correct emissivity for the materials (e.g. 0.95 for opaque painted surfaces).
      • Scan the entire panel, focusing on visible doors, vents and connection points.
      • Result A: Localized Hot Spots (ΔT > 15-20°C compared to the surrounding area or adjacent component)
        Go to Section 5.1: Localized Hot Spot Diagnosis.
      • Result B: Diffuse and uniform overheating of the entire panel (ΔT > 10°C compared to the surrounding environment) or high internal temperatures.
        Go to Section 5.2: Diffuse Overheating Diagnosis.
      • Result C: No hot spots or significant overheating detected externally.
        Proceed to Section 5.3: Anomalous Diagnosis (if suspicion persists).
2. 5.1. Localized Hot Spot Diagnosis (ΔT > 15-20°C)

   1. DANGER: Apply BLOCK/CUT.

   2. Internal Visual Inspection (with the panel sectioned off):
      • Look for discoloration of the cables or insulation, signs of burns, soot deposits, oxidation on the contacts or terminals.
      • Verify that the internal cooling fans (if present) are clean and functioning.
   3. Detailed Electrical Measurement (with panel sectioned off):
      • Voltage Drop Measurement: With a multimeter (mV AC/DC range), measure the voltage drop between the suspected connection points (e.g. input-output of a switch, terminal-conductor). A >50mV (AC) or >10mV (DC) drop under load indicates excessive contact resistance.
        If significant drop, the probable cause is Loose/Corroded Connections. Proceed with troubleshooting.
      • Measuring Contact Resistance: With a micro-ohmmeter (or multimeter in very low resistance mode), measure the resistance across switch contacts or busbar junctions. Values >0.1 Ohm are abnormal.
        If high resistance, the probable cause is Loose/Corroded Connections or Defective Component. Proceed with troubleshooting/replacement.
   4. If the electrical measurements are inconclusive but the hot spot persists:
      • Assess the Localized Overload of the specific component. Measure the current flowing through it (after power supply has been restored and with due precautions) and compare it with its nominal current (In).
        If current > In, the probable cause is Overload. Proceed with the resolution.
3. 5.2. Diffuse Overheating Diagnosis (ΔT > 10°C)
   1. Inspection of the Cooling System:
      • Check the cleanliness of the ventilation filters and the functionality of the extraction/intake fans (hearing, touch, anemometer). A blocked fan or saturated filters drastically reduce heat exchange.
        If anomalies, the probable cause is Inadequate Ventilation. Proceed with the resolution.
   2. Measuring Phase and Neutral Currents (with TRMS current clamp, with the panel closed and under load):
      • General Overload: Measure the current on each phase (L1, L2, L3) and compare it with the nominal current (In) of the panel and the main power cables. If one or more phases exceed In, the panel is overloaded.
        If phase current > In, the probable cause is General Overload. Proceed with the resolution.
      • Load Imbalance: Calculate the percentage of unbalance between the phase currents (max - min) / average * 100. An unbalance > 10% (CEI reference EN 50160, max 2% per voltage) indicates an unequal distribution of the loads.
        If unbalance > 10%, the probable cause is Load Imbalance. Proceed with the resolution.
      • Anomalous Neutral Current: In three-phase systems with non-linear loads, measure the current on the neutral conductor. If the neutral current is significantly high (>50% of the phase current), it indicates the presence of third harmonics.
   3. Power Quality Analysis (with network analyzer):
      • Harmonic Distortion: Measure the Total Harmonic Distortion in Current (THD-I) and in Voltage (THD-V). THD-I values > 5% and THD-V > 3% (according to CEI EN 61000-2-4) indicate harmonic problems.
        If THD-I/V is high, the probable cause is Harmonic Distortion. Proceed with the resolution.
4. 5.3. Abnormal Diagnosis / If a suspicion persists (no hot spot or significant external overheating)
   1. Check the ambient temperature of the room. An excessively hot working environment can raise the internal temperature of the panel even without intrinsic problems of the panel itself.
   2. Review the history of alarms and protection trips. Sporadic errors may not appear during a single thermographic inspection.
   3. Consider faulty temperature sensors (if present) within the panel generating false alarms.

6. Fault-Cause Matrix

The following matrix maps observed symptoms to probable causes, specific diagnostic tests, and expected results for confirmation.

7. Root Cause Analysis for Each Failure

Understanding the 'why' of a failure is essential to implementing lasting solutions.

7.1. Loose or Corroded Connections

Why it happens: Electrical connections can become loose due to several factors:

• Vibrations: Industrial environments with heavy machinery generate mechanical vibrations that can progressively loosen tightenings.
• Thermal Cycles: Continuous heating and cooling of conductors and terminals (due to current draw) causes differential expansion and contraction of materials, leading to a loss of contact pressure.
• Inadequate Torque: An initial installation with tightening torques that are insufficient or do not meet manufacturer specifications.
• Oxidation and Corrosion: Exposure to moisture, chemicals or air pollutants can lead to the formation of oxides and corrosive compounds on contact surfaces, increasing resistance.

How to confirm it: The primary evidence is a localized hot spot, detectable with a thermal imager, with a ΔT > 20°C. Final confirmation is obtained by measuring the voltage drop across the connection under load (a multimeter in the mV range). Values ​​>50mV in AC or >10mV in DC are unacceptable. Alternatively, with the panel sectioned off, measuring the contact resistance with a micro-ohmmeter will reveal values ​​>0.1 Ohm (often in the order of milli-ohms for optimal connections).

Damage if left unresolved: Increasing contact resistance generates heat due to the Joule effect (P = R * I²). This heat degrades the insulation of cables and adjacent components, reduces current-carrying capacity, increases energy losses and, over time, can lead to arcing, short circuits and, in the worst case scenario, fire. Melting of the conductors or terminals is a common outcome.

7.2. Overload of the Panel or Specific Circuits

Why it happens: An electrical panel or circuit becomes overloaded when the absorbed current exceeds its nominal capacity or that of its components and conductors:

• Additional Loads: The installation of new machines or equipment without a redesign or verification of the capacity of the existing switchgear.
• Using Beyond Limits: The intentional or unintentional operation of machinery above its rated specifications (e.g. motors operating at excessive torque).
• Machine Failures: A stuck motor, worn bearing, or seized gear can cause significantly higher than normal current draw.
• Incorrect Sizing: A calculation error in the design phase or undersizing of cables and protections.

How to confirm it: Confirmation occurs by measuring the total current absorbed on each phase (with TRMS current clamp) and comparing it with the nominal current (In) of the panel, switches and cable sections. If the measured current consistently exceeds In, the overload is confirmed. The reference values ​​are the component plates and the cable capacity tables (CEI UNEL 35024/1).

Damage if left unresolved: Overload causes widespread overheating of conductors and components. This drastically reduces the useful life of the insulation, leads to premature wear of switches and contactors, and causes untimely tripping of thermal protections, resulting in machine downtime. The risk of fire is high, especially if the protections are undersized or altered.

7.3. Harmonic Distortion

Why it happens: Harmonics are currents and voltages with frequencies that are multiples of the fundamental frequency (50 Hz in Europe). They are mainly generated by non-linear loads, which do not absorb a pure sinusoidal current, but rather a distorted one. Common examples include:

• Frequency converters (VFDs) for motors.
• Switching power supplies (computers, PLCs, servers, LED lights).
• UPS (Uninterruptible Power Supply).
• Arc welding machines and ovens.

Harmonics add, especially third harmonics (150 Hz), on the neutral conductor in three-phase systems, causing severe overheating even with balanced phase loads.

How to confirm it: You need a mains analyzer to measure Total Harmonic Distortion in Current (THD-I) and Voltage (THD-V). According to the CEI EN 61000-2-4 standard, THD-I values ​​> 5% and THD-V > 3% are indicative of a harmonic problem. It is critical to analyze the harmonic spectrum to identify the dominant harmonics (e.g. 3rd, 5th, 7th). In three-phase systems, measure the current on the neutral with a current clamp; if the neutral current is greater than 50% of the phase current with balanced loads, third harmonics are present.

Damage if left unresolved: Harmonics cause excessive overheating of transformers (especially no-load), conductors (particularly neutral), motors (additional losses, vibrations) and power factor correction capacitors (risk of resonance). They can cause malfunctions or failures of sensitive electronics, untimely tripping of RCDs and a general reduction in the energy efficiency of the system.

7.4. Load Imbalance

Why it happens: In a three-phase system, a load imbalance occurs when the currents drawn on each of the three phases are significantly different. The causes can be:

• Unequal Distribution: A suboptimal distribution of single-phase loads across three phases during design or subsequent modifications.
• Partial Faults: A fault in a part of the circuit or in a load (e.g. a broken heating element in a group of three, broken fuse on one phase).
• Anomalous Three-Phase Loads: Three-phase machinery with intrinsically unbalanced absorption due to internal defects or particular operating conditions.

How to confirm this: The current on each phase (L1, L2, L3) is measured with a TRMS current clamp, under operating load. Subsequently, the imbalance percentage is calculated: (I_max - I_min) / I_media * 100. A current imbalance greater than 10% is an alarm. The CEI EN 50160 standard recommends a maximum voltage unbalance of 2%.

Damage if left unresolved: Load imbalance causes asymmetric overheating of conductors and equipment. In three-phase motors, it generates negative sequence currents that cause rapid overheating of the winding, vibrations and a drastic reduction in efficiency and useful life. It can also cause overheating of the primary side of transformers and untimely tripping of protections.

7.5. Environmental Factors and Inadequate Ventilation

Why it happens: Even a correctly sized electrical panel can overheat if the surrounding environment or its cooling system are not adequate:

• High Ambient Temperature: Panels installed in non-air-conditioned rooms, near heat sources (ovens, hot machinery) or directly exposed to sunlight.
• Ventilation Obstruction: Accumulation of dust, dirt, debris or obstruction of ventilation openings (grills, filters).
• Faulty/Undersized Fans: Blocked cooling fans, faulty motor, or ventilation system not sized correctly for the internal dissipated power of the panel (according to UNI EN 61439-1).
• Dirty Filters: Saturated air filters preventing adequate airflow.

How to confirm it: Measure the room temperature in the room where the panel is installed. Perform a visual inspection of the grilles, fans and filters: check for cleanliness, the absence of obstructions and the correct operation of the fans (noise, air flow). Measure the internal temperature of the panel with a portable infrared thermometer.

Damage if left unresolved: The generalized overheating due to these causes reduces the useful life of all the components inside the panel, in particular the electronic devices (PLC, power supplies, drives), which are sensitive to high temperatures. It increases the likelihood of premature electronic failure and can lead to operational instability.

8. Step-by-Step Resolution Procedures

The following procedures are described for the identified root causes. REMEMBER TO ALWAYS APPLY BLOCK/CUT (LOCKOUT/TAGOUT) BEFORE ANY PHYSICAL INTERVENTION ON THE ELECTRICAL PANEL.

8.1. Resolving Loose/Corroded Connections

1. Identification: With thermographic inspection, identify the critical hot spots (ΔT > 20°C).
2. Securing: Apply BLOCK/CUT on the panel. Check the absence of voltage on all phases and on the neutral.
3. Detailed Inspection: Open the panel and visually inspect the identified connections. Look for signs of overheating (discolored insulation, melted plastic), oxidation, or corrosion on conductors and terminals.
4. Cleaning: Clean oxidized or corroded contact surfaces using a non-abrasive brush (e.g. fibreglass) and, if necessary, a specific spray for electrical contacts (e.g. Kontaktspray 60, Kontakt WL) to remove residues and improve conductivity.
5. Torque Tightening: Retighten all suspect connections (terminals, conductor fixing screws, busbar bolts) using a torque wrench. The tightening torque must comply with the specifications of the component manufacturer (switch, contactor, terminal block). In the absence of specifications, consult the regulatory tables (e.g. UNI EN 61439-1 or CEI guides for specific cables and terminals). Example: for 16 mm² cables, a typical torque is 4-5 Nm; for 35 mm², 8-10 Nm. Do not overtighten as this may damage the conductors or terminals.
6. Final Check: After tightening, check the electrical continuity and the low contact resistance (with a multimeter or micro-ohmmeter) to confirm the effectiveness of the intervention (resistance < 0.1 Ohm).
7. Recommissioning: Close the panel, remove the BLOCK/CUT and restore power.
8. Functional Check: Perform a new thermographic inspection under load to confirm the elimination of hot spots. The temperature should now be aligned with adjacent components or lower than ΔT < 5°C.

8.2. Overload Resolution

1. Measurement: With a TRMS current clamp, measure the phase currents (L1, L2, L3) and compare them with the rated currents (In) of the protection devices (circuit breakers, fuses) and the maximum current ratings of the conductors.
2. Analysis: If the overload is confirmed (measured current > In), analyze the loads connected to that circuit or panel.
3. Corrective Action (priority):
   1. Load Redistribution: If possible, move non-essential loads to other panels or circuits with residual capacity.
   2. Power factor correction: For inductive loads, correct power factor correction of the system can reduce the total current absorbed, improving the power factor and efficiency.
   3. Load Reduction: If it is not possible to redistribute or rephasing, evaluate reducing operating loads or modifying work cycles.
   4. Redesign: If the overload is structural and persistent, it is necessary to redesign the affected section. This includes increasing the section of the cables (consult CEI UNEL 35024/1 tables), replacing the protection devices with larger sizes and, if necessary, installing an additional distribution panel. All modifications must comply with CEI EN 60364.
4. Final Check: Monitor the phase currents after the trip to ensure they are within nominal limits. Perform a thermography to confirm the absence of overheating.

8.3. Harmonic Distortion Resolution

1. Source Identification: Use the network analyzer to identify the non-linear loads that generate the most significant harmonics (THD-I > 5%).
2. Corrective Intervention (priority):
   1. Passive Harmonic Filters: Install passive filters resonant for the dominant harmonics (e.g. 5th, 7th, 11th). These are effective for fixed harmonics but may resonate with the system.
   2. Active Harmonic Filters: For more complex situations or with variable harmonics, install active harmonic filters. These inject currents equal and opposite to the harmonics, canceling them out. Examples: ABB PQF active filters, Siemens Active Filter.
   3. Neutral Oversizing: In case of high third harmonics on the neutral conductor (neutral current > 50% phase), consider oversizing the neutral up to twice the section of the phase conductors.
   4. Isolation/K-Type Transformers: Use isolation transformers to isolate nonlinear loads or K-rated transformers designed to handle harmonic currents without overheating excessively.
   5. Low THD Equipment: Where possible, replace existing non-linear loads with new generation equipment that has low input THD.
3. Final Verification: Perform a new network analysis after the intervention. The objective is to reduce the THD-I to less than 5% and the THD-V to less than 3%, and the neutral current to acceptable values ​​(less than 10-20% of the phase current with balanced loads).

8.4. Load Imbalance Resolution

1. Measurement: Use a TRMS current clamp to measure the currents on each phase (L1, L2, L3) of the affected panel or circuit under load.
2. Analysis: Calculate the percentage imbalance. If the imbalance exceeds 10% (current) or 2% (voltage), action is required.
3. Corrective Intervention:
   1. Single-phase Load Rebalancing: With the panel disconnected and safe (BLOCK/CUT), manually redistribute the single-phase loads between the phases to obtain the most balanced current absorption possible. The goal is an imbalance < 5%.
   2. Three-Phase Load Inspection: If the imbalance persists or comes from a three-phase load, inspect the load itself (e.g. motor, resistors) to identify partial failures or malfunctions.
4. Final Verification: After rebalancing, restore power and repeat phase current measurements to confirm that the unbalance has been reduced to an acceptable level.

8.5. Resolving Environmental Factors and Inadequate Ventilation

1. Environment Inspection: Evaluate the ambient temperature of the room. If excessive (>35°C), consider improvements in the air conditioning of the room or the installation of thermal screens.
2. Cleaning and Checking Filters: Apply BLOCK/CUT. Remove and carefully clean the fan filters with compressed air or replace them if too dirty or damaged.
3. Checking and Replacing Fans: Check the operation of the existing fans (power supply voltage, rotation, noise). Replace faulty or noisy fans. If the panel is thermally undersized, install fans or additional air conditioning systems. The sizing must consider the internal dissipated power and the environmental conditions (UNI reference EN 61439-1 for the calculation of thermal dissipation).
4. Obstruction Removal: Make sure all ventilation openings are free of obstructions (wires, dirt, buildup).
5. Radiation Protection: If the panel is exposed to direct sunlight or radiant heat sources, install reflective screens or move the panel to a more suitable position.
6. Final Check: After the interventions, monitor the internal temperature of the panel with thermal sensors to ensure that it falls within the established operating limits.

9. Preventive Measures

Adopting a preventative maintenance strategy is essential to avoid the recurrence of overheating problems.

10. Spare parts and components

Having essential spare parts available significantly reduces recovery times in the event of a breakdown. UNITEC-D offers a wide range of components for electrical panels.

Visit our e-catalog for the complete range of certified spare parts and electrical components, ideal for ensuring maximum reliability and compliance with UNI, CEI, EN standards for machine tools: UNITEC E-Catalog.

11. References

• CEI Standard EN 61439-1: Assembled protection and switching equipment for low voltage (LV switchboards) – Part 1: General rules.
• CEI standard EN 60364 (part 4 and 5): User electrical systems.
• CEI standard EN 50160: Characteristics of the voltage supplied by public electricity distribution networks.
• CEI standard EN 61000-2-4: Electromagnetic compatibility (EMC) – Part 2-4: Environment – ​​Compatibility levels for low frequency conducted pollution in industrial plants.
• Norma CEI 11-27: Work on electrical systems.
• UNI EN standard ISO 9001: Quality management systems.
• Component manufacturer (OEM) specific installation and maintenance manuals.