Guide to Diagnostics and Resolution of Thermal Overloads in Industrial Electrical Panels

1. Description of the Problem and Purpose

This diagnostic guide addresses the problem of anomalous overheating inside industrial electrical panels, a phenomenon that can seriously compromise the reliability, safety and durability of the systems. Unmanaged overheating leads to insulation degradation, service interruptions, premature component failures and, in extreme cases, fires. This guide is applicable to distribution panels, motor control panels (MCC), and automation panels typically found in machine tools and manufacturing production systems.

The severity classification of the problem is as follows:

• Critical: Temperatures exceeding the operating limits specified by the manufacturer (e.g. CEI EN 61439-1 for internal temperatures) or in the presence of smoke, burning smell, melting of materials. Requires immediate intervention and safety of the system.
• Major: High but stable temperatures, 15-20°C above normal values ​​(e.g. > 50°C in a 25°C environment) or significant thermal differences (>10°C) between similar connection points. Requires short-term intervention planning.
• Minor: Temperatures slightly above normal (<15°C deviation) or slight thermal differences. It requires in-depth monitoring and analysis.

2. Safety Precautions

ATTENTION: RISK OF ELECTRIC SHOCK AND BURNS.

The inspection and diagnostics of live electrical panels present high risks. All operations must be carried out by qualified and trained personnel, in compliance with current regulations (e.g. Legislative Decree 81/08, regulation CEI 11-27). ALWAYS take the following precautions:

• LOCKOUT/TAGOUT PROCEDURE (LOTO): Before any intervention involving contact with live parts or the removal of protections, electrically isolate the entire panel or the section involved, lock the switches in the open position and affix the non-operational labels. Check the absence of voltage with suitable tools.
• PERSONAL PROTECTIVE EQUIPMENT (PPE): ALWAYS use insulating gloves, helmet, protective glasses, fireproof clothing and safety footwear, compliant with CE regulations. For live thermographic inspection, consider face shields and additional arc flash protection (Arc Flash PPE).
• RESIDUAL ENERGY: Components such as capacitors can retain electrical charge even after isolation. Be sure to safely discharge these components before handling them.
• SAFETY ZONES: Delimit the work area and ensure that only authorized and protected personnel are present.

3. Diagnostic Tools Required

The effectiveness of the diagnosis depends on the use of adequate and calibrated instrumentation. Below is a summary table:

4. Initial Assessment Checklist

Before starting any instrumental diagnostics, perform a careful visual assessment and gather operational information. This phase can highlight obvious anomalies or narrow the field of investigation.

5. Systematic Diagnostics Flowchart

Follow the following decision logic to identify the root cause of overheating.

1. Initial Symptom: Thermal Overload of the Electrical Panel
   1. Perform Thermographic Inspection (under power and with adequate safety measures):

      Use the thermal imager with an emissivity set to ~0.95 for painted or oxidized surfaces, or ~0.80 for polished copper/aluminium. Optimal shooting distance: 0.5 - 2.0 meters.

      1. Identification of Localized Hot Spots (ΔT > 5-10°C compared to equivalent adjacent points):
         1. Hot spots on terminals, connections, cable lugs, busbars:
            • Probable Cause: Loose or oxidized connections (high contact resistance).
            • Action: Proceed with the diagnostics for Loose/Oxidized Connections (section 6, step 1).
         2. Hot spots on contacts of remote control switches, switches, disconnectors, fuse holders:
            • Probable Cause: Worn contacts, pitting, weakened spring, high internal contact resistance.
            • Action: Proceed with diagnostics for Defective Components (section 6, point 4).
         3. Hot spots on conductors, cables, transformer/reflance windings:
            • Probable Cause: Conductors undersized for the load, line overload, load imbalance between phases.
            • Action: Proceed with the diagnostics for Overload or Load Imbalance (section 6, point 2 and 3).
      2. Identification of Diffuse/Generalized Heating of the Panel (without distinct hot spots or with a homogeneous increase in internal temperature):
         1. The entire panel or large sections have high temperatures:
            • Probable Cause: Bad ventilation, high ambient temperature, high harmonic distortion, general overload of the panel.
            • Action:
              1. Check the ventilation of the panel and the ambient temperature (see initial checklist). If inadequate, correct.
              2. If ventilation and environment are optimal, proceed with the Energy Quality (Harmonic Distortion) analysis (section 6, point 3).
              3. At the same time, perform current measurements on all the main branches to verify the Global Switchgear Overload (section 6, point 2).

6. Fault-Cause Matrix

This table provides a link between observed symptoms, probable causes (ordered by probability), diagnostic tests to perform, and expected results to confirm the cause.

7. Root Cause Analysis for Each Fault

7.1. Loose or Oxidized Connections

Explanation: The most common cause of local overheating is an increase in contact resistance. Over time, mechanical vibrations, thermal expansion/contraction cycles, or oxidation (corrosion) of contact surfaces can loosen fasteners. Even minimal contact resistance (e.g. from 0.01 Ω to 0.1 Ω) generates heat due to the Joule effect (P = I²R), where P is the power dissipated in heat, I is the current flowing through the connection and R is the contact resistance. The higher the current, the more heat is generated. This phenomenon is often localized and identifiable via thermography.

How to Confirm:

• Thermography: Detection of a localized hot spot with ΔT > 10°C compared to a similar spot under normal operating conditions.
• Measure Contact Resistance: (After LOTO) Using a micro-ohmmeter, values ​​above 100 µΩ for power connections are indicative of a problem.
• Visual Inspection: (After LOTO) Evidence of discoloration, melting of insulation material, oxidation on conductor or terminal.

Damage if left unresolved: Heat degrades conductor insulation, weakens terminal springs, accelerates oxidation and can lead to complete connection failure, risking arc flash, fire and service interruption.

7.2. Current Overload

Explanation: It occurs when a circuit, conductor or component is required to carry or handle a current greater than its rated capacity for an extended period. This can result from an increase in machine load, insufficient initial sizing, a downstream fault or an imbalance between phases in unbalanced three-phase systems. Excessive current causes a general increase in temperature along the path of the conductor or inside the component.

How to Confirm:

• Measure Current: With a current clamp (True RMS), measure the current on each conductor. If the measured current is consistently higher (e.g. > 110%) than the rated current of the conductor or protective device.
• Thermal imaging: Homogeneous heating along the conductor or on a protection switch.
• Sizing verification: Comparison between the cable section (mm²) or the rated current of the components and the calculated actual load.

Damage if not resolved: Degradation of insulation, melting of conductors, untimely intervention of protection devices, damage to motors or other downstream loads, potential fire risk.

7.3. Harmonic Distortion

Explanation: Non-linear loads (e.g. inverters, switching power supplies, PLCs, motor drivers) absorb a non-sinusoidal current from the network, introducing harmonic components (multiples of the fundamental frequency, 50 Hz). These harmonics flow through conductors, transformers and protective devices, increasing the effective RMS current and generating additional heat, even without apparent fundamental overload. Harmonics can also cause significant heating in the neutral conductor in three-phase, 4-wire systems.

How to Confirm:

• Power Quality Analysis: Use of an analyzer to measure THD-I (Total Harmonic Distortion in Current) and THD-V (Total Harmonic Distortion in Voltage) and individual harmonics. THD-I values ​​> 15% (in particular 3rd, 5th, 7th harmonics) are indicative.
• Thermal imaging: Diffuse heating on transformers, power conductors (particularly the neutral), and reactors.

Damage if not resolved: Overheating of transformers and cables (especially the neutral), reduction in the useful life of equipment, untimely tripping of RCDs, malfunctions of sensitive equipment and increased energy losses.

7.4. Load Imbalance between Phases

Explanation: In a three-phase system, a significant imbalance of currents between phases can lead to overheating of conductors and protective devices, even if the total current is within limits. This is especially true for three-phase motors, where current imbalance can cause excessive overheating and reduced efficiency. Voltage imbalance can result from uneven distribution of single-phase loads or a problem in the power supply network.

How to Confirm:

• Measure Phase Currents: With a current clamp, measure the currents on each of the three phases. Calculate the percentage imbalance: (I_max - I_min) / I_average * 100%. An imbalance > 10-15% is critical.
• Measure Phase-Phase and Phase-Neutral Voltages: Check for any voltage imbalances.
• Thermography: Differential heating between the three phases on conductors, contactors or switches.

Damage if not resolved: Overheating of three-phase motors (with consequent reduction in useful life), overheating of conductors and protections, increase in losses and reduction in the energy efficiency of the system.

7.5. Defective or Obsolete Components

Explanation: Circuit breakers, contactors, relays, terminals, fuse holders and other electrical components wear out over time. Contacts can oxidize, degrade due to repeated arcing (pitting), springs can weaken and insulating materials age. This increases the internal resistance of the component, generating additional heat even at rated loads.

How to Confirm:

• Thermography: Identification of a specific component as the main hot spot (ΔT > 15°C compared to similar components or internal ambient temperature).
• Measure Voltage Drop: (Under Load and Under Power, Safely) Measure the voltage drop across the component terminals. A high voltage drop (e.g. > 50 mV AC for a switch) indicates abnormal internal resistance.
• Visual Inspection: (After LOTO) Signs of arcing, discoloration, deformation or mechanical failure of the component.

Damage if not resolved: Total component failure, arc flash, service interruption, possible cascading damage to other components in the panel.

8. Step-by-Step Resolution Procedures

WARNING: ALWAYS PERFORM THE COMPLETE LOTO PROCEDURE BEFORE STARTING ANY MECHANICAL OR ELECTRICAL WORK.

8.1. For Loose or Oxidized Connections

1. Securing: Run LOTO on the affected circuit or panel. Check the absence of voltage.
2. Detailed Visual Inspection: Examine the connection identified by the thermal imaging camera. Look for signs of oxidation, pitting or overheating (discoloration).
3. Cleaning: If oxidation is present, carefully clean the contact surfaces (conductor and terminal) using a fine wire brush or fine-grained sandpaper. Use a specific product for electrical contacts if necessary and be sure to remove any residue.
4. Reconnection and Tightening: Re-insert the conductor and tighten the terminal to the torque specified by the manufacturer (consult manuals or tables EN 60999-1). Use a calibrated torque wrench. For tightening power conductors (e.g. 50 mm²), the torques can vary from 15 Nm to 30 Nm.
5. Check Contact Resistance: (Optional, but recommended) Use the micro-ohmmeter to check the resistance of the connection. Expected value < 50 µΩ.
6. Functional check: Remove LOTO and restore voltage. Monitor the connection with the thermal imaging camera under load to verify hot spot resolution.

8.2. Due to Current Overload

1. Securing: Run LOTO on the affected circuit to check the sections.
2. Load Evaluation: Analyze the actual measured load and compare it with the plate data of the components and conductors.
3. Sizing: If the conductor is undersized or the component is operated above its rated current, it is necessary to:
   • Replace the conductor with one of adequate section (e.g. according to CEI 64-8 for the current, length and type of installation) or
   • Replace the component (e.g. switch) with one of higher rated capacity, or
   • Rebalance the loads to reduce the current on the affected circuit, or
   • Reduce the operational load on the machine if the problem is systemic.
4. Verification and Testing: After modification, remove LOTO and restore. Measure the current under load again and check for overheating with the thermal imaging camera.

8.3. For Harmonic Distortion

1. Security: The installation of filters requires LOTO on the entire panel or on the affected line.
2. Detailed Analysis: Confirm the magnitude and order of harmonics with the power quality analyzer.
3. Solutions:
   • Passive Harmonic Filters: Installation of LC filters tuned to specific harmonics (e.g. 5th, 7th) on the main non-linear loads or at the switchgear input.
   • Active Harmonic Filters: For more complex situations or with variable harmonics, an active filter injected in parallel can effectively compensate for distortion.
   • K-Rated Transformers: Use K-rated transformers to power loads with high harmonics, as they are designed to withstand the additional heat.
   • Increasing Neutral Section: If the overheating affects the neutral, consider installing a neutral conductor with a section double that of the phases.
4. Effectiveness check: After installing the filters, perform a new energy quality analysis to verify the reduction of THD-I and THD-V below the standard limits (e.g. THD-I < 10%, THD-V < 5%).

8.4. Due to Load Imbalance between Phases

1. Securing: Run LOTO on the entire machine or on the affected panel.
2. Identification of Single-Phase Loads: Map and identify all single-phase loads connected to the panel.
3. Rebalancing: Redistribute single-phase loads between the three phases to minimize the current difference. The objective is to obtain a percentage imbalance between the phase currents of less than 5%.
4. Functional Check: Remove LOTO and restore. Remeasure the phase currents under load to confirm balance.

8.5. For Defective or Obsolete Components

1. Securing: Run LOTO on the affected circuit or panel.
2. Component Replacement: Remove the faulty component (e.g. switch, contactor, terminal block) and install a new spare part of equal or higher technical specification, compliant with regulations (e.g. CEI EN 60947 for LV equipment).
3. Connections: Tighten all connections to the new component according to the torque torques indicated by the manufacturer.
4. Functional Check: Remove LOTO and restore. Test the operation of the component and monitor it with the thermal imager under load to verify the absence of overheating.

9. Preventive Measures

10. Spare parts and components

Timely supply of quality spare parts is essential to reduce downtime. UNITEC-D offers a wide range of components for industrial electrical maintenance.

For availability and detailed technical specifications, consult the UNITEC-D electronic catalogue: www.unitecd.com/e-catalog/

11. References

• CEI Standard EN 61439-1: Low voltage assembled electrical panels - Part 1: General rules.
• Standard CEI 11-27: Work on electrical systems.
• Legislative Decree 81/08: Consolidated Law on Health and Safety at Work.
• EN 50160 Standard: Characteristics of the voltage supplied by public electricity distribution networks.
• CEI Standard EN 60947-2: Low voltage equipment - Automatic switches.
• CEI Standard EN 60947-4-1: Low voltage equipment - Contactors and motor starters.
• Service manuals and wiring diagrams provided by the machine manufacturer.
• Related UNITEC-D maintenance guides on thermal management of components.