Diagnosis and elimination of overheating in control cabinets

1. Problem description & scope of application

This guide serves as an accurate diagnostic tool for electricians and maintenance personnel to systematically identify and resolve overheating problems in electrical panels. Overheating is a critical issue that significantly shortens the lifespan of components, leads to unplanned downtime and, in the worst case scenario, poses the risk of fire. The area of ​​application includes low-voltage switchgear (up to 1000 V AC) in industrial manufacturing environments, especially in the DACH region (Germany, Austria, Switzerland), taking into account relevant standards such as DIN EN 61439 (low-voltage switchgear assemblies) and VDE 0100 (construction of low-voltage systems).

The symptoms covered in this guide relate to thermal anomalies detected through thermographic inspections or manual temperature measurements. Components such as circuit breakers, contactors, terminal blocks, busbars, busbar distributors, transformers, frequency converters and cable connections are typically affected. A classification of the severity of overheating can be made as follows:

• Critical: Temperature difference (ΔT) of > 30 K to the reference value or adjacent components. Immediate need for action.
• Major: Temperature difference (ΔT) of 15 K – 30 K. Immediate remediation planning required.
• Minor: Temperature difference (ΔT) of < 15 K. Observation and root cause analysis recommended.

2. Safety precautions

WARNING: Work on electrical systems may only be carried out by qualified electricians in accordance with VDE 1000-10. Before starting any diagnostic or repair work, the five safety rules according to DIN VDE 0105-100 must be adhered to:

1. Disconnect: Disconnect the system from the power supply at all poles (e.g. by turning off the main switch, removing fuses).
2. Secure against being switched on again: Prevent accidental or unauthorized switching on again (e.g. by locking the main switch, attaching warning signs).
3. Determine the absence of voltage: Check that there is no voltage at all poles on all active parts of the work site using a suitable and tested voltage tester (e.g. according to DIN EN 61243-3).
4. Grounding and short-circuiting: For systems with nominal voltages above 1 kV or for inductively/capacitively powered system parts, grounding and short-circuiting is mandatory.
5. Cover or fence off adjacent live parts: Protect yourself and others from accidentally touching live parts.

In addition, the prescribed personal protective equipment (PPE) must be worn, which corresponds to the arc flash hazards of the work area (e.g. in accordance with DGUV rule 103-011). This includes flame-retardant clothing, VDE-tested protective gloves (e.g. Class 0 for low voltage), face protection (visor), safety shoes and hearing protection. Always observe the manufacturer's specifications for the components and the control cabinet.

CAUTION: Capacitors can store residual charges. Before touching them, they must be properly discharged. When working on frequency converters, the discharge times must be observed.

3. Required diagnostic tools

Effective diagnosis of overheating in control cabinets requires the use of specific measuring devices. The selection and correct use of these tools is crucial for accurate results.

4. Checklist for initial assessment

Before starting a detailed diagnosis, a systematic initial assessment of the control cabinet and its surroundings must be carried out. This helps to narrow down potential causes early on and make the diagnosis more efficient.

5. Systematic diagnostic flow chart

Diagnosis of cabinet overheating follows a structured decision tree, starting with non-invasive thermography and branching out based on the measurement results.

1. Visual Inspection & Initial Assessment:
   1. Performing the “Initial Assessment Checklist” (Section 4).
   2. Result: Are serious defects such as traces of smoke, melted insulation or blocked fans discovered?
   3. IF YES: Immediate shutdown of the system in accordance with the safety precautions (Section 2) and direct troubleshooting (Section 8) of the obvious defects.
   4. IF NO: Continue with thermographic examination.
2. Thermographic examination (control cabinet during operation):
   1. Measurement: Use of the thermal imaging camera (section 3) to scan the entire control cabinet - inside and outside - under operating conditions. Pay particular attention to circuit breakers, contactors, terminals, busbars, transformers and cable junctions.
   2. Rating: Identify hot spots and measure the temperature difference (ΔT) to adjacent or reference components.
   3. Result A: Local hot spots with ΔT > 20 K detected.
   4. IF YES (A):
      1. Suspicion of loose electrical connections or overloading of a component.
      2. Continue with: Physical test after activation and load current measurement (Step 3).
   5. Result B: No local hot spots, but general heating of the entire control cabinet or diffuse temperature increase (e.g. all phase conductors of an outgoing feeder are equally elevated).
   6. IF YES (B):
      1. Suspected insufficient ventilation/cooling, general overload, harmonic distortion or uneven load distribution.
      2. Proceed to: Analysis of Environmental Conditions and Power Distribution (Step 4).
   7. Result C: No significant thermal anomalies detected.
   8. IF YES (C):
      1. Possibly sporadic error or environmental factor. Install long-term monitoring and, if necessary, order recurring thermography tests. Problem not critical for now.
3. Physical test after activation and load current measurement:
   1. WARNING: Activate and secure the system! (Section 2)
   2. Measure 1 (load current measurement): Use of the clamp ammeter (Section 3) on all three phase conductors of the affected outlets (with three-phase current) and on the feed cables.
   3. Rating 1: Are the measured currents higher than the rated current of the component (according to the nameplate) or the cable load capacity (according to DIN VDE 0298-4)? Are there significant current imbalances between phases (>10%)?
   4. IF YES (1):
      1. Suspicion of Overload or Uneven load distribution.
      2. Proceed to Resolving Congestion/Load Imbalance (Section 8).
   5. Measure 2 (torque check & visual inspection): After activation! Check the screw connections (terminals, busbars) with a calibrated torque wrench (Section 3) for correct tightening torques according to the manufacturer's instructions or DIN 43670/43671. Visual inspection for corrosion, material fatigue, signs of arcing.
   6. Rating 2: Are connections loose or do they show corrosion/damage?
   7. IF YES (2):
      1. Suspected Loose connections.
      2. Proceed to Fixing Loose Connections (Section 8).
   8. IF NO (2):
      1. The cause of the error is probably not loose connections.
      2. Proceed to Analysis of Environmental Conditions and Power Distribution (Step 4) if not already done.
4. Analysis of environmental conditions and power distribution:
   1. Measure 1 (ventilation): Checking the function of all fans, cooling devices and the cleanliness of all filters. Measurement of the inlet and outlet temperatures of the control cabinet.
   2. Rating 1: Is the cooling performance insufficient or blocked?
   3. IF YES (1):
      1. Suspected Inadequate ventilation/cooling.
      2. Proceed to Fixing Ventilation Issues (Section 8).
   4. Measure 2 (power quality analysis): Use of the power quality analyzer (Section 3) on the main feed of the control cabinet or affected outlets. Focus on THD-I (Total Harmonic Distortion of Current), individual harmonics and phase current imbalances.
   5. Evaluation 2: Are there THD-I values ​​above 5% (according to EN 50160) or significant harmonics? Are there phase current imbalances greater than 10% for no apparent reason?
   6. IF YES (2):
      1. Suspicion of Harmonic distortions or Uneven load distribution.
      2. Proceed to Troubleshooting Power Quality Issues (Section 8).
   7. Measure 3 (component test): Individual testing of suspicious components (e.g. contactors, circuit breakers) according to manufacturer specifications, if necessary by replacing them for diagnostic purposes.
   8. Rating 3: Is a component overheating despite the correct load and connection?
   9. IF YES (3):
      1. Suspected component failure.
      2. Proceed to Component Replacement (Section 8).

6. Error-cause matrix

This matrix presents the most common symptoms of panel overheating, their likely causes, and the diagnostic tests required. The probability of causes is sorted in descending order.

7. Root cause analysis for each error

A thorough understanding of the causes of overheating is essential to implement sustainable solutions and avoid repeat errors.

7.1 Loose electrical connections

Cause: Loose connections are primarily caused by insufficient tightening torque during assembly, vibrations during operation, thermal cycles (heating and cooling lead to material expansion and contraction) or material fatigue of the screws or spring elements. Leakage currents caused by contamination can also attack the surface.

Confirmation: A thermographic inspection under load shows a clear hot spot directly at the terminal point (e.g. ΔT > 20 K). After activation and no voltage, a torque test with a calibrated torque wrench (Section 3) can confirm that the target torque has not been reached. Visual inspection may reveal discoloration, traces of smoke or beginning corrosion.

Damage if not repaired: An increased contact resistance at the contact point leads to power conversion into heat (P = I²R). This accelerates the degradation of the insulation, causing arcing, which in turn leads to material erosion and increased fire risk. Arc flashes can also cause electromagnetic interference (EMC) that affects sensitive control electronics. In the long term, this can lead to a failure of the connection, the connected device or a fire in the control cabinet.

7.2 Overload

Cause: Overload occurs when a circuit, conductor, or component carries more current than it is designed for. This can be caused by unexpected increases in machine performance, the addition of new consumers without appropriate adjustment of the electrical system, or by a defect in a connected device (e.g. short winding in a motor). Incorrectly dimensioned components or conductor cross-sections according to DIN VDE 0298-4 are also a cause.

Confirmation: A current measurement with a clamp-on ammeter (Section 3) shows a value that is permanently above the rated current of the component or the permissible load capacity of the conductor. A comparison with the nameplate or the calculation documents (according to VDE 0100) confirms the overload. A thermographic image often shows uniform heating of the overloaded conductor or component.

Damage if not repaired: The continuous overload leads to a constant increase in the operating temperature. This accelerates the aging of the insulation of cables and windings, making them brittle and susceptible to short circuits. Circuit breakers may trip more frequently, resulting in lost production. Motors or transformers can suffer permanent damage or even complete failure due to overheated windings. The efficiency of the system decreases due to increased losses.

7.3 Harmonic distortions (harmonics)

Cause: Harmonic distortions in the power grid are caused by non-linear loads that do not draw the current sinusoidally. Typical culprits include frequency converters (VFDs), switched-mode power supplies (SMPS), LED lighting, UPS systems and computers. These devices generate harmonics that propagate across the network and result in additional currents, especially in the neutral conductor (at the 3rd harmonic and its multiples).

Acknowledgment: A power quality analyzer (Section 3) is required to measure the Total Harmonic Distortion of Current (THD-I) and individual harmonics. Values ​​of THD-I > 5% (according to EN 50160) or high proportions of the 3rd, 5th, 7th harmonics confirm the problem. A current measurement in the neutral conductor can show a disproportionately high current in the event of an imbalance or 3rd harmonic.

Damage if not rectified: Harmonic currents generate additional losses (eddy currents, hysteresis losses) in transformers, motors and cables, which leads to their overheating. This is particularly critical for transformers and the neutral conductor, which can even be loaded beyond its rated current at high 3rd harmonics. This leads to insulation damage, premature aging of equipment and unnecessary tripping of circuit breakers. The overall network quality deteriorates, which can disrupt other sensitive devices on the network.

7.4 Uneven load distribution

Cause: In three-phase networks, uneven load distribution occurs when single-phase consumers are not evenly distributed across the three phase conductors (L1, L2, L3). This can result from ill-considered planning, subsequent changes to the system or the failure of consumers in one phase. The result is an asymmetrical load on the network.

Confirmation: A current measurement on all three external conductors (L1, L2, L3) and the neutral conductor with a clamp ammeter (section 3) shows clear differences in the phase currents (> 10-15% deviation from the mean). An increased current in the neutral conductor (> 50% of the highest phase conductor current without significant harmonics) is also a strong indication.

Damage if not rectified: Uneven load distribution leads to uneven voltage drop in the phases, which can affect the performance and lifespan of connected devices. The neutral conductor can be overloaded even if the phase conductors are within their limits because the phase currents do not completely cancel each other out. This leads to overheating in the neutral wire, losses in transformers and possibly failure of the neutral wire, which can lead to overvoltages in the remaining phases (especially dangerous for single-phase loads).

7.5 Inadequate ventilation/cooling

Cause: Insufficient heat dissipation from the control cabinet can be caused by various factors: clogged or dirty fan filters, defective or blocked fans, failure of an air conditioning unit, ambient temperatures that are too high, incorrectly dimensioned cooling systems or the blockage of ventilation openings by dust, foreign bodies or subsequently installed components. An unsuitable installation location (e.g. direct sunlight) also plays a role.

Confirmation: Visual inspection shows dirty filters or blocked vents. A functional test confirms the failure of fans or air conditioning units. Temperature measurements with a contact thermometer or thermal imaging camera show a small temperature difference between the inside cabinet temperature and the ambient temperature, but a high absolute inside temperature. The airflow at the ventilation openings is noticeably low or non-existent.

Damage if not repaired: Components in the control cabinet are designed for certain operating temperatures. A permanently elevated internal temperature accelerates the aging of all electronic and electromechanical components exponentially (Arrhenius equation). The lifespan of capacitors, relays, switching power supplies and frequency converters is drastically shortened, leading to premature failures and high maintenance and spare parts costs. The reliability of the entire system decreases.

7.6 Component Failure

Cause: Even under optimal operating conditions, individual components can overheat or fail due to material defects, manufacturing defects, aging processes (end of service life), internal short circuits or mechanical damage. This can manifest itself in the form of increased internal resistance or a partial short circuit.

Confirmation: Thermographic examination shows abnormal heating isolated to a single component, even when the load and connections appear correct. After activation, a resistance measurement with the multimeter (Section 3) or an insulation measurement (Section 3) may provide suspicious values. Sometimes the defect can also be recognized visually (burst capacitors, deformations). Replacing the suspect component and repeating the thermographic test under load can definitively confirm the error.

Damage if not corrected: A defective component can lead to cascade errors, in which the overheating or failure of one component damages neighboring components. This not only increases repair costs, but can also bring the entire system to a standstill. In the worst case, a short circuit in the defective component can lead to an arc and a fire.

8. Step-by-step troubleshooting procedures

The following procedures must be carried out after the system has been activated and secured in accordance with Section 2.

8.1 Troubleshooting loose electrical connections

1. System security: Activate and secure the system in accordance with the “Five Safety Rules” (Section 2). Determine the absence of voltage.
2. Visual inspection: Specifically inspect the hot spots from thermography. Pay attention to discoloration, traces of smoke or corrosion.
3. Cleaning: If corrosion or contamination is present, clean the contact surfaces thoroughly with a suitable contact cleaning spray and a brush (non-metallic).
4. Tighten: Tighten all screw connections (terminals, busbars, connection bolts) in the affected area using a calibrated torque wrench (Section 3) to the torque specified by the component manufacturer or the applicable standard (e.g. DIN 43670 for copper busbar connections). Documentation of torque.
5. Restoration: Close the control cabinet again and put it into operation.
6. Verification: Carry out another thermographic inspection immediately after restarting and under load. The hot spots must be eliminated or significantly reduced (ΔT < 5 K).

8.2 Resolving congestion

1. System safety: Operation under voltage may be necessary for temporary load measurements. Particular caution is required and PPE (Section 2) must be worn. Complete activation is required for changes to the system.
2. Load analysis: Use the clamp ammeter (Section 3) to measure the actual current curve of the affected feeders over a longer period of time (e.g. using the data logger function). Comparison with nominal currents and cable load capacities (DIN VDE 0298-4).
3. Causal clarification: Determine which consumers are causing the overload (e.g. new machines, changed production processes).
4. Measure planning:
   • Option A (reducing the load): Adjusting the operating mode to reduce the load.
   • Option B (system adjustment): Redistribution of the load to other, less busy circuits.
   • Option C (Sizing): Increasing the conductor cross-section of the affected cables and/or replacing the circuit breaker/components with higher-dimensioned types according to VDE 0100 and VDE 0298-4.
5. Implementation & Verification: After implementing the measures (option A, B or C), put the system under load again and carry out a thermographic inspection and current measurements to confirm the successful correction.

8.3 Fix harmonic distortion

1. System safety: Operation under voltage is required for the power quality analysis. Particular caution is required and PPE (Section 2) must be worn. Complete activation is required for installation work.
2. Power quality analysis: Use of the power quality analyzer (Section 3) at the power supply to the control cabinet and at the outputs to the non-linear loads identified as the cause. Measurement of THD-I and the individual harmonic components (especially 3rd, 5th, 7th).
3. Causal analysis: Identify the main causes of harmonics (e.g. frequency converters without chokes, large numbers of switching power supplies).
4. Measure planning:
   • Option A (passive filter): Installation of passive harmonic filters (e.g. chokes, LC filters) in front of the non-linear loads or at central points in the network.
   • Option B (Active Filter): Installation of active harmonic filters that compensate for harmonic currents. These are usually more flexible and effective.
   • Option C (devices with low harmonics): When purchasing new consumers, pay attention to devices with integrated harmonic compensation or lower THD-I (e.g. frequency converters with AFE rectifiers).
5. Implementation & Verification: After installing the filters or components, perform another power quality analysis to confirm the reduction in harmonics (THD-I < 5%) and neutral currents. A thermographic test can also show reduction in thermal stress.

8.4 Fixing uneven load distribution

1. System safety: Operation under voltage is required for load current measurement. Particular caution is required and PPE (Section 2) must be worn. Complete activation is required for changes to the wiring.
2. Load current measurement: Use the clamp ammeter (Section 3) to measure the currents in all three phase conductors (L1, L2, L3) and in the neutral conductor (N) of the affected outlets.
3. Analysis: Compare the measured values ​​of the phase currents. A deviation of over 10-15% between phases indicates an uneven distribution. Also note an excessively high neutral conductor current without significant harmonics.
4. Measure planning:
   • Option A (reassignment): In the activated state, distribute the single-phase consumers across the three phase conductors so that the loads are as even as possible. Update circuit diagrams.
   • Option B (balancing calculation): For larger systems, a detailed load flow calculation may be required to determine the optimal distribution.
5. Implementation & Verification: After rewiring, put the system back into operation and measure the phase currents again. The currents should now be approximately the same (difference < 5-10%). The neutral conductor current should reduce accordingly. A thermographic test can confirm the elimination of diffuse heating of the neutral conductor.

8.5 Correcting inadequate ventilation/cooling

1. System safety: For the maintenance of fans and the replacement of filters, the control circuits of the cooling devices must be activated. For work in the control cabinet, a complete activation of the entire control cabinet (Section 2) is required.
2. Visual Inspection & Functional Testing: Inspect all ventilation openings, filters and cooling devices. Are the filters dirty? Are the fans spinning? Are the air conditioning units working?
3. Cleaning/replacement:
   • Filter: Clean or replace dirty filters. Recommendation: Filter mats according to DIN EN 779.
   • Fans: Replace defective fans with new, equivalent or more powerful models (according to manufacturer specifications and required air flow).
   • Air conditioning units: If an air conditioning unit fails, have it serviced or repaired by qualified personnel (refrigerant charge, clean evaporator/condenser).
4. Optimization:
   • Ensure that no objects are blocking the ventilation openings.
   • If necessary, install additional fans or a more powerful air conditioning unit if the heat in the cabinet has increased.
   • Checking the location of the control cabinet. Avoid direct sunlight.
5. Verification: After the measures, put the control cabinet back into operation and monitor the internal temperature over an operating cycle with an IR thermometer (Section 3). The temperature should be within the permissible range (e.g. according to the manufacturer's specifications, often < 40 °C). A thermographic test may show overall warming to be reduced.

8.6 Fixing Component Failure

1. System security: Activate and secure the system in accordance with the “Five Safety Rules” (Section 2). Determine the absence of voltage.
2. Component inspection: Visually inspect the component identified as defective (e.g. circuit breaker, contactor, power supply) for damage. If necessary, carry out electrical measurements (resistance, insulation resistance).
3. Exchange: Replace the defective component with an identical or technically equivalent spare part. Pay attention to correct dimensioning (rated current, switching capacity, rated voltage).
4. Wiring: Wire the connections of the new component correctly according to the circuit diagram and tighten the screw connections with the correct torque.
5. Recommissioning: After completing the work, close the control cabinet again and put it into operation.
6. Verification: Perform a thermographic inspection under load to ensure that the new component is not exhibiting abnormal heat and that the problem is resolved.

9. Preventive measures

Implementing preventative measures is crucial to minimize the recurrence of overheating problems and increase operational safety and equipment lifespan.

10. Spare Parts & Components

The availability of spare parts is crucial to minimize downtime in the event of a fault. The components listed here are common spare parts that can be found in the UNITEC-D e-catalogue.

You can find a comprehensive selection of high-quality spare parts and components that meet the applicable DIN, VDE and TÜV standards in our UNITEC-D e-catalog.

11. References

• DIN VDE 0100 (VDE 0100): Setting up low-voltage systems
• DIN VDE 0105-100 (VDE 0105-100): Operation of electrical systems
• DIN EN 61439 (VDE 0660-600): Low-voltage switchgear assemblies
• DIN VDE 0298-4: Use of cables and wires for power systems - Recommended values for the current carrying capacity of cables and wires in systems
• DIN EN 60898-1 (VDE 0641-11): Circuit breaker for household and similar purposes
• DIN EN 60947-4-1 (VDE 0660-102): Low-voltage switching devices - contactors and motor starters - electromechanical contactors and motor starters
• DIN EN 50160: Characteristics of voltage in public electricity supply networks
• DGUV Rule 103-011: Selection and use of protective gloves
• VDE 1000-10: Requirements for people working in the field of electrical engineering
• Manufacturer manuals for specific components (e.g. Siemens, Eaton, Schneider Electric)