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Elimination of overheating of electrical cabinets: thermographic inspection, detection of bad contacts, harmonic distortion and load balancing

Technical analysis: Troubleshooting electrical panel overheating: thermographic inspection, loose connection detection,

Усунення перегріву електричних шаф: термографічна інспекція, виявлення поганих контактів, гармонійні спотворення та балансування навантаження - UNITEC-D Industrial MRO
Цей посібник надає систематичний підхід до діагностики та усунення причин перегріву електричних шаф, зосереджуючись на термографічній інспекції, виявленні поганих контактів, гармонійних спотвореннях т

1. Description of the problem and scope of application

This manual is intended for diagnosing and troubleshooting problems associated with overheating of electrical cabinets, switchboards, motor starter panels, and other industrial electrical equipment. Overheating of electrical components is a critical symptom that can lead to: premature equipment failure, insulation degradation, reduced system efficiency, unplanned production downtime, and significant fire and personnel safety risks. This guide covers diagnosing common causes of overheating, such as poor electrical connections, overloaded circuits, harmonic distortion, and unbalanced loads.

Classification of the severity of the malfunction:

  • Critical: The temperature of the components exceeds the maximum allowable operating temperature specified by the manufacturer (eg above 90°C for copper conductors in air). Immediate shutdown of the equipment is mandatory.
  • Serious: The temperature of components is 15-30°C higher than the temperature of similar components under identical load or exceeds 70°C. Requires immediate corrective action planning.
  • Minor: The temperature of the components is 5-15°C higher than the temperature of similar components or exceeds 50°C. Requires monitoring and preventive maintenance planning.

2. Safety measures

CAUTION: Working with electrical cabinets carries a high risk of electric shock, arc flash, and other serious injury or death. Before starting any diagnostic or repair work, strictly follow all established safety procedures.
  • Lockout/Tagout (LOTO): Always apply LOTO procedures in accordance with the company's internal rules and the requirements of DSTU EN 50110-1:2017 (Operation of electrical installations), DSTU EN 61439-1:2018 (Complete low-voltage distribution devices). Make sure all power sources are disconnected, locked out, and tested for voltage before touching the equipment.
  • Personal Protective Equipment (PPE): Use appropriate PPE to protect against arc flash and electrical current. This includes: non-flammable clothing, safety glasses, face shield, dielectric gloves (class appropriate for the voltage), safety shoes. The level of PPE must correspond to the arc flash hazard category defined for this electrical cabinet.
  • Check for no voltage: After applying LOTO and before starting work, always check for no voltage with a working and tested measuring device on all phases and between phases and ground.
  • Stored Energy Discharge: Capacitors can store dangerous levels of energy even after the power is turned off. Ensure capacitors are fully discharged before touching.
  • Work under voltage: If diagnostics require work under voltage (for example, thermographic inspection or current measurement with clamps), this should only be performed by qualified personnel, observing all necessary PPE and safe distances established by regulations.

3. Necessary diagnostic tools

Name of the tool Specification/Model (examples) Measuring range Purpose
Thermal imaging camera (thermograph) FLIR E-series, Testo 883 from -20°C to +650°C, sensitivity <0.05°C Detection of hot spots, visualization of temperature anomalies by non-contact method.
Digital multimeter Fluke 179, Testo 760-3 Voltage: up to 1000 V AC/DC; Resistance: up to 50 MΩ; Current: up to 10 A. Measurement of voltage, resistance, circuit integrity.
Electrical measuring clamps (current) Fluke 376 FC, Chauvin Arnoux F407 Current: up to 1000 A AC/DC; Voltage: up to 1000 V AC/DC; Power, power factor. Measurement of load current without breaking the circuit, measurement of current imbalance.
Power quality analyzer Fluke 435 II, Chauvin Arnoux Qualistar+ Voltage, current, power, THD, harmonics up to the 50th. Detection of harmonic distortions, monitoring of power grid parameters.
Microohmmeter (Milliohmmeter) Megger DLRO10, AEMC 6250 from 0.1 μOhm to 2000 Ohm Accurate measurement of transient resistance of contacts, buses, connections.
Non-contact pyrometer (infrared) Fluke 561, Testo 835-T2 from -30°C to +900°C Fast spot temperature measurement of surfaces.

4. Initial evaluation checklist

Before starting a detailed diagnosis, it is important to collect as much information as possible about the operating conditions and history of the electrical cabinet.

Checkpoint What to observe/record Note
External inspection of the cabinet The presence of dust, dirt, damage to the case, signs of overheating (discoloration, deformation), blocked ventilation holes. Are there any visible signs of trouble?
Working conditions Ambient temperature in the room, relative humidity. High ambient temperature impairs cooling.
Recent changes Were new loads added? Have any repairs or modifications been made to the system? Changes often cause new problems.
Alarm/fault history View the event log of the automation system or logger. Have there been previous reports of overload or high temperature?
Nominal and actual loads Compare design loads with actual currents and voltages (if possible non-contact). Exceeding the nominal values ​​indicates an overload.
Sound and smell signs Unusual noises (hissing, cracking), the smell of burnt insulation. Signs of rapid development of a serious malfunction.

5. Systematic diagnostic algorithm

  1. Initial Overheat Detection (Thermographic Inspection)
    1. Perform a thermographic inspection of all electrical cabinets and their components under working load.
    2. Use a thermal imaging camera set to the appropriate emissivity of the materials (e.g. 0.95 for matte paint, 0.20 for shiny metal).
    3. Record the temperatures of the hottest spots.
    4. If hot spots are detected (>50°C): Go to point 2.
    5. If hot spots are absent or minor: Overheating may be caused by a general increase in ambient temperature or insufficient cabinet ventilation. Go to point 6.
  2. Locating the hot spot and its identification
    1. Determine which component is overheating (contact, conductor, circuit breaker, contactor, transformer, harmonic filter, etc.).
    2. If this is a contact or connection: Go to point 3.
    3. If it is a circuit breaker or protective device: Go to point 4.
    4. If it is a transformer, reactor or capacitor: Go to point 5.
    5. If it is a conductor along its entire length: Go to point 4.
  3. Diagnosing Bad Pins/Connections (After Power Off)
    1. Lockout/Tag (LOTO): Turn off power according to LOTO procedures.
    2. Visually inspect the suspicious connection: signs of corrosion, weakening, burning.
    3. Use a multimeter or microohmmeter to measure the resistance of the connection.
      1. Expected result: The resistance should be less than 100 µOhm (0.0001 Ohm) for power connections.
      2. If the resistance is high: This is a bad contact. Go to Section 8 (Troubleshooting: Bad Contacts).
    4. Check the torque of the terminals with a torque wrench according to the manufacturer's recommendations.
  4. Diagnosis of overload and unbalanced load (under voltage)
    1. Use of PPE: Provide full protection against arc flash and electric shock.
    2. Using a clamp meter, measure the load current on each phase (L1, L2, L3) of the overheated component.
    3. Compare the measured currents with the nominal values ​​of the component and the permissible currents for the conductor section (according to PUE, DSTU IEC 60364).
    4. If the current exceeds the nominal (>100% of the nominal) or permissible: This is an overload. Go to Chapter 8 (Troubleshooting: Overloading).
    5. If the phase currents are significantly different (>10% unbalance between phases): This is an unbalanced load. Go to Chapter 8 (Troubleshooting: Unbalanced Load).
  5. Diagnosis of harmonic distortions (under voltage)
    1. Use of PPE: Provide full protection against arc flash and electric shock.
    2. Using a power quality analyzer, measure the total harmonic distortion current (THDI) and voltage (THDU) at the entrance to the cabinet and at the taps to the suspect components.
    3. Expected result: According to EN 50160 and DSTU EN 50160:2014, THDU should not normally exceed 8% for systems up to 1 kV. Acceptable THDI levels depend on the type of equipment, but often >15-20% can cause problems.
    4. If the level of harmonics is high (THDI > 15-20%): This is harmonic distortion. Go to Chapter 8 (Troubleshooting: Harmonic Distortion).
  6. Diagnosing ventilation/cooling problems
    1. Check the operation of the fans: are they spinning, are the filters not clogged.
    2. Measure the temperature inside the cabinet and the ambient temperature.
    3. If ventilation is insufficient or blocked: This is the cause of overheating. Go to Section 8 (Troubleshooting: Ventilation).

6. Malfunction-cause matrix

Symptom (hot spot) Probable causes (by probability) Diagnostic test Expected result if the cause is confirmed
Localized overheating on terminals, buses, conductor connections 1. Bad (weakened/corroded) contact
2. Insufficient conductor cross-section for this load
3. Low-quality crimping of the tip		 1. Measuring the connection resistance with a microohmmeter (LOTO)
2. Measuring load current with clamps (under voltage)		 1. Resistance > 100 μΩ
2. Current > nominal for section/terminal
Overheating of the automatic switch, fuse, contactor 1. Circuit overload
2. Internal defect of the device (aging, contact fatigue)
3. Bad contact on the input/output terminals of the device		 1. Measurement of load current with clamps (under voltage)
2. Comparison of temperature with similar devices (thermograph)
3. Terminal resistance measurement (LOTO)		 1. Current > nominal for the device
2. The temperature is much higher than normal
3. Resistance > 100 μΩ
Overheating of the conductor along its entire length 1. Circuit overload
2. Insufficient conductor cross section
3. Grouping of conductors without taking into account reduction factors		 1. Measurement of load current with clamps (under voltage)
2. Checking the cross-section of the conductor according to PUE, DSTU IEC 60364		 1. Current > admissible for cross section
2. The section does not correspond to the rated current
General overheating of the cabinet without localized hot spots 1. Insufficient ventilation/cooling
2. High ambient temperature
3. A collection of minor heats that are added		 1. Checking the operation of fans, condition of filters
2. Measuring the temperature inside and outside the cabinet		 1. Dirty filters, non-working fans
2. The temperature inside > ΔT specified by the manufacturer
Overheating of transformers, reactors, capacitors (especially in systems with frequency converters) 1. Harmonic distortions in the network
2. Overloading the device
3. Low power factor		 1. Analysis of power quality (THDI, THDU) (under voltage)
2. Measuring load current with clamps (under voltage)		 1. THDI > 15-20%
2. Current > nominal
Overheating of one phase in a three-phase system 1. Unbalanced load
2. Equipment defect of one phase
3. Zero sequence harmonics (for neutral)		 1. Measurement of currents on each phase with clamps (under voltage)
2. Analysis of harmonics (under voltage)		 1. Imbalance of currents between phases > 10%
2. High harmonics

7. Root cause analysis for each malfunction

7.1. Bad (weakened/corroded) contacts

Explanation: Transient resistance at the point of electrical connection (terminals, clamps, crimping places) should usually be in the microohm range. Tightening loosening, vibration, temperature cycling, oxidation or corrosion all increase this resistance. According to the Joule-Lenz law (P = I2R), even a slight increase in resistance (R) when a large current (I) flows leads to a significant release of heat (P = power loss). This heat causes local overheating.

Confirmation: A thermal imaging camera will show a localized hot spot and a microohmmeter will confirm a high resistance connection after power is off. A visual inspection may reveal traces of burning, discoloration of insulation or metal.

Consequences if not eliminated: Further increase in resistance and temperature, destruction of conductor insulation, deformation of plastic parts, melting of metal, short circuit, arc flash, complete shutdown of equipment, fire.

7.2. Circuit/equipment overload

Explanation: An overload occurs when the current flowing through a conductor or electrical component exceeds its nominal or permissible value calculated by the manufacturer or standards (PUE, DSTU IEC 60364) for safe long-term operation. This leads to excessive heat generation along the entire length of the conductor or inside the device.

Confirmation: The clamp meter will show a current that exceeds the nominal value. A thermographic camera will show overheating of a component or conductor along its entire length, not just at the connection point.

Consequences, if not eliminated: Accelerated aging of insulation, reduction of service life of equipment, frequent tripping of protective devices (automatic circuit breakers), fire. Currents close to the circuit breaker rating can cause the circuit breaker to overheat even if it does not trip.

7.3. Harmonic distortions

Explanation: Harmonics are sinusoidal voltages and currents, the frequency of which is a multiple of the main frequency of the network (50 Hz). They are created by non-linear loads such as frequency converters, rectifiers, switching power supplies, computers, LED lighting. Harmonic currents flow through the network, causing additional losses and overheating: transformers (increased eddy currents), capacitors (resonance), conductors (especially the neutral conductor, where harmonics of the third order can add up), motors (additional heating, vibration).

Confirmation: The power quality analyzer will detect high current (THDI) and/or voltage (THDU) values. According to DSTU EN 50160:2014, THDU should not exceed 8%.

Consequences, if not eliminated: Overheating and damage to transformers, reactors, capacitors, motors. Malfunctions of sensitive electronics, malfunctions of protective devices, increased network losses.

7.4. Unbalanced load

Explanation: In three-phase systems, an unbalanced load occurs when the currents in the three phases differ significantly. This leads to the appearance of a current in the neutral conductor (even if the load is linear, but asymmetrical) and to an uneven load of the phase conductors. The phase with a higher current will overheat more than the others. In addition, the asymmetry of currents causes additional heating of three-phase motors.

Confirmation: Electrical measuring clamps will show a significant difference in currents between phases (>10% of the average current value). For example, if IL1 = 100A, IL2 = 90A, IL3 = 80A, then the unbalance will be significant.

Consequences, if not eliminated: Overheating of individual phase conductors and devices connected to them. Reduction of efficiency and service life of three-phase motors, their overheating. Increasing losses in the system.

7.5. Insufficient cabinet ventilation/cooling

Explanation: Electrical cabinets are designed taking into account the removal of a certain volume of heat released by internal components. If the vents are blocked, the filters are clogged with dust, the fans are not working or their performance is insufficient, heat accumulates inside the cabinet, causing a general increase in the temperature of all components.

Confirmation: A visual inspection will show clogged filters, non-working fans. The thermograph will record the overall temperature rise inside the cabinet without obvious localized hot spots on individual components. Measuring the temperature inside the cabinet can show an excess of ΔT (the temperature difference between inside and outside) relative to the design.

Consequences, if not eliminated: General accelerated aging of all components, reduced reliability, more frequent failures, activation of thermal protection.

8. Step-by-step troubleshooting procedures

8.1. Troubleshooting: Bad contacts

  1. SECURITY: Perform a full lockout/tagout (LOTO) procedure for the affected electrical cabinet. CHECK NO VOLTAGE!
  2. Open the protective covers and visually inspect the connections. Pay attention to the change in color of insulation, burning, melting of plastic, loosening of bolts.
  3. Loosen the clamping screws or bolts of the suspect connection.
  4. Thoroughly clean the contact surfaces from oxidation, dirt and corrosion with the help of fine sandpaper or special means for cleaning contacts.
  5. Make sure that the ends of the conductors are properly crimped and correspond to the cross section of the conductor. If necessary, crimp the tip.
  6. Reconnect the conductor by tightening the screws or bolts with a torque wrench to the torque recommended by the component manufacturer (eg for a 2.5mm terminal block2 it might be 0.8-1.2 Nm, for power busbars much more according to EN 60947).
  7. After tightening, perform a control measurement of the connection resistance with a microohmmeter. Expected value <100 µOhm.
  8. Restore power and perform a thermographic check of the connection under load to confirm that the overheating has been resolved.

8.2. Troubleshooting: Circuit/equipment overload

  1. Measure the load currents with a clamp meter on all taps from the circuit breaker or overheated component. Follow the PPE!
  2. Compare the actual currents with the nominal data of the devices and the permissible currents for the cross-section of the cables according to the PUE.
  3. If a significant excess is found:
    1. Split the load into several separate circuits by adding new circuit breakers and conductors of the appropriate cross-section.
    2. Replace conductors and/or circuit breakers with devices with a higher current rating if permitted by design documentation and downstream circuits can also handle the increased current.
    3. Determine if there are any temporary or unauthorized connections causing congestion.
    4. Check the operating mode of the equipment: it may be operating in a mode for which it is not designed (for example, a motor with a locked shaft).
  4. Perform a thermographic check after making the changes to confirm that overheating has been resolved.

8.3. Troubleshooting: Harmonic distortion

  1. Perform a detailed power quality analysis with the analyzer by measuring THDI and THDU at various points in the network starting from the cabinet entry. Follow the PPE!
  2. Identify sources of harmonics (eg large frequency converters, thyristor regulators, induction furnaces).
  3. To reduce harmonics:
    1. Install passive harmonic filters (such as chokes) at the input to harmonic generating equipment.
    2. Use active harmonic filters that compensate for harmonic currents.
    3. Switch to equipment with lower harmonic content (such as frequency converters with multi-pulse rectifiers).
    4. Increase the power of the transformer or add an isolation transformer, which will help mitigate the effect of harmonics on the general network.
  4. After installing filters or other measures, re-measure harmonic levels and component temperatures to confirm effectiveness. THDU should be less than 8% (according to DSTU EN 50160).

8.4. Troubleshooting: Unbalanced load

  1. Measure the load currents on each phase (L1, L2, L3) using the clamp meter. Follow the PPE!
  2. Calculate the coefficient of asymmetry of the currents (phase imbalance) - the deviation of the current of a separate phase from the average value. If the imbalance exceeds 10%, it needs correction.
  3. To eliminate unbalanced load:
    1. Redistribute single-phase loads between phases to achieve the greatest possible symmetry of currents.
    2. Check the condition of three-phase motors: an unbalanced load can be caused by an internal defect in the motor (for example, an open winding).
    3. If the imbalance is caused by the specifics of the technological process (for example, asymmetric operation of welding machines), consider installing balancing devices.
  4. Remeasure the currents after redistributing the load and perform thermographic control.

8.5. Troubleshooting: Insufficient ventilation/cooling

  1. Perform a visual inspection of the vents and filters.
  2. Check the operation of the fans: whether they rotate, whether there are no extraneous noises.
  3. Clean or replace dirty air filters. SAFETY: Make sure fans are turned off before cleaning to avoid injury.
  4. Replace faulty fans.
  5. Ensure that airflow is not blocked inside the cabinet due to misalignment of components or cables.
  6. If natural convection and existing forced ventilation are not sufficient (for example, after adding new equipment), consider installing additional fans or an air conditioning system for the cabinet.
  7. Perform a thermographic check after correcting the ventilation problem.

9. Preventive measures

The root cause Prevention strategy Monitoring method Recommended interval
Bad contacts Use of quality terminals and tips. Regular inspection and tightening of connections with a torque wrench. Thermographic inspection. Measuring the resistance of connections (after LOTO). Annually or every 6 months for critical equipment.
Circuit/equipment overload Project calculations with margin. Control of connection of new loads. Current monitoring. Measurement of load current with clamps. Analysis of event logs. Quarterly or when the configuration/load changes.
Harmonic distortions Installation of harmonic filters. Selection of equipment with a low level of harmonics. Analysis of power quality (THDI, THDU). Once every 2-3 years or when overheating/failure problems occur.
Unbalanced load Uniform distribution of single-phase loads. Control of the connection of new equipment. Measurement of phase currents with clamps. Monthly for systems with a significant number of single-phase loads.
Insufficient ventilation/cooling Regular cleaning/replacement of filters. Checking the operation of the fans. Visual inspection. Thermographic inspection. Monthly (filters), annually (fans).

10. Spare parts and components

Timely replacement of faulty or worn components is critical to prevent overheating and ensure smooth operation. UNITEC-D offers a wide range of industrial electrical components.

Part description Specification When to replace Category UNITEC
Terminal blocks Appropriate conductor cross-section (for example, 2.5 mm2, 6 mm2, 16 mm2), rated current, material (copper/brass). When burning, deformation, loss of mechanical integrity, corrosion is detected. Electrical components / Connectors
Automatic switches Rated current (eg 10A, 16A, 63A), characteristic type (B, C, D), number of poles, manufacturer. In case of activation without apparent reason, overheating of the housing, deformation, visual damage. Electrical components / Means of protection
Contactors/Relays Rated current (for example, AC-3 32A), coil voltage (24V DC, 230V AC), number of contacts. In case of overheating of contacts, unclear operation, burning of power contacts, coil noise. Electrical components / Switching devices
Conductors/Cables Section (for example, 1.5 mm2, 4 mm2, 10 mm2), insulation type, rated voltage. In case of damage to the insulation, visual signs of overheating (color change), when the current section does not correspond to the load. Electrical components / Cable products
Cabinet fans Productivity (m3/h), dimensions, power supply voltage (24V DC, 230V AC), degree of protection (IP). In the event of malfunction (does not rotate, makes noise), reduced performance, physical damage. Cooling systems / Fans
Air filters Size (for example, 200x200 mm), filtration class (G2, G3). In case of severe pollution, which prevents the flow of air. Cooling systems / Filters
Harmonic filters (active/passive) Rated current/power, harmonic suppression level, voltage. When detecting high harmonics and overheating of sensitive equipment. Control systems / Filters

To order quality components and spare parts, visit our electronic catalog UNITEC-D.

11. Links

  • DSTU EN 50110-1:2017. Operation of electrical installations.
  • DSTU EN 61439-1:2018. Complete low-voltage distribution devices. General requirements.
  • DSTU EN 50160:2014. Characteristics of power supply voltage in general-purpose electrical networks.
  • PUE (Rules for the arrangement of electrical installations).
  • ISO 18434-1:2008. Condition monitoring and diagnostics of machines. Thermography.
  • IEEE Std 519-2014. IEEE Recommended Practice and Requirements for Harmonic Control in Electric Power Systems.
  • OEM service manuals for specific equipment.
  • Related UNITEC-D Service Manuals.

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