Troubleshooting electrical distribution overheating: thermography, detection of loose connections, harmonic distortion and load distribution

1. Problem description & scope of application

Overheating of electrical distributions represents a critical risk in industrial plants. It is defined as an increase in the operating temperature of components or the entire distribution above the nominal value specified by the manufacturer or the permissible limit temperatures specified in standards such as DIN EN 61439. Such thermal stress leads to accelerated aging of insulation materials, a reduced service life of equipment and poses a significant risk of unexpected failures, production interruptions and, in the worst case, fires or arcing incidents.

Symptoms of overheating:

• Visible discoloration or signs of melting on insulation materials (cables, busbar insulation, housings of components).
• A characteristic burning smell or the smell of scorched insulation in the electrical distribution area.
• Inexplicable and repeated tripping of circuit breakers, motor protection switches or fuses, often without obvious overload of the connected consumers.
• Unusual noises such as humming, crackling, or flashing.
• Reduced function or malfunction of electronic components within the distribution.

Affected system types:

This guide is applicable to all types of low-voltage switchgear assemblies according to DIN EN 61439, including:

• Main distributions and feeds
• Sub-distributions and machine distributions
• Engine Control Center (MCC)
• Control cabinets for control and automation technology (PLC, frequency converter)
• Capacitor banks for reactive power compensation

Severity classification:

The classification of the thermal problem identified is crucial for prioritizing the repair measures:

• Critical: A temperature increase of more than 20 Kelvin (K) above ambient temperature at connections or components, visible discoloration, or a burning smell. This requires the affected parts of the system to be shut down immediately and rectified immediately. The risk of fire, complete equipment failure and long production shutdowns is very high.
• Major: A temperature increase between 10 K and 20 K above ambient temperature. No immediate danger, but accelerated aging of the insulation and increased energy consumption. A cause analysis and resolution should be carried out within the next planned maintenance period or at short notice to avoid consequential damage.
• Minor: A temperature increase of less than 10 K above ambient temperature. Hotspot monitoring and detailed root cause analysis should be prioritized as part of the next regular scheduled maintenance. There is usually no immediate need for action, but the anomaly should be documented and tracked.

2. Safety precautions

ATTENTION: Danger to life due to electric shock, arcing faults and fire! Work on or near electrical systems may only be carried out by qualified electricians in accordance with VDE 0105-100 and in strict compliance with the accident prevention regulations (UVV). Failure to follow these safety instructions can result in serious injury or death.

Personal protective equipment (PPE):

• Arc-tested protective clothing: Protection level (APC, Arc Protection Class) must correspond to the risk assessment of the work area, e.g. class 1 (4 kA) or class 2 (7 kA) according to IEC 61482-1-2.
• Face protection: Arc-tested visor with helmet according to EN 166 and EN 170.
• Insulating protective gloves: Suitable for the maximum operating voltage of the system (e.g. class 0 for 1000 V AC) according to EN 60903. Check for integrity before each use.
• Safety shoes: With electrically insulating sole (SB/S1P/S3) according to EN ISO 20345.

Unlock and secure (LOTO – Lockout/Tagout):

Before any intervention that requires direct contact with live parts, the "Five Safety Rules" according to VDE 0105-100 must be strictly observed:

1. Disconnect: Disconnect the system from the mains on all poles, i.e. all external conductors and, if necessary, the neutral conductor.
2. Secure against being switched on again: Lock safety elements (e.g. circuit breakers) and provide a warning (“Do not switch! Work is underway!”).
3. Determine the absence of voltage: Use a two-pole voltage tester (e.g. Duspol) to check the absence of voltage on all previously live parts. The voltage tester must be tested for functionality before and after the measurement on a known voltage source.
4. Grounding and short-circuiting: Especially in medium and high-voltage systems, but also in large low-voltage systems with the risk of rear feed, ground and short-circuit the system.
5. Cover or fence off adjacent live parts: If not all parts of the system can be isolated, the remaining live areas must be safely isolated or fenced off.

Residual energy and hot surfaces:

• Capacitors: Larger capacitor banks or interference suppression capacitors in frequency converters and UPS systems can still store dangerous charges after they have been disconnected. Allow sufficient unloading time or use suitable unloading devices.
• Hot Surfaces: Overheated components maintain their temperature for some time even after switching off. There is a risk of burns if there is direct contact. Check thermographically before touching or wait for cooling time.

3. Diagnostic tools required

Specific measuring and testing devices are essential for precise and efficient troubleshooting. The selection of tools should be based on their accuracy, reliability and suitability for industrial use.

4. Checklist for initial assessment

Before beginning detailed measurements, a comprehensive visual inspection and data collection is essential. These steps help to quickly assess the situation and focus the diagnostic process.

5. Systematic diagnostic flowchart

This decision tree guides the technician through systematic troubleshooting based on the primary symptoms and measurement results.

1. Symptom: Increased temperature detected on the electrical distribution or individual components.
   1. Safety first: Before any direct measurement or contact: Disconnect the system on all poles, secure it against being switched on again and ensure that there is no voltage (according to VDE 0105-100). Wear PPE!
   2. Thermographic examination (under rated load, if possible safely and safely):
      1. Identify hotspots:
         • IF hotspot at terminal points, bolt connections or fuse holders (ΔT > 15 K to the surrounding component) → Probable cause: High contact resistance due to loose connections or corrosion.
           • Next step: Go to 6.1 (Loose Connections).
         • IF hotspot on circuit breakers, contactors, relays or transformers (ΔT > 20 K to the environment) → Probable cause: overloading of the component, internal damage or aging.
           • Next step: Go to 6.2 (Overloading of components).
         • IF Hotspot on cables/lines (uniform heating over the length of the line) → Probable cause: Overloading of the line or insufficient dimensioning.
           • Next step: Go to 6.3 (Overloading of lines).
         • IF general heating of the entire cabinet interior without a clear hotspot on individual components (ΔT cabinet interior > 10 K to the ambient air) → Probable cause: Insufficient cooling, harmonic distortions or unbalanced load.
           • Next step: Go to 6.4 (Insufficient cooling, harmonics or unbalanced load).
   3. Electrical measurements (under rated load, if safe, or after activation):
      1. Load distribution and current measurement (with True-RMS current clamp or network analyzer):
         • IF unequal currents on the phases (unbalanced load, > 10% deviation between phase currents) → Probable cause: Unbalanced Load distribution.
           • Next step: Go to 6.5 (Unbalanced load).
         • IF total current above the rated current of the feed point or individual components → Probable cause: overall overload.
           • Next step: Go to 6.2 (overload of components) or 6.3 (overload of lines).
         • IF Current in the neutral conductor > Phase current with symmetrical load or generally high neutral conductor current → Probable cause: Harmonic distortions (especially 3rd harmonics).
           • Next step: Go to 6.6 (Harmonic distortions).
      2. Measure harmonic distortions (with power analyzer):
         • IF THD-I (Total Harmonic Distortion Current) > 5% or individual harmonics (e.g. 3rd, 5th, 7th) > 3% (according to EN 50160) → Confirmation: Harmonic distortions caused by non-linear consumers.
           • Next step: Go to 6.6 (Harmonic Distortions).
      3. Measure contact resistances (after activation, with low-resistance tester):
         • IF resistance at terminal points or busbar connections > 100 µΩ (or > 20% above the reference value for a new connection) → Confirmation: High contact resistance.
           • Next step: Go to 6.1 (Loose connections).
      4. Insulation test (after activation, with insulation measuring device):
         • IF insulation resistance < 1 MΩ (for systems < 1000 V) or significantly below the manufacturer's specifications → Probable cause: Damaged insulation due to aging or thermal overload.
           • Next step: Go to 6.3 (overloading of cables) or 6.2 (overloading of components), depending on the component affected.

6. Error-cause matrix

This matrix presents the most common symptoms with their most likely causes, corresponding diagnostic tests, and expected results.

7. Root cause analysis for each error

7.1. Loose connections / high contact resistance

Why it happens: Loose electrical connections occur due to insufficient tightening torque during assembly, vibrations, thermal cycles (expansion and contraction of materials), or material fatigue of the spring contacts in terminals. Corrosion or oxidation on the contact surfaces also increases the contact resistance. According to Ohm's law and the power formula P = I² * R, any increased resistance results in local heat generation. This heating can further increase resistance (positive feedback), which speeds up the process and can lead to thermal runaway.

How to confirm: Thermography identifies hotspots with a temperature difference (ΔT) of over 15 K to the surrounding material under load. After the system has been activated, a low-resistance measurement (e.g. with a test current of 10 A) confirms the increased resistance; Values ​​above 100 µΩ for screwed connections or above the manufacturer's specifications are critical. A visual inspection may show discoloration (tarnishing from yellow to blue-black on copper) or even signs of melting. Checking the tightening torque with a calibrated torque wrench reveals insufficient tightening forces.

What damage it causes: The local overheating leads to degradation and destruction of the cable insulation, to contact fire at the terminal points, to component failures and can trigger an arc fault or fire. This leads to unplanned production downtime and high repair costs.

7.2. Overloading of components/cables

Why it happens: An overload occurs when the current flowing through a component or line exceeds the rated current specified by the manufacturer or the continuous current permitted by VDE 0298-4. The causes can be incorrectly dimensioned components, an unexpected increase in load (e.g. due to new machines), uncontrolled system expansions or inadequate protection (incorrect fuse selection). The increased current leads to excessive I²R losses and therefore increased thermal stress.

How to confirm: A current measurement with a true-RMS current clamp or power analyzer under real operating conditions shows whether the actual current exceeds the nominal values ​​or the permissible limits. The comparison with the respective data sheet of the component or the tables of VDE 0298-4 is critical.

What damage it causes: Permanent overload accelerates the aging of the insulation, significantly reduces the service life of the components and can lead to their premature failure. In extreme cases, thermal circuit breakers, insulation defects or fires can occur.

7.3. Inadequate cooling/ventilation

Why it happens: Control cabinets are designed for a certain maximum power loss and heat dissipation. Insufficient cooling occurs when filter mats are dirty, ventilation openings are blocked, fans are defective, or the ambient temperature where the cabinet is installed is above the permissible operating range. Improper placement of heat-emitting components in the cabinet can also lead to hotspots that are not dissipated effectively.

How to confirm: A visual inspection reveals dirty filters and blocked vents. The function of the fans can be assessed visually or acoustically. Measuring the interior temperature of the cabinet (e.g. with an infrared thermometer or thermologger) in comparison to the ambient temperature provides information about the efficiency of the cooling. Thermography of the entire cabinet shows generalized warming without individual, prominent hotspots.

What damage it causes: Inadequate cooling leads to generalized overheating of all internal components. This accelerates the aging of all electronic components, capacitors and insulation, reduces the MTBF (Mean Time Between Failures) and increases the general risk of failure of the system.

7.4. Harmonic distortions

Why it happens: Harmonic distortions are caused by non-linear consumers (e.g. frequency converters, switching power supplies, UPS systems, LED lighting) that do not draw sinusoidal currents from the network. These non-sinusoidal currents contain harmonics that overlap in the network and lead to additional current flows. The increased effective values ​​of these currents lead to additional losses (heat) in cables, transformers, capacitors and especially in neutral conductors, which can then be overloaded, even if the phase currents are within the nominal range.

How to confirm: A power analyzer is the primary tool for measuring THD-I (Total Harmonic Distortion current) and individual harmonics up to the 50th order. THD-I values ​​of over 5% or individual harmonics of over 3% of the fundamental (according to EN 50160) are a clear indicator. A current measurement on the neutral conductor with a true-RMS current clamp, which shows a higher current than the phase conductors, also confirms the presence of harmonics.

What damage it causes: Harmonic distortions lead to overheating of transformers, motors and neutral conductors, malfunction of protection and measuring devices, increased reactive and active losses, shortened life of capacitors and can lead to resonance problems that cause further damage.

7.5. unbalanced load

Why it happens: An unbalanced load occurs in a three-phase system when the three phases are loaded unevenly, i.e. the currents on the individual phases are different. This can arise from the distribution of single-phase consumers across the three phases if the load distribution is not optimal or has shifted over time due to changes in the system. An unbalanced load results in a current in the neutral wire, even without the presence of harmonics, and increases losses in the system.

How to confirm: A simultaneous measurement of the phase currents (L1, L2, L3) with a true-RMS current clamp or a network analyzer shows the inequality. A deviation of more than 10% between the phase currents is to be assessed as a significant unbalanced load. At the same time, measuring the neutral conductor current can provide information.

What damage it causes: Unbalanced loads lead to unnecessary losses in lines and transformers, increase the thermal load and can reduce the efficiency of the system. Motors can run less efficiently and develop additional vibrations. Under certain conditions, overloading of the neutral conductor can result.

8. Step-by-step fix procedure

8.1. Corrective measures for loose connections

1. Safety: UNLOCK THE SYSTEM ON ALL POLES AND SECURE IT AGAINST SWITCHING ON AGAIN (LOTO)! DETERMINE NO VOLTAGE! WEAR PPE! Ensure verifiable discharge of residual energy.
2. Identification: Accurately locate the hotspot that was identified thermographically.
3. Cleaning: Carefully clean contacts and terminal points mechanically. Remove oxidation layers or corrosion using fine emery cloth (P400 or finer) or special contact cleaners. Make sure no residue remains.
4. Resistance check: Use a low-resistance meter to check the contact resistance between the parts to be connected. The target value should be below 100 µΩ or correspond to the value of a comparable, intact connection.
5. Tightening: Tighten the connecting elements (screws, bolts) with a calibrated torque wrench according to the tightening torques specified by the component manufacturer (e.g. for an M8 screw in the range of 20-30 Nm, depending on the material pairing and clamp type). For spring-loaded terminals, check the correct insertion technique of the conductor.
6. Verification: After restarting the system, carry out another thermographic check under nominal load. The temperature difference of the repaired area to the surrounding material should ideally be < 5 K and the absolute temperature should be within the permissible limits.

8.2. Corrective measures for overloading of components/cables

1. Safety: Activate the affected circuit or system if necessary.
2. Analysis: Collect a detailed load profile of the affected components and lines using a network analyzer over a representative period of time in order to determine the actual utilization.
3. Replanning / resizing: If the rated or continuous current is permanently exceeded: redistribute loads to less loaded circuits, replace components with those with higher rated power or create additional feed points or circuits. Cable cross-sections must be dimensioned accordingly in accordance with VDE 0298-4 and EN 61439-1, Table 1 (permissible current carrying capacity).
4. Protective device: Circuit breakers or circuit breakers must be correctly matched to the rated current of the line and the consumer. If necessary, adjust response values ​​or replace components.
5. Verification: Carry out current measurements again after the adjustments. The measured operating current must remain permanently below the rated/continuous current of the respective component or cable.

8.3. Corrective measures for inadequate cooling/ventilation

1. Security: If necessary, unlock the system for work on the cooling system.
2. Cleaning: Clean or replace filter mats. This should be done according to the maintenance schedule, typically quarterly. Clear ventilation openings in the cabinet housing from dust, dirt or other obstructions.
3. Testing and replacement: Check the function of the fans (speed, noise level). If there is a defect or insufficient performance, replace the fan with a model with the same or higher performance (m³/h air flow) and appropriate protection class (IP).
4. Optimization: If temperatures continue to be too high, consider installing additional fans, roof fans, heat exchangers or air conditioning units. The placement of heat-generating components in the cabinet can be improved by optimizing the layout so as not to impede air circulation.
5. Verification: Continuous temperature measurement inside the cabinet (e.g. with temperature sensor with alarm function). The temperature should remain below the allowable operating range of the most critical component.

8.4. Corrective measures for harmonic distortions

1. Security: If necessary, activate the system for installation work.
2. Analysis: Use the power analyzer to identify the exact sources and magnitude of the harmonics. Prioritize actions based on the most dominant harmonics.
3. Filtering: Installation of passive or active harmonic filters. Active filters (e.g. from Schaffner, Comsys) are more flexible and effective with varying loads and can compensate for several harmonics. Passive filters are suitable for constant loads with specific harmonics. Install the filter at the feed point of the causing load or centrally in the distribution.
4. Chokes: Use of line chokes (commutation chokes) in front of frequency converters and other non-linear consumers significantly reduces harmonic emissions.
5. Consumers: Where possible, use low-harmonic consumers or frequency converters with AFE technology (Active Front End).
6. Verification: Measurement again with the network analyzer after installing the filters. The THD-I should be below 5% (according to EN 50160) and the individual harmonics should be below the permissible limits.

8.5. Corrective measures for unbalanced load

1. Security: If necessary, activate the system for switching.
2. Analysis: Carrying out a detailed load analysis with phase current measurement.
3. Load distribution: Distribute loads as evenly as possible across the three phases. This often requires switching single-phase consumers from a heavily loaded phase to a less loaded phase. A balanced phase current ratio should be aimed for (< 10% deviation).
4. Neutral conductor sizing: Ensure that the neutral conductor cross-section is correctly dimensioned. If harmonics are present, the neutral conductor may need to have the same or even a larger cross-section than the phase conductors because the third and multiples of the third harmonics add up in the neutral conductor.
5. Verification: After redistribution, carry out phase current measurements again to confirm successful load balancing.

9. Preventive measures

Implementing a proactive maintenance strategy is crucial to avoid overheating problems and ensure the operational safety and lifespan of electrical distribution systems.

10. Spare Parts & Components

In order to be able to react quickly in the event of an error, proactive storage of critical spare parts is crucial. The following components are typically affected by overheating or require repair.

You can find further spare parts and components for your electrical distribution in our comprehensive e-catalogue:

UNITEC-D e-catalogue

11. References

• DIN EN 61439-1: Low-voltage switchgear assemblies - Part 1: General specifications.
• VDE 0105-100: Operation of electrical systems - Part 100: General provisions.
• VDE 0100: Construction of low-voltage systems (various parts).
• EN 50160: Characteristics of voltage in public electricity supply networks.
• VDE 0298-4: Use of cables and insulated lines for power systems - Part 4: Values ​​for the current carrying capacity of cables and lines.
• VDI 3822: Technical diagnosis – general principles and procedures.
• IEC 61482-1-2: Protection against the thermal hazards of an arc flash - Method 1-2: Testing the material for clothing using a directed arc flash (box test).
• Manufacturer documentation: For specific components and systems (e.g. Siemens, Schneider Electric, Eaton, ABB).
• UNITEC maintenance guide: “Thermography in maintenance – basics and application”.
• UNITEC maintenance guide: “Basics of power quality analysis – detecting and eliminating network faults”.