When the cross-sectional area of the wire does not match the operating current, the resistance thermal effect will cause continuous temperature rise. For instance, a certain agricultural irrigation pump (with a power of 5.5kW) wrongly selected 2.5mm² wires (with a rated current-carrying capacity of only 21A), and the actual working current reached 25A. The temperature of the conductor rose to 98℃ under an ambient condition of 30℃, exceeding the heat resistance limit of the PVC insulation layer (70℃) by approximately 40%. In 2023, a rice cooperative in Jiangsu Province suffered a fire due to this, resulting in direct losses of 480,000 yuan. The NFPA investigation report indicates that overloading of the lines was the main cause.
Oxidation and corrosion of the connection terminals cause an abnormal increase in contact resistance. According to the IEEE Std 815 test standard, the resistance value of the rusted contact points can reach 50 times that of the clean state. After the connection terminal of the ship Fuel Pump operated in the salt spray environment for 800 hours, the contact resistance soared from 0.5mΩ to 25mΩ, and the heat generation power increased from 72mW to 3.6W at the same 12A current (a 50-fold increase). This phenomenon led to an accident where the fuel supply of a certain cargo ship was interrupted, and the Marine diesel engine stopped running, causing a loss of approximately 120,000 US dollars at the port.
Flaws in the heat dissipation design will cause heat to accumulate continuously. The cooling pump unit of a certain data center adopts an IP55 sealed cabinet, but is not equipped with a forced air cooling system. The measured data shows that the air flow rate in the cable trench is only 0.2m/s. Under the full-load current of 80A, the temperature of the wire harness rises by 8℃ every 15 minutes and reaches 127℃ after 3 hours, triggering overheat protection. After comparison and improvement, four 120mm axial flow fans (with an air volume of 85CFM) were installed, and the temperature rise was stabilized within +22℃ of the ambient temperature.
The additional losses caused by voltage fluctuations cannot be ignored either. There is a ±15% deviation in the power supply voltage of the deep well pump in a certain mining area (the nominal voltage is 380V and the actual minimum is 323V), which causes the working current of the motor with a rated power of 37kW to increase from 70A to 82A. According to Joule’s Law Q=0.24I²Rt, the heat generation per hour rose from the designed value of 425kcal to 580kcal, which is equivalent to an increase of 17% in heat load per second. The continuous operation led to the deformation of the plastic shell of the junction box.
High-frequency harmonic pollution (THD > 12%) can significantly intensify the skin effect. The measured 5th harmonic content of the variable frequency drive pump set in the painting workshop of a certain automobile manufacturing plant reached 38%. Detection using a FLIR thermal imager revealed that under the influence of 400Hz harmonics, the surface temperature difference of the 50mm² power cable reached 14.2℃ (the maximum temperature for the core was 86℃, while that for the skin was only 71.8℃). This temperature gradient accelerated the aging process of the insulation layer. ISO 13849 certification requires that output reactors must be installed in such scenarios.
The synergistic effect of environmental temperature and humidity can form a vicious circle. The insulation resistance of the hydrochloric acid circulation pump in a certain chemical plant in South China dropped from 500MΩ to 0.5MΩ due to the infiltration of condensate water in the environment of 35℃/85%RH during summer. The leakage current reaches 15mA (the safety limit is 5mA). According to the thermal power formula P=I²R, the abnormal current causes the connection point temperature to increase by 32℃ compared with the design value. The subsequent IP68-level sealing transformation carried out by the factory extended the service life of the equipment by 3.8 years.
Local overheating caused by the fatigue fracture of conductors is particularly dangerous. The Fuel Pump for water injection in a certain oilfield bent the cable more than 10⁷ times due to mechanical vibration (exceeding the limit of IEC 60245 standard), and microscopic observation showed that the copper wire breakage rate was approximately 27%. Under a load of 60A, the effective cross-sectional area decreases and the current density rises to 9.6A/mm² (the standard limit is 6A/mm²), and the peak temperature at the break point reaches 216℃, approaching the 20% danger threshold of the melting point of copper wire (1083℃).