Dangerous temperatures are caused by overheating of electrical equipment, which is mainly caused by the heat generated by electric current. Electrical equipment always emits heat when it is operating. Only when the normal operating conditions of electrical equipment are destroyed, its heat generation increases and the humidity increases, which can cause a fire. ?
The abnormal operation of electrical equipment that causes excessive heating can generally be summarized in the following situations. ?
1. Short circuit?
When a short circuit occurs, the current in the line increases to several times or even dozens of times of normal, and the heat generated can be proportional to the square of the current. The temperature rises sharply, greatly exceeding the allowable range. If the temperature reaches the auto-ignition point of self-igniting materials or the ignition point of combustible materials, it will cause combustion and lead to fire. Situations that are prone to short circuits include:
(1) The insulation of electrical equipment ages and deteriorates, is mechanically damaged, and is damaged by high temperature, moisture or corrosion. ?
(2) The insulation breakdown is caused by voltages such as lightning strikes. ?
(3) Due to wiring and operation errors during installation and maintenance work. ?
(4) Due to lax management or untimely maintenance, dirt accumulates, small animals get in, etc. ?
In addition, the lightning discharge current is extremely large, much larger than the short-circuit current, which may cause fire and explosion. ?
2. Overload?
Overload can also cause heating of electrical equipment. The causes of overload are generally as follows:?
(1) Design , The selected lines or equipment are unreasonable, resulting in overheating under the rated load;
(2) Unreasonable use, such as overload operation, continuous use time exceeding the design value of the line or equipment, resulting in overload;?
(3) Equipment fault operation causes equipment and circuit overload, such as single-phase operation of three-phase motors and asymmetric operation of three-phase transformers, which can cause overheating. ?
3. Poor contact?
(1) Poor connection of non-detachable joints, poor welding, or impurities mixed in the joints will increase the contact resistance and cause the joints to overheat. ?
(2) Detachable joints that are not tightly connected or loose due to vibration can also cause overheating; ?
(3) Movable contacts, such as those of knife switches , contacts of contactors, contacts of plug-in fuses and other movable contacts, if there is insufficient contact pressure or the contact surface is rough and uneven, it will cause the contacts to overheat;?
(4) The brush There is not enough pressure at the sliding contact or the contact surface is dirty or not smooth, which can also lead to overheating;?
(5) For copper and aluminum joints, due to their different properties, the joints are susceptible to corrosion due to electrolysis. This can lead to overheating. ?
4. Poor heat dissipation?
All kinds of electrical equipment are designed and installed with certain heat dissipation or ventilation measures in mind. If the measures are damaged, the equipment may overheat. ?
In addition to the above points, electrical equipment such as electric lamps and electric furnaces that directly use the heat energy generated by current to operate have relatively high operating temperatures. Improper installation and use may cause fires.