Electrical equipment always generates heat during operation, but for correctly designed, constructed, and operating electrical equipment, the maximum temperature and the difference between it and the surrounding environment temperature (i.e. the maximum temperature rise) will not exceed a certain allowable range. For example, the maximum temperature of bare wires and plastic insulated wires generally does not exceed 70 degree ; The maximum temperature of rubber insulated wire shall not exceed 65 degree ; The upper oil temperature of the transformer shall not exceed 85 degree ; The temperature of the outer shell of power capacitors shall not exceed 65 degree , etc. This means that normal heating of electrical equipment is allowed. But when the normal operation of electrical equipment is disrupted, the heat generation increases, the temperature rises, and under certain conditions, it may cause a fire. Abnormal operation that causes overheating of electrical equipment generally includes the following situations:

1. Short circuit: When a short circuit occurs, the current in the circuit increases several times or even tens of times that of normal, and the heat generated is directly proportional to the square of the current, causing the temperature to rise sharply, greatly exceeding the allowable range. If the temperature reaches the self ignition point of the combustible material, it will cause combustion, leading to a fire.

2. Overload: Overload can cause electrical equipment to heat up. There are generally two reasons for overload: firstly, the unreasonable selection of circuits or equipment during design, resulting in overheating under rated load. The second is unreasonable use, that is, the load of the circuit or equipment exceeds the rated value, or the continuous use time is too long, exceeding the design capacity of the circuit or equipment, resulting in overheating.

3. Poor contact: The contact part is the weak link in the circuit and a key area where overheating occurs.

Non detachable joints are called loose connections, poor welding, or oxidation of the joints, which can increase contact resistance and cause the joints to overheat; Loose detachable joints or loose connections due to vibration can also cause the joints to heat up; Active contacts: such as the contacts of knife switches, contactors, plug-in fuses, pins, etc. If there is not enough contact pressure or the contact surface is rough and uneven, it can cause the contacts to overheat; For copper aluminum joints, due to the different electrical properties of copper and aluminum, corrosion occurs at the joint due to electrolytic oxidation, resulting in overheating of the joint.