Causes and preventive measures of fire caused by t

Causes and preventive measures of fire caused by grounding

electrical fire is on the rise at present, and there are many fires caused by grounding fault. In order to prevent fire caused by grounding fault, the causes are analyzed here, and preventive measures are put forward

I. the cause of fire caused by ground fault

ground fault is phase line and electrical. Let's see how to pull the black knob sample when buying a pressure testing machine in order to buy a high-quality and reasonably priced pressure testing machine. The exposed conductive parts of the device (including the metal shell of electrical equipment, wiring duct slot and framework, etc.) The short circuit between the external conductive parts (including metal water, heating, gas, air conditioning pipes and metal structures of buildings) and the earth is shown in Figure 1. This kind of fault is different from the single-phase short-circuit fault between phase line and neutral line, and the phase to phase short-circuit between phase lines

compared with general short circuit, ground fault has greater danger and complexity in case of fire. Generally, the fire caused by short-circuit is mainly caused by the high temperature on the short-circuit current, while the ground fault has the following three reasons to cause the fire, which is more dangerous, and its prevention work is also very complex

1. Fire caused by grounding fault current

generally, the short-circuit current path is the metal conductor of the line. The short circuit metal circuit current seen by people is large, and the short-circuit point metal is often fused and welded, and its impedance can be ignored. This short circuit is easy to be quickly cut off by overcurrent protection appliances (fuses, low-voltage circuit breakers) without causing fire. However, the current path of grounding fault includes equipment shell, wiring duct slot and multiple connection terminals of grounding circuit. TT system (grounding system) also takes the earth as the path. The grounding resistance of the earth is large, and the resistance of the connecting terminals of PE and pen lines (grounding wires) is often large due to various reasons. Therefore, the grounding fault current is smaller than the general short-circuit current, which often cannot make the overcurrent protection appliances cut off the fault in time, and there are many fault points without fusion welding, resulting in arcs and sparks. The local high temperature of 0.5A current arc and electric spark can burn combustible materials and cause a fire. For example, in March, 1994, the warehouse of a factory in Beijing was burned. The reason is that the electrical circuit of the warehouse is close to the heating inlet pipe in the incoming trench, and it has been roasted for many years. The insulation level has decreased, resulting in grounding fault arcing and fire. Workers in the factory reported that the light in the workshop flickered for a period of time before the fire, indicating that the fault was not cut off, and the arc lasted for a period of time. After the circuit was cut off, the fire had become a potential

in addition, in the past, we did not pay attention to the thermal stability of PE and pen lines in TN system under fault conditions, and often chose too small cross-section. When the larger ground fault current in TN system passes, it is easy to cause high-temperature line fire

2. The fire is caused by the loose connection of PE and pen terminals.

the PE line of equipment grounding usually does not pass the load current, and only passes the fault current in case of grounding fault. If the connection is loose and the contact resistance increases due to vibration, corrosion and other reasons, it is usually not easy to detect. Once a ground fault occurs, when the ground fault current needs to return to the power supply through the PE line, the large contact resistance of the PE line limits the fault current, so that the protective appliance cannot act in time, but the high temperature or arc, electric spark generated at the connection terminal due to the large contact resistance can lead to the occurrence of fire

in TN-C system, pen line usually passes through three-phase unbalanced current, but in some enterprises with three-phase balanced load, such as machinery and textile, due to the small three-phase unbalanced current, the connection of pen line terminals is not tight, and the hidden dangers of measuring the physical properties of various materials according to international standards or industry standards are not easy to find. When the large fault current passes, it can also lead to fire

in March, 1994, a row of shops in Xibianmen, Beijing, caught fire. At the beginning, a container truck broke two single-phase wires at the end of the store, and the wires burst out electric sparks to the ground. A few minutes later, the store caught fire. After investigation, the fire was on the back wall of the store, where a flexible cable was divided into three single-phase circuits, one of which was the broken wire. Due to years of exposure to the sun and rain, the wire has been similar to a bare wire, and the connection between the cable core and the wire is poor, causing a fire when passing through the grounding fault current

3. Fire caused by fault voltage

these electrical accidents often occur: the voltage of equipment metal shell or n-line to ground is dozens of generations; The hand-held equipment itself was not damaged, but the user was electrocuted to death; The power supply has been cut off, but when carrying out maintenance, the enclosure or n-wire has a voltage to ignite, resulting in fire or explosion. This kind of accident caused by unknown voltage is mostly caused by another grounding fault. The fault voltage conducted everywhere after a ground fault occurs is a dangerous ignition source, which causes a fire through electric sparks and arcs to the ground. The breakdown of 10mm air gap requires 30KV voltage. Once the conductors with different potentials touch and pull up the arc, the voltage to maintain the arc in the same gap only needs 20V. At this time, the local temperature of the arc of 2A current can exceed 2000 ℃