When the metal shell of electrical equipment is charged (the phase wire collides with the shell or the insulation of the equipment is damaged and leaks), the risk of electric shock can be greatly reduced because of grounding protection. However, the low-voltage circuit breaker (automatic switch) may not trip, resulting in the leakage equipment shell voltage higher than safe voltage, which is a dangerous voltage. When the leakage current is small, even if there is a fuse, it will not necessarily blow, and it needs leakage protector to protect it, which makes TT system difficult to popularize. ? TT system grounding device consumes a lot of steel, which is difficult to recover, time-consuming and laborious. IT system: When the power supply distance is far away, the distributed capacitance of the power supply line to the ground cannot be ignored. When the load is short-circuited or the equipment shell is electrified, the leakage current will form a loop through the earth, and the protection equipment may not necessarily act, which is dangerous.

1, for some reason, although the danger can be reduced, it is still dangerous because of the small leakage current (leakage current I=220V/Rpe+Re=27.5A, in which the grounding resistances of pe and N lines are 4 Ω respectively).

2. Leakage current cannot guarantee fusing Fu; High sensitivity leakage protector must be used for protection;

3. Laying PE equipment is exquisite and expensive. IT system, as mentioned above, means that every time the relative insulation of three-phase power supply decreases, the complex impedance "Z" of three-phase power supply may form a loop with human body. In this case, the leakage current through the human body is not enough to turn off the circuit breaker, but it greatly exceeds the current that the human body can bear. So there is danger. The main reason is that the fault voltage will not spread, the protection grounding point is far away from the equipment, and the disconnection is easy to find. Disadvantages: high cost; When the shell collides, the shell potential is higher than safe voltage, which is unsafe.