First, the principle and function of protective grounding
Protective grounding is to prevent the metal shell of electrical device, the frame of power distribution device and line tower from being charged and endangering personal and equipment safety. The so-called protective grounding is a protective wiring method that reliably connects the metal parts of electrical appliances that are not normally charged but may be charged after the insulation material is damaged or otherwise (that is, the metal structure parts insulated from the charged parts) with the wires and grounding bodies.
Grounding and zero connection
Second, the principle and function of protection to zero
The metal shell of electrical equipment is connected to the neutral line of the power grid to protect personal safety. In the low-voltage neutral network, if the metal shell of electrical equipment is charged due to insulation damage or unexpected circumstances, a single-phase short circuit will be formed between the phase line and the neutral line, then the protective device on the line will quickly cut off the power supply, so that the metal part of the equipment will not have dangerous voltage for a long time, ensuring personal safety.
Third, the reason why protective grounding and protective grounding cannot be mixed.
Protective grounding and protective grounding are measures to ensure personal safety, that is, the shell of live equipment is reliably grounded or grounded, and when it fails, the protective equipment is quickly started to cut off the power supply to ensure personal safety. However, in the same transformer system, the two can never be used at the same time (on the same equipment). What is the principle?
Because when the protective grounding equipment shell is charged, if its grounding resistance R is large and the fault current Id is not enough to make the protection device act, the voltage U0 = ID * rd will always exist on the neutral line due to the existence of the working resistance RD. At this time, there is a dangerous voltage U0 on the protective grounding equipment shell for a long time, which endangers personal safety.