The capacity of rural distribution network is small, and the load distribution is dispersed. But on the whole, the low voltage power grid can be divided into three layers: distribution transformer station, branch distribution box and user end.

Three-level protection is a form of combination of direct contact protection and indirect contact protection. According to the rural low voltage power grid, different types of protectors are installed at the first, the middle and the end of the power supply line, and the three layers are respectively equipped with different protectors.

For the protection of direct contact at the end of the line, the general type (no time delay) protector is installed with an action current of 30MA. The first and the middle section shall be protected by indirect contact, and a protective device shall be installed for coordination of operation characteristics. The first end as the first level protection, value rated residual current protection, should use adjustable grade, the rules of the maximum value of the power grid: small leakage current is 75mA/200mA, the larger the 100mA/300mA leakage current, the perfect protection system can be increased to 500mA. level as a shunt protection value, rated residual current protector and choose between superior and subordinate protector rated residual operating current value.

Select off action time, a  residual current circuit breakers should be used to delay the action type protector, the breaking time should be increased by at least 0.2s. than the protector of the next level of breaking time

This network of three protected with each other in the operating current and operating time, terminal electrical equipment failure, third action level protection; network fault occurs at the end of action, intermediate protection; network middle fault, total protection, in order to reduce the accident stop power range.

In short, the residual current circuit breakers in rural low voltage power grid is a firewall that prevents leakage of electric equipment and causes casualties in the network". In addition, because the protector can isolate the fault equipment in time, it becomes a technical measure to prevent the electrical fire and equipment damage caused by leakage. The safety of the protector is related to the safety of the human body and the equipment. In the daily management of the low voltage power grid, the maintenance of the three stage protector is a very important task. The operation rate and reliability of the residual current circuit breakers are directly related to the safety status and management level of the low voltage power grid.