5.4　Service Reliability

5.4.1　Urban HV distribution networks shall be designed to meet requirements of N-1 safety criterion.No component（except for busbar）in an HV distribution network shall affect the normal availability of power in case of fault or outage for maintenance.

5.4.2　Urban MV cable lines shall be designed to meet requirements of N-1 safety criterion，so should MV overhead lines.

5.4.3　Urban LV distribution networks may be designed to allow for load loss when LV lines fail.

5.4.4　The service reliability indicators for urban MV customers should not be inferior to those specified in Table 5.4.4.

Table 5.4.4　Service reliability indicators

Note:1　RS-3 refers to service reliability without considering power outages caused by brownout due to insufficient capacity.
2　Industrial parks can be treated as suburban zones at their early time and ordinary zones when they are developed.

5.4.5　MV customers of different capacities and requirements for reliability shall be powered in different ways.When customers suffer from outage due to network fault，the acceptable loss of capacity and power resumption target shall meet the following requirements:

1　Dual-circuit powered customers shall not lose load when one circuit is lost；

2　Three-circuit powered customers shall not lose load when one circuit is lost，and shall maintain at least 50% load when two circuits are lost；

3　The time it takes for multi-circuit powered customers to resume power supply is the time to handle fault on a circuit when all circuits are out of service；

4　For customers in an open loop network，when the network fails，the power resumption time for healthy portions shall be the time required for switching operation.