GB 311 specifies the insulation coordination requirements for high-voltage transmission and transformation equipment in China. Insulation coordination is the process of selecting the dielectric strength of equipment so that it withstands the voltages that occur in service, both the continuous power frequency voltage and the transient overvoltages produced by lightning and by switching operations, while retaining an acceptable margin and an economic design. The standard establishes the relationship between the highest voltage for equipment, the rated short-duration power frequency withstand voltage, the rated lightning impulse withstand voltage and the rated switching impulse withstand voltage, and it gives the standardised values for each system voltage level that equipment must be designed to meet. It also defines the representative overvoltages used in the studies, the required protective levels of surge arresters and the altitude correction that applies when equipment is installed above the reference elevation. The document is applied by manufacturers and utilities when specifying transformers, switchgear, bushings and insulators.