Insulators corona rings installation notes from a electric engineer，he wrote as following:

the size of the Grading "corona" rings have tended to get bigger. In our case we had parrallel insualtors on all of our deadends for 2-conductor bundle line. The minimum size I approved were 10 inch rings. The bid was awarded to a supplier that included 11 inch rings. I also now ask for 3-D analysis of the voltage stress by the manufacturer for the designs we are using.

I also found that at 345kV and above it is now recommended that the structure end of a NCI now include a grading ring.

Corona grading ring should be designed to reduce the critical dielectric voltage gradient (typ. 20 to 30 kVrms/cm) to prevent corona effect, internal discharge and reduce E-field in live parts and fitting that cause radio/ TV interference (RIV), audio noise and losses. Corona ring could also help to smooth the voltage profile distributing the voltage more uniform along the insulator preventing concentration of over stresses.

For porcelain post insulators, China manufacturer recommend one corona ring and for 500 kV and above two rings. However, for composite insulator the corona ring is recommended for 220/230 kV. Most equipment manufacturer provide corona ring base on testing such surge arrester, switches, CT’s/PTs, etc.

For bus, dead-end assembly and connectors, the enclosed guide could be helpful. The selection of the corona ring should consider the performance not only in fair weather, but also in raining fogging, snowing and icing conditions.Consult the EPRI’s Transmission Line Handbook, 345 kV and Above, IEC Pub.61109 Amendment 1 and check the enclose sites for additional reference.

composite insulators used at transmission voltages have to be equipped with corona rings to reduce the risk of severe erosion to their polymeric housings. Yet, there are still cases where such insulators are either installed without rings or where the rings become detached and fall off due to factors such as conductor vibration or improper installation.