Xi'an Shendian is the earliest manufacturer to develop metal-oxide surge arrester with series gap, which was granted national patent (Patent No.: 03218812.9). The arrester is used in urban mass transit system to protect the insulators of overhead contact system from the damage of over-voltage and ensure the safe operation of the power supply system. The arrester is composed of arrester body, discharge gap and metal fixture, and installed in parallel with the insulator of overhead contact system. In normal operation, due to isolation of the discharge gap, no electric current flows through the surge arrester body and thus the device will not age. When the lines are damaged by over-voltage such as lightning strike, the gap discharges and the arrester body acts reliably to limit the amplitude of over-voltage within the range that the insulator can bear to ensure the normal operation of the system. The arrester has passed the comprehensive type test of the National Quality Supervision and Inspection Center of Electrical Products, exclusively passed the short-circuit current test (that is explosion-proof current test), and passed the test for 2 million times of mechanical vibrations in the Product Quality Supervision and Inspection Center of the Ministry of Railways according to General Technical Specification of Fittings for Overhead Contact System in Electrification Railway (TB/T 2073) and Test Methods of Fittings for Overhead Contact System in Electrification Railway (TB/T 2074). The tests proved that the product has reasonable structure and reliable anti-vibration performance. In order to record the action of the arrester, Xi'an Shendian has also developed metal-oxide surge arrester with series gap and counting function (Patent No: 201720466627.5), which does not need to increase the residual voltage of the arrester. It is reliable in functionality, free of external power supply, and very convenient for installation. Featured by reliable protection performance and stable operation, the product has been used in the rail transit systems in Shenzhen, Beijing, Nanjing, Changsha, Guangzhou, Nanning and Zhengzhou and other major cities for many years.