IEC 60099-4 is the performance standard for gapless metal-oxide surge arresters used in alternating-current power systems. It applies to arresters from distribution voltage classes up to the highest transmission voltages, whether they are porcelain-housed, polymer-housed or gas-insulated, and it defines the quantities by which a surge arrester is specified so that purchasers and manufacturers use the same language. The principal characteristics treated are the rated voltage and continuous operating voltage, the nominal discharge current, the residual voltage at specified impulse currents, the rated short-circuit withstand, and the energy absorption capability expressed through long-duration and operating-duty tests. Type tests, routine tests and acceptance tests are each listed and their procedures described, including the high-current impulse test at 4/10 microseconds, the switching impulse test, the steep current impulse test and the artificially polluted withstand test. Guidance is given on the selection of a rated voltage from the system earthing arrangement and on the safety margins to be maintained.