Applies to thyristor valves with metal oxide surge arresters directly connected between the valve terminals, for use in a line commuted commutated converter for high voltage d.c. power transmission or as part of a back-to-back link. Retricted to electrical type and production tests. Tests are based on air insulated valves. This consolidated version consists of the first edition (1998) and its amendment 1 (2003). Therefore, no need to order amendment in addition to this publication.