This part of IEC 60700 applies to thyristor valves with metal oxide surge arresters directly connected between the valve terminals@ for use in a line commutated converter for high voltage d.c. power transmission or as part of a back-to-back link. It is restricted to electrical type and production tests. The tests specified in this standard are based on air insulated valves. For other types of valves@ the test requirements and acceptance criteria can be agreed.