IEC Guide 111 is a horizontal publication which gives guidance for the harmonization of product and system standards within substations where the highest voltage for equipment is higher than 1 kV. It primarily addresses conventional a.c. equipment which is found in highvoltage (HV) substations in most cases. This Guide contains recommendations for common specifications for all HV substation product standards, each of which is augmented by the technical background specific to each TC, which naturally retains freedom in its technical choices. Where HV power electronic equipment is part of an HV substation, for example, HVDC or SVC,economics and technology dictate a deviation from common clauses as standardized for a.c. equipment. However, when developing product standards for HV power electronic equipment for use in HV substations, the common recommendations of this Guide should be adopted as far as possible. Clearly, any specification in this guide which is inapplicable to a certain type of product,because of that product's inherent characteristics, for example, should not be taken into account when writing standards for that type of product.