This guide, covering three-phase ac systems from 1 kV to 800 kV, provides recommended electrical operating, safety clearances, and insulation levels in air-insulated electric supply substations; addresses insulation coordination procedures; provides design procedures for the selection and coordination of the insulation levels within the station as they relate to substation clearances; and addresses how reduced clearances in highvoltage ac substations will allow for compact bus arrangements and substation voltage uprating applications. This guide addresses insulation coordination procedures, including the choice of insulation levels and arrester specification, in limited detail and only as relevant to clearance requirements. Detailed and expanded coverage of insulation coordination procedures is provided in other ANSI and IEEE guides and standards (see Clause 2). This guide focuses on open-air bus assemblies and configurations and excludes apparatus clearances (i.e., bushing clearances for transformers, and breakers). Detailed coverage of apparatus clearances is provided in other applicable guides and standards.