For this guide@ a transmission line is any overhead line with a phase-to-phase voltage exceeding 69 kV and an average conductor height of more than 10 m. The transmission line is usually shielded by one or more overhead ground wires (OHGWs)@ at least for a short distance from a substation. While reference is primarily made to ac transmission characteristics@ the guide is also relevant for high-voltage direct-current (HVDC) overhead lines.The guide is written for the transmission-line designer. When given the problem of designing or redesigning a transmission line@ the designer should consider certain limiting factors such as the voltage level@ the beginning and ending points for the transmission line@ and the desired ampacity of the line. Sometimes the exact route@ and the type of conductor and structure have already been determined. Usually the designer may choose structural details@ the geometry of the structure@ the structure height@ the exact placement of the OHGWs@ the amount and type of insulation@ the type of grounding@ and other design features of a line. This guide is written to show the designer which choices will improve or degrade lightning performance. Sections of the guide discuss the effect of routing@ structure type@ insulation@ shielding@ and grounding. An additional section discusses several special methods@ which may be used to improve lightning performance. Finally@ in Annex B@ a listing and description of the FLASH program is presented.The line designer should be aware that lightning performance is not of primary importance in the economics of line designing. Other factors@ such as line length@ right-of-way costs@ construction costs@ material costs@ and losses affect the economics of a line design much more than lightning performance. The designer should always balance the costs of higher insulation levels@ improved grounding@ better shielding@ or line relocation against the benefits of improved reliability.