The On-Site Guide is an essential guide to BS 7671. It incorporates the extensive changes to BS 7671, making this a vital guide for keeping up to date. It enables the competent installer to deal with installations (up to 100 A, 3-phase) providing essential information in a convenient, easy-to-use format. This publication has been further updated to include all four amendments to the 18th Edition and aligns with BS 7671:2018+A4:2026 The IET Wiring Regulations. BS 7671:2018+A4:2026 incorporates changes from all four amendments to BS 7671:2018 and the On-Site Guide includes new topics, such as bidirectional and unidirectional protective devices; Stationary Secondary Batteries; amended rational for maximum Zs values for protective devices; guidance on selection of RCDs and guidance on prospective fault current for protective devices.