IEC 62040-1:2008 applies to uninterruptible power systems (UPS) with an electrical energy storage device in the d.c. link. It is used with IEC 60950-1, which is referred to in this standard as "RD" (reference document). It is applicable to UPS which are movable, stationary, fixed or for building-in, for use in low-voltage distribution systems and intended to be installed in any operator accessible area or in restricted access locations as applicable. It specifies requirements to ensure safety for the operator and layman who may come into contact with the equipment and, where specifically stated, for the service person. It cancels and replaces the first edition of IEC 62040-1-1, published in 2004 and IEC 62040-1-2, published in 2004 and constitutes a technical revision. This standard merges all requirements of previous IEC 62040-1-1 and IEC 62040-1-2, with the addition of the following: - update of normative references including IEC 60950-1 as Reference Document (RD); - harmonization and alignment with current world recognized best practices; - enhancement of backfeed protection, definition of ground-fault, revision of temperature rise tables and of hydrogen concentration in battery compartments. The contents of the corrigendum of September 2008 have been included in this copy.