The latest (17th) edition of the IEE Wiring Regulations (BS7671:2008), effective from 1 July 2008, now requires that Static UPS must be capable of operating circuit protective devices when in battery (emergency) mode. This means that it is no longer acceptable for system designers to rely on the argument that for fault clearing, the static UPS bypass is generally available (normal mode) and that battery mode is a rare operating condition where special circumstances apply and circuit discrimination can be treated differently.