standard nominal supply voltage in domestic single-phase 50 Hz installations in the UK is still 240 V AC (RMS), but since 1 January 1995 (Electricity Supply Regulations, SI 1994, No. 3021) this has an asymmetric voltage tolerance of (216.2–253.0 V), which covers the same voltage range as continental 220 V supplies to the new unified 230 V standard. This was supposed to be widened to 230 V ±10% (207–253 V), but the time of this change has been put back repeatedly and as of December 2012 there is no definitive date. The old standard was 240 V ±6% (225.6–254.4 V), which is mostly contained within the new range, and so in practice suppliers have had no reason to change voltages. Note that 240 V AC RMS means a momentary peak voltage of plus or minus 339 volts (see this explanation), occurring 50 (either positive or negative) or 100 (both, opposing) times per second. At the tolerances mentioned above, the peak voltage could be (momentarily) larger still. Three-phase supply Three phase power is usually supplied as needed, for commercial and industrial premises. While three phase loads take balanced power from the three phases, any single phase loads are distributed to ensure equal loading of the three phases. Each row of breakers in the distribution board is fed from a different phase (L1, L2, and L3), to allow 3-pole common-trip breakers to have one pole on each phase. Isolating devices Single-pole switches are most commonly used to control circuits. These switches isolate only the conductor feeding the load and are used for lighting and other smaller loads. For larger loads like air conditioners, cookers, water heaters and other fixed appliances a double-pole switch is used, which isolates also the neutral, for more safety. A three-pole isolator or circuit breaker is used for three-phase loads, for devices with both permanent and switched supplies (such as bathroom extractor fans) and also at the distribution board to isolate all the phases as well as the neutral. Outlets and accessories Many accessories for electrical installations (e.g., wall sockets, switches) sold in the UK are designed to fit into the mounting boxes defined in BS 4662:2006—Boxes for flush mounting of electrical accessories—Requirements, test methods and dimensions, with an 86 mm × 86 mm square face plate that is fixed to the rest of the enclosure by two M3.5 screws (typ. 25 mm or 40 mm long) located