The master socket contain a surge arrestor and an R/C circuit filter these are used by your service provider to get a fixed value resistance when they run line checks. Standard BT type socket with screw type terminals. Made from high quality premium grade material. Complete with integral gasket. Compliant to all relevant British safety standards. Minimum mounting depth: 25mm. Dimensions: 86mm x 86mm. Supplied in retail blister with full fitting instructions. 15 year guarantee.