IEC 61557-8:2014 specifies the requirements for insulation monitoring devices (IMD) which permanently monitor the insulation resistance RF to earth of unearthed a.c. IT systems, of a.c. IT systems with galvanically connected d.c. circuits having nominal voltages up to 1 000 V a.c., as well as of unearthed d.c. IT systems with voltages up to 1 500 V d.c. independent from the method of measuring. IT systems are described in IEC 60364-4-41 amongst other literature. Additional data for the selection of devices in other standards should be noted. IMDs according to this part of IEC 61557 can also be used for de-energized TT, TN and IT systems or appliances. This third edition cancels and replaces the second edition published in 2007. This edition constitutes a technical revision which includes the following significant technical changes with respect to the previous edition:- terms and definitions have been complemented;- abbreviations are listed and explained;- requirements have been revised;- mandatory and optional functions and their terminology have been adapted from IEC 61557-15;- mechanical requirements have been added;- information on operating instructions has been added;- type tests and routine tests have been complemented;- an Annex C: 'Insulation monitoring devices for photovoltaic systems (PV-IMD)' has been added; and- an Annex D: 'Insulation monitoring function of a photovoltaic inverter (PV-IMF) or in a charge controller' has been added.