Defines 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 1000 V a.c., as well as of unearthed d.c. IT systems with voltages up to 1500 V d.c. independent from the method of measuring.