This part of IEC 61557 specifies the requirements for insulation monitoring devices (IMD) which permanently monitor the insulation resistance to earth of unearthed IT a.c. systems, for IT a.c. systems with galvanically connected d.c. circuits having nominal voltages up to 1 000 V a.c., as well as of unearthed IT d.c. systems with voltages up to 1 500 V d.c. independent from the method of measuring. This second edition cancels and replaces the first edition published in 1997. This edition constitutes a technical revision. The following changes were made with respect to the previous edition: a) definitions complemented; b) revision of some requirements; c) addition of information on operating instructions; d) sections "type tests" and "routine tests" complemented; e) modification of Table 1; f) addition of Annexes A and B.