This part of IEC 61557 specifies the requirements for insulation monitoring devices 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.
NOTE 1 IT systems are described in IEC 60364-4-41 amongst other literature. Additional data for a selection of devices in other standards should be noted.
NOTE 2 Various standards specify the use of insulation monitoring devices in IT systems. In such cases, the objective of the equipment is to signal a drop in insulation resistance below a minimum limit.
NOTE 3 Insulation monitoring devices according to this part of IEC 61557 may also be used for de-energized electrical systems.