Note: An insulation monitoring device will signal a drop in insulation resistance without an "actual" insulation fault being present. This can be due, for example, to moisture (if the installation has been out of service for a long period of time). Once the installation has been put back into operation, the insulation resistance usuallyincreases automatically due to the moisture disappearing as the operating temperature is reached.The values listed in the table below are recommended minimum values. IEC 60364-5-53/DIN VDE 0100-530 (VDE 0100-530) con- tains practical instructions regarding the selection of response values for insulation monitoring devices. A value of 50 Ω/V ofsystem voltage is used as a guide value. The response values listed in the table below have been taken from the corresponding standards. Technical aspects affecting the use of insulation monitoring devices A-ISOMETER® 5.4 Fuse protection for insulation monitoring devices Most insulation monitoring devices are connected to the system to be monitored via two leads. In principle, any type of fuse protection for an insulation monito-ring device must be considered as line protection. According to IEC 60364-4-43/DIN VDE 0100-430, fuse protection is always required if the current carrying capacity is reduced by a decrease in cable cross section, a change in cable installation method, a change in conductor insulation or a change in the number of conductors. Protective devices for overload protection can be omitted from lines and cables in which overload currents will not occur as long as they feature no taps or plug-and-socket devices. It can generally be assumed that these conditions can be met both where the power supply specification connection is concerned and in re-spect of the supply voltage connection on insulation monitoring devices. The situation as regards short-circuit protection is very different. The supply voltage terminals must be fitted with over- current releases for short-circuit protection. We recommend the use of 6 A fuses. Fuse protection also makes it easier to accessdevices for service operation. Protective devices for short-circuit protection do not have to be used for the power supply connec- tion if the risk of a short-circuit is minimal (see IEC 60364-4-43/ DIN VDE 0100-430). Therefore, we recommend at least one cable installation protected against short-circuits and earth faults. Reducing the risk of short-circuits to a minimum and deciding whether this has been achieved is often very difficult. In cases of doubt, we recommend installing protective devices for short-cir- cuit protection in the power supply link in the form of 6 A fuses. On devices with built-in connection monitoring, the blowing of a fuse is signalled as an open circuit. 5.5 Setting response values When operating an insulation monitoring device, make sure that the total insulation resistance of the relevant IT system is mea-sured. This total resistance is calculated by adding together the resistive leakage of all outgoing lines connected in parallel to earth.The response value of the insulation monitoring device should be set to approx. 100 Ω/V. Example: System voltage 500 V A-ISOMETER® IRDH275 set to 50 kΩIf an insulation monitoring device is being used in an existing complex installation, we recommend setting the response value to approx. 50 Ω/V.
Annex 1.9.1 168 Main catalogue part 1 – 08.2007/Insulation monitoring