Technical aspects affecting the use of insulation monitoring devices A-ISOMETER® 2.3.2 Earthing of loads According to IEC 60364-4-41/DIN VDE 0100-410 the loads in an IT system can be earthed individually or in groups, or they can share a common connection to earth. 2.3.1 System leakage capacitances C
e in IT systems The prevailing system leakage capacitance C
e is an importantcriterion when selecting an insulation monitoring device. The
system leakage capacitance is the total capacitive leakage of all live conductors to earth. The value of the system leakage capaci-tance C
e comprises the cable capacitances C
e of the individual conductors to PE plus the interference suppression capacitances C
L of the electrical equipment in the loads to earth. Cemax is the value up to which the insulation monitoring device will operate as intended.
Figure 2.7 All exposed-conductive parts connected to a singleprotective earth conductor 2.3.1.1 Natural system leakage capacitance (A) The natural system leakage capacitance is the proportion of the total capacitance C
Figure 2.5 Capacitance ratings in an IT system e resulting from the natural (geometrical) structure of the system's cables to earth. Line capacitances are determined by the insulation strength between conductors,distance h, material constant εf, electrical field constant ε0 and area of insulation between conductors. This value is usually
approx. 150 pF/m. 2.3.1.2 Artificial system leakage capacitances (B) In order to eliminate EMC faults caused by non-linear loads, diffe-ring frequencies or even high frequency harmonics and meet the requirements of EMC standards, loads are wired with interference suppression capacitances. These
professional filters lead to an increase in sy-
stem leakage capacitances and allow higher leakage currents to flow. Radio interference suppression
manufacturer of capacitors in electronicdevices are in the range 10…100 nF.
Figure 2.8 Exposed-conductive parts earthed individually or in groups Figure 2.6 Addition of system leakage capacitances withrefereence to the insulation monitoring device Annex 1.9.1 162 Main catalogue part 1 – 08.2007/Insulation monitoring