Справочник Пользователя для Benning IT 120 B 044102
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11 Appendix B IT Systems
11.1 Applicable standards
EN 60364-4-41, EN 60364-6, EN 60364-7-710, BS 7671
11.2 Basic information
The IT system is an power supply network which is isolated from the protective conductor. It is
an ungrounded power supply network. Either the network is not connected to earth directly or it
is connected to earth via a relatively high impedance. It is mainly used in areas requiring addition
protection against electrical accidents. A typical area of application are medical operating rooms.
an ungrounded power supply network. Either the network is not connected to earth directly or it
is connected to earth via a relatively high impedance. It is mainly used in areas requiring addition
protection against electrical accidents. A typical area of application are medical operating rooms.
Normally, the network is connected to earth via the high impedance and consists of the capaci-
tances of the lines connected to earth as well as of the capacitances between the primary and
secondary windings of the IT transformer. Occasionally, Y capacitors (EMC) are used in the mains
area of the connected devices. The maximum leakage current can be limited by choosing an ap-
propriate transformer as well as the laying of lines and choosing an optimum earthing with high
impedance.
Depending on the area of application, additional impedance to earth can be generated as shown
tances of the lines connected to earth as well as of the capacitances between the primary and
secondary windings of the IT transformer. Occasionally, Y capacitors (EMC) are used in the mains
area of the connected devices. The maximum leakage current can be limited by choosing an ap-
propriate transformer as well as the laying of lines and choosing an optimum earthing with high
impedance.
Depending on the area of application, additional impedance to earth can be generated as shown
in figure 11.1 or by means of specific devices for charge control. The minimum impedance value
should be 100 Ω.
An IT network offers additional protection against electrical accidents. If the insulation of a line to-
wards the protective conductor fails due to device failures, incorrect use or improper proceeding,
the network will continue to be safe, but will be converted into a TN/TT system. However, the oc-
currence of another fault would be dangerous. For this reason, the insulation must be monitored
permanently and must be restored immediately after a failure has been detected.
In addition to other protective mechanisms, the IT system normally contains an insulation monito-
ring device (IMD) or a system which emits a warning, if the insulating resistance or the impedance
value falls below the preset limiting value. The limiting value depends on the respective environ-
wards the protective conductor fails due to device failures, incorrect use or improper proceeding,
the network will continue to be safe, but will be converted into a TN/TT system. However, the oc-
currence of another fault would be dangerous. For this reason, the insulation must be monitored
permanently and must be restored immediately after a failure has been detected.
In addition to other protective mechanisms, the IT system normally contains an insulation monito-
ring device (IMD) or a system which emits a warning, if the insulating resistance or the impedance
value falls below the preset limiting value. The limiting value depends on the respective environ-
ment. A typical value for installations in medical rooms is 55 kΩ.
In some countries, it is not sufficient to monitor only the insulating resistance of the IT system. In
this case, the network capacity must be continuously monitored as well.
this case, the network capacity must be continuously monitored as well.
IEC 60364-4-41 (©IEC): In IT systems, the live parts must be isolated from earth or must be con-
nected to earth via a sufficiently high impedance. This connection can be made either at the neut-
ral conductor, at the star point of the network or at an artificial star point. The latter can be connec-
ted directly to the earthing, if the resulting impedance to the earthing is high enough for the given
system frequency. If there is no neutral conductor or star point available, the mains conductor can
be connected to earth via a high impedance.
ted directly to the earthing, if the resulting impedance to the earthing is high enough for the given
system frequency. If there is no neutral conductor or star point available, the mains conductor can
be connected to earth via a high impedance.