Bedienungsanleitung Benning IT 110 (Seite 69 von 80) (Englisch)

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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.

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

ingure11.1orbymeansofspecic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-

ment.Atypicalvalueforinstallationsinmedicalroomsis55kΩ.

Insomecountries,itisnotsufcienttomonitoronlytheinsulatingresistanceoftheITsystem.In

this case, the network capacity must be continuously monitored as well.

nectedtoearthviaasufcientlyhighimpedance.Thisconnectioncanbemadeeitherattheneut-

ralconductor,atthestarpointofthenetworkoratanarticial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.

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