Bedienungsanleitung Metrel MI 3102 BT - Eurotest XE (Seite 89 von 104) (Englisch)

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MI3002 MI3100 MI3102 IT supply systems

B IT supply systems

In order for operator to be familiar enough with measurements in and their typical

applications in IT supply system it is advisable to read Metrel handbook Measurements

on IT power supply systems.

B.1 Standard references

EN 60364-4-41, EN 60364-6, EN 60364-7-710, BS 7671

B.2 Fundamentals

IT supply system is mains supply system that is insulated from ground (PE) – it is

ungrounded supplying system. The system is without direct connection to the ground or

the connection is provided through relatively high impedance. It is applied mostly in

areas where additional protection to electric shock is required. Typical places are

medical surgery rooms.

IT supply system also omits any grounding currents except leakages, and in this way

there aren’t problems with step voltage, i.e. voltage drop in one step, neither high

energy sparking in Ex areas.

In normal cases the high impedance to the ground exists and is formed by capacitances

of supply wires to the ground plus capacitances between primary and secondary

windings of IT supply transformer. Minor part is formed by Y capacitors (EMC) in mains

section of connected equipment. Selecting appropriate transformer, installation cabling

and selection of optional high impedance connection to ground can control maximum

leakage current.

Depending on application area additional impedance to the ground can be applied as

presented in figure D.1 or through special loading equipment. Value of the impedance

should begin from 100 Ω.

IT system represents additional level of protection to electric shock. In the case of a

failure of any line insulation to the PE through equipment failure, wrong application or

procedure, this system is still safe but converted to TN / TT type. However, additional

failure is hazardous, which means that the insulation has to be continuously checked

and repaired immediately after detected failure.

Supplementary to other protection devices the IT system normally contains insulation

monitoring device (IMD) or system that alarms when insulation resistance or impedance

is below set threshold. Threshold value depends on environment. Typical value for

medical installations is 55 kΩ.

In some countries it is not enough to trace insulation resistance of IT supply system to

the ground, they require tracing of system capacitance, too.

connected to earth through sufficiently high impedance. This connection may be made

either at the neutral point or at midpoint of the system or at an artificial neutral point.

The latter may be connected directly to earth if the resulting impedance to earth is

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