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Metrel MI 3102 BT - Eurotest XE Manual

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MI3002 MI3100 MI3102 IT supply systems
89
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.
IEC 60364-4-41 (©IEC): In IT systems live parts shall be insulated from earth or
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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