MI 3101 EurotestAT: Appendix D
IT supply systems
107
D Appendix D - 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
.
D.1 Standard references
EN 60364-4-41, EN 60364-6, EN 60364-7-710, BS 7671
D.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
.
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 k
.
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 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 sufficiently high at the
system frequency. Where no neutral point or mid-point exists a line conductor may be
connected to earth through high impedance.
