MI 3000 EasiPLUS IT supply systems
58
B IT supply systems
In order for the operator to be familiar with the measurements and their typical
applications on IT supply systems, it is advisable to read Metrel’s 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
An IT supply system is a mains supply system that is insulated from ground (PE) (i.e. it
is ungrounded supplying system). The system either has no direct connection to ground
or the connection is provided through a relatively high impedance. IT systems mainly
occur in areas where additional protection against electric shock is required. Typical
places are medical surgery rooms.
IT supply systems also has no grounding currents except leakages, and in this way they
are not suceptable to problems with step voltages, (e.g. voltage drops in a step) or high
energy sparking in Extrinsic areas.
In normal situations, a high impedance to ground exists and is formed by the
capacitances of the supply wires to ground and the capacitances between the primary
and secondary windings of the IT supply transformer. A minor part of the capacitance is
formed by Y capacitors (EMC) in the mains section of any connected equipment.
Selecting the appropriate transformer, installation cabling and the selection of an
optional high impedance connection to ground can control the maximum leakage
current of the system.
Depending on the application, additional impedances to ground can be applied or can
occur through special loading equipment (as presented in Figure B.1). The value of the
impedance should be greater than 100 .
The IT system can provide an additional level of protection against electric shock. In the
situation where a failure occurs in any of the insulation between the line conductors and
PE (e.g. through equipment failure, wrong application or procedure), this system will
remain safe but will be converted into a TN / TT type system. However, additional failure
can be hazardous, which means that the insulation must be continuously checked and
repaired immediately after a failure has been detected.
Supplementary to other protection devices, an IT system normally contains insulation
monitoring devices (IMD) or systems that sound alarms when insulation resistance or
impedance drops below set threshold. The threshold value usually depends on
environment and the application. (e.g. Typical value for a medical installations is 55 k
.
In some countries, it is not enough to trace the insulation resistance of the IT supply
system to 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
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