Download The IT (unearthed) power supply system

The IT (unearthed) power supply system
A typical application for an unearthed system is in industrial power supply, e. g. in chemistry, where security of
supply is important, and insulation faults must be avoided. The use of IT power supply systems in medical
locations is important for successful diagnosis and therapy, by ensuring the smooth operation of electrical
medical devices.
IT power supply systems have no intentional connection between active conductors and earth. An insulation
monitoring device is used to monitor the insulation resistance; this is installed between the active conductors and
earth and superimposes a measuring voltage. If an insulation fault occurs, the measuring circuit closes and a
small measuring current flows. The measuring current is proportional to the insulation resistance. This current is
registered and evaluated. An alarm is activated as soon as the insulation resistance falls below a set value.
The inherent advantage of the IT system is the continuity of the power supply and the resulting continued
operation of the facility on a first fault. Because of the reduced flow of fault current no fuse is activated. The
insulation monitoring device (IMD) provides an early warning of insulation failure, which enables the insulation
fault to be eliminated early, e.g. during maintenance.
Insulation monitoring devices are required in all IT power supply systems, according to standards IEC 60364-441: 1992. The product standard of the devices is described in IEC 61557-8:1999.
Standard for insulation fault location systems
A major factor for safe operation and availability of electrical power supplies is the insulation resistance; this is
also valid for IT power supply systems. The standard requires that IT power supply systems eliminate insulation
faults when they occur. In the past fault location has only been possible through switching off circuits. Modern
insulation monitoring devices allow fault location during operation and thus allow fault elimination without
interrupting operation. The international standard IEC 61557-9 has now been developed for insulation fault
location systems, which consist of various modules. The original title of the complete series of these standards is
called ”Electrical safety in low voltage distribution systems up to 1 000 V AC and 1 500 V DC – Equipment for
testing, measuring or monitoring of protective measures”. The English title of part 9 is: ”Equipment for insulation
fault location in IT power supply systems”.
The development of the series of IEC 61557 standards part 1-10 has been initiated by Germany. The basis for
this development was a request from CENELEC, to initiate European standards based on the German standards
DIN VDE 0413-1 to DIN VDE 0413-8. Under German guidance the draft standard prEN50197 was supplied to
the technical committee of IEC TC85, in order to develop the international standards. An international working
group was formed, consisting mainly of members from Germany, France, UK, Sweden and Austria. In a short
time an international draft was prepared and presented. CENELEC then stopped its work and presented the
international drafts to the IEC for parallel voting for acceptance as International and European standards. Part 1
to 5 and 7 to 8 were ratified in March 1997, part 6 in June 1998.
All standards in this series comprise of a table with all-important technical data. IEC 61557-1 (part 1) contains
joint definitions, which are used for all test, measure or monitoring devices according to IEC 61557-2 to IEC
61557-10 (parts 1 to 10). They are:
• Protection against extraneous voltages;
• Protection class II (except insulation monitoring devices);
• Specifications and safety precautions against hazardous touch voltages at the measuring device;
• Specifications for the assessment of connection configurations or wiring errors of tested systems;
• Special mechanical requirements;
• Measuring methods;
• Measured quantity, nominal range of use;
• Specification of the maximum operating error;
• Specification for testing the influencing quantity and the calculation of the operational error;
• Errors of the measuring device at the thresholds specified in the respective standards;
• Specification of the nature of type and routine tests with the required conditions for testing.
Special requirements
Part 8 specifies the special requirements for insulation monitoring devices which permanently monitor the
insulation resistance to earth of unearthed IT AC systems, for IT AC systems with galvanically connected DC
circuits having nominal voltages up to 1000 V AC, as well as of unearthed IT DC systems with voltages up to
1500 V DC independent from the method of measuring.
Part 9 specifies the requirements for insulation fault location systems ”which localise insulation faults in any part
of the system”. This means systems that can locate sections with insulation faults in unearthed IT power supply
systems up to 1000 V AC and 1500 V DC, independent of the measuring principle.
Insulation fault location systems, normative structure
The international standard IEC 61557-9: 1999, section 3.1 defines an insulation fault location system as that
”which permanently monitors the insulation resistance of an unearthed IT system and additional insulation fault
location modules, which are activated if an insulation fault is detected. The insulation fault location modules
simultaneously may perform the function of injecting a test current between the electrical system and earth to
locate the sections of the IT power supply system with insulation faults. The insulation fault location system may
consist of several functional modules combined in one device” e.g.:
Insulation fault location systems in practice
Usually a test device is connected between the active conductor and earth. If an Insulation fault occurs, the
insulation monitoring device reports this as soon as the set-value falls short. The insulation fault location system
can now be started either manually or through the output contact from an insulation monitoring device. In this
case the test device produces a test current limited to a low value, by connecting the system conductors
alternately and in intervals with earth. If a system conductor insulation fault to earth occurs, a circuit is
developed, which consists of the source of the monitored system and the insulation resistance. This means, that
the test current is dependent on the circuit tension as well as insulation resistance. The test unit will have a
maintenance value, e.g. 1 mA, to prevent the test current reaching an uncontrollable value.
Figure 2 shows an example of this.
Test current in an IT system flows through insulation fault points or rather through test current output. In order to
pursue the path of the test current or rather to locate the point of fault occurrence, all important current output in
the IT system is equipped with measuring transformers, which register the test current. The measuring
transformers likewise are connected with an evaluation device. The evaluation device periodically tests and
indicates the flow of the test current in the respective transformers (output).
A central control device is connected to all evaluation devices. Through the circuit measuring transformers
(output) the insulation fault location can now be easily traced (figure 3). This information can be reported via
intersections or report contacts.
The advantages in terms of operation safety of the IT system are altogether enhanced through this principle of
testing. Because the search is being done during operation, fault repairs can be carried out at planned time, e.g.
during regular maintenance routines. The described insulation fault location system is modular. This means that
it can be adapted to facilities in terms of several current outputs. Split-core measuring transformers are available
which allow upgrading of existing facilities without the need to open cable connections. The use of modern bustechnology reduces the cabling requirements within the system/facility. This is not only advantageous in terms of
cost, but also saves complicated changes to the wiring when up-grading.
Limits of the insulation fault location
Naturally there are certain physical limits in this system. The smaller the test current, or rather the higher the
insulation resistance, the more difficult it is to register the test current through the measuring transformers.
However modern electronics switching techniques allow insulation fault location systems to register insulation
faults in a distribution system, e.g. 230 V, as small as 10 kΩ or less. This is a value which has proven itself in
practice and which can be used for most operations.
Conclusion
IT power supply systems with insulation monitoring have become an integral part in many areas that require high
operational safety. With the standardized definition of insulation fault location system a gap has been filled in the
international standards and an important contribution has been made to safety and security in electrical facilities.
Contact Wiebe Visser, Anglo Allied Engineering, Tel (011) 766-1180, [email protected]