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There has been substantial effort in Austria to examine the use of
RCDs and their shortcomings in some applications. This paper describes the
problem and the reworking of the electrical supply system to fix it. This
IEC Working Group paper is by Prof Ing Dr phil Gottfried Biegelmeier now an
electrotechnical engineering consultant to Cooperative Testing Institute,
Wien and, formerly, a long time employee of OVE in Austria.
The electrical systems are discussed further in IEC 364 and in IEC
950 plus other standards.
- - - - -
Ref: Austrian Electricity Supply Undertakings - Change of measures of fault
protection (protection against indirect contact)
For historical reasons up to now in the western part of Austria the
TN-system (protective neutral earthing) and in the eastern part the
TT-system with RCDs for disconnection are used as measures of fault
protection.
In view of the fact that reliable statistics prove that in the areas of
eastern Austria which are supplied with electric energy using the TT-system
for fault protection approx. 100,000 installations are in imminent danger
due to non functioning RCDs (ref Document
64/wg30/London/Austrian Expert/01/95). The Austrian Ministry of Economy
being responsible for the safety of application of electric energy has
taken the necessary measures in order to protect the consumer.
At first it was tried to motivate the consumer to operate the test-device
of the RCD regularly. But even propagating the testing of RCDs at the
conversion of the daylight saving time and vice versa with considerable
costs in broadcasting and television proved to
be a failure.
It was therefore clear that in order to eliminate the danger only the
change of the TT-system to the TN-system in the eastern areas of Austria
would be an appropriate measure which has to be carried out by the
electricity supply undertakings concerned step by step
within the next ten to fifteen years.
It should also be mentioned that expensive and time-consuming field
investigations have been carried out by the electricity supply undertakings
of Upper-Austria and Salzburg including measurements of fault voltages,
prospective touch voltages and touch voltages
in agricultural installations.
Details and results of these field investigations have been published in a
special issue of the periodical of the Austrian Electricity Supply
Undertakings, VEO Journal 11/96, in German language.
The change form TT- to TN-system involves the connection of the consumers
earth electrode to the PEN-conductor of the supply system. RCDs in the
installations will be tested and if in working order remain where they are.
If their rated tripping fault current
Ideltan <= 30 mA they ensure additional protection, if it is higher they
will serve for fire protection in a kind of insulation monitoring function.
It has to be mentioned that in Austria for final circuits with socket
outlets up to a rated current of 25 A and for special locations (e.g.
bathrooms) additional protection with RCDs with a rated tripping fault
current Ideltan <= 30 mA is obligatory.
Before these measures are carried out it has to be ensured that the supply
system and the installations comply with the requirements for the TN-system
(protective neutral earthing, see VEO-Journal 11/96).
By carrying out the change from the TT-system to the TN-system safety of
application of electricity is considerably increased in case of an
insulation fault because:
-) high reliability of disconnection is insured by overcurrent
devices, usually at least two in series
-) fault voltages and touch voltages usually remain below half of
the values occurring in TT-systems
-) lower earthing resistances of the PEN-conductor of the supply
system due to the connection of the earthing electrodes of the consumer
installations to the PEN-conductor.
(Field measurements indicate that the total earthing resistance Rb of the
supply system in a TN-system is lower than half of the value of a similar
TT-system.)
-) better protection against overvoltages due to the low resulting
earthing resistance of the PEN-conductor
-) simple and cheap fault protection even for installations with
high current ratings and equipment in distribution systems and public
systems (e.g. street lighting).
After that change in the future years the whole of Austria has to be marked
in red color (TN-system 3N~400/230V) in the world-wide survey on network
systems.
IEC 64/WG1(Definitions)/Biegelmeier/05/96
IEC 64/WG4(Effects of current passing thru the human body & thru
livestock)/Biegelmeier/07/96
IEC 64/WG9(Disconnecting times & related matters)/Biegelmeier/07/96
