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Test terminal block
Preliminary fuses
4.1.2
Connection to medium and high voltage (Aron circuit)
Grounding
Three-phase
three-wire meter
© Landis+Gyr
The secondary circuit of a current transformer must always be closed when
current is flowing in the primary. Opening of the secondary circuit can
destroy the transformer. The test terminal block shown in figure 46 allows
the secondary circuits to be short-circuited and the voltage circuits to be
opened in order to exchange the meter without interrupting operation.
The preliminary (supply) fuses in the voltage circuit (max. 10 A) protect
against short-circuits in the measuring circuit. The voltage circuits are
connected directly to the primary and would only be protected by the main
fuses of 100 A or more if the preliminary fuses were not fitted. A short-
circuit would certainly destroy the measuring device and cause fire and
other damage.
This circuit is used above all in the medium voltage range (3 to 30 kV) and
rarely with high voltage (from 30 kV).
It is also recommended here to insert a test terminal block between
transformer and measuring device (meters and others). This permits simple
exchange without interrupting operation. On the other hand, no preliminary
supply fuses are necessary in the voltage circuits, since the voltage
transformers cannot produce a high short-circuit capacity on the secondary
side.
On medium and high voltage the secondary side of all voltage transformers
should be grounded for safety reasons. Otherwise potentials could develop,
which could result in dangerous contact voltages.
Grounding
All following drawings can be used without potential transformers but in any
cases fuses must protect the voltage lines (see section 4.1.1). Then
grounding neutral meter point is not required.
Figure 47. Connection to medium and high voltage (Aron circuit)
Instead of the three-phase three-wire meter ZFD400 the power supply
company can also use a three-phase four-wire meter ZMD400. The
following two connections are then possible:
with 2 voltage transformers and open star point at the meter
with 3 voltage transformers and star point connected at the meter
D000062026 en a – E650 Series 4 – ZMD400AT/CT, ZFD400AT/CT – User Manual
Installation and uninstallation
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