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BETA Monitoring
Monitoring of Electrical Values
7
LQ3 insulation monitors for medical premises
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Siemens ET B1 · 10/2008
13
Insulation monitoring
The 7LQ3 354 and 7LQ3 355 insulation monitors are used for the
monitoring of the insulation resistances of non-grounded
IT systems in medical premises. They also simultaneously
monitor the load current and the temperature of the IT isolating
transformer. The devices can monitor both three-phase and
AC systems.
Temperature measurement: The temperature in the transformer
development is recorded over PTC thermistor or NC contacts.
As well as an adjustable response value of 50 … 500 k , the
7
LQ3 355 insulation monitor also has an 11-step LED chain for
displaying the current insulation resistance of the system. A
range of different colored LEDs indicate the insulation resistance
within the range of 20 k … 1 M . This allows insulation
deteriorations to be detected even before an alarm is triggered.
The device is also equipped with an additional relay for
connection of a test and signaling combination. This means that
the 7LQ3 360 test and signaling combination and the previously
unavailable 7XV9 306, 7XV9 304 and 7XV9 302 test and signal-
ing combinations can be connected to the 7LQ3 355
insulation monitors
(
see the diagram on page 13/36 "Connection
example: 7LQ3 355 insulation monitor to 7XV9 306 test and
signaling combination")
.
Load current sensing: The 7LQ3 358 current transformer detects
the load current of a phase. Evaluation is carried out over the
7
LQ3 354 and 7LQ3 355 insulation monitors.
Evaluation: If one of the values is outside the limit values, an
alarm is triggered. The LED for the relevant fault lights up and the
alarm relay switches. The information is made available over the
changeover contacts and can be displayed on the 7LQ3 356
and 7LQ3 357 test and signaling panels.
Voltage monitoring
In the case of undervoltage, there is no guarantee that medical
equipment will continue to function. Because of the risk this
presents to patients, e.g. during operations, the changeover unit
must reliably switch to a second power supply in the event of an
undervoltage in the preferred power supply.
The voltage relays switch when the voltage falls below 90 % of
the rated voltage. The 5TT3 411 relays serve to monitor a
single-phase infeed. Three-phase infeeds can be monitored
using 5TT3 412 relays. These relays also offer asymmetry,
reverse voltage and phase failure detection.
© Siemens AG 2008