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2.16.2 Software Monitoring
Watchdog
For continuous monitoring of the program sequences, a watchdog timer is provided in
the hardware (hardware watchdog) which will reset and completely restart the proces-
sor system in the event of processor failure or if a program falls out of step.
A further software watchdog ensures that any error in the processing of the programs
will be recognized. This also initiates a restart of the processor system.
If such a malfunction is not cleared by the restart, an additional restart attempt is
begun. If the fault is still present after three restart attempts within 30 s, the protection
system will take itself out of service, and the red LED "ERROR" lights up. The Device
OK relay drops off and signals the malfunction by its life contact.
Ambient Tempera-
If ambient temperature sensing by an external device has been configured, the func-
ture Sensing
tioning of the ambient temperature sensing is monitored. In case of malfunctions (am-
bient temperature sensing missing or defective, broken connection wire to the device
or to the temperature sensor), the indication „Fail.TEMPSENS" (FNo. 158) is gen-
erated.
2.16.3 Fuse Failure Monitor
In the event of measured voltage failure due to a short circuit or a broken conductor in
the voltage transformer secondary circuit, certain measuring loops may mistakenly
see a voltage of zero, which due to the load current may result in an unwanted pickup
or even trip.
If fuses are used instead of a secondary miniature circuit breaker (VT mcb) with con-
nected auxiliary contacts, then the fuse failure monitoring can detect problems in the
voltage transformer secondary circuit. Of course the miniature circuit breaker and the
fuse failure monitor can be used at the same time.
A fuse failure is characterized by missing voltages but flowing current. Figure 2-48
shows the logic diagram of the fuse failure monitor.
If a substantial voltage drop is detected in the measurement quantities without an in-
crease in current flow being registered simultaneously, this indicates a fault in the sec-
ondary circuit of the voltage transformer.
The voltage drop is detected when the voltage drops below 5 V while an operating
current between 0.06 In and 1.2 In is flowing. Currents above 1.2 In are considered
not as normal operating currents but as short-circuit currents.
If a measuring-circuit voltage failure is detected, the distance and the undervoltage
protection function are blocked after expiry of an overfunction protection period of 5
sec. The emergency overcurrent-time protection is activated, provided that it has been
configured accordingly (see Section 2.5).
During measured value monitoring with active Fuse-Failure-Monitor (MEASURE.
SUPERV=With FFM), the same conditions as for standard monitoring are being veri-
fied. However, a blocking of the distance protection and release of the emergency
overcurrent function occurs immediately if the conditions are complied with (no delay).
In order to avoid triggering of the Fuse-Failure-Monitoring in case of a network error,
the Fuse-Failure-Monitor is only triggered by a non-recognised current step. A network
error always occurs in conjunction with a current step. The Fuse-Failure Monitor
remains blocked for 10 s after triggering of the jump detector.
7ST6 Manual
E50417-G1176-C251-A3
2.16 Monitoring Function
167
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