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3.6 Functional safety
The STL50 safety temperature limiter was developed according to the specifications of the
IEC 61508. This standard describes the functional safety of safety-related programmable
electrical and electronic systems.
The devices is a Class B subsystem with the SIL2 requirement (single-channel). The
safety function of the device relates to the detection and evaluation of the temperature and
the resulting mandatory contact position of the installed relay.
Safe state
The safe state of the device is only provided when the relay is in idle position (closed cur-
rent principle). If the internal diagnostic system recognises an error, the relay switches to
idle position. Therefore, a normally-open contact can be used for integration of the relay
changeover into the monitoring device.
Temperature probe
Connected temperature probes are monitored for cable breaks or short-circuits. With ther-
mocouples, this is only physically possible for double thermocouples. It is not permissible
to use simple thermocouples and connect the inputs in parallel with jumpers. If separate
probe fittings are used, they must be installed next to each other so that they both detect
the same temperature.
In case of faults and errors
If a fault occurs in the system, the cause must be rectified immediately. If the STL50 safety
temperature limiter is decommissioned for this purpose, the process must be safeguarded
in another manner. If there is a device error, we request that you send the device to the
factory with a brief description of the error.
STL50 safety indicators for functional safety
Safety-related output signal
Relay output
Test standard
IEC 61508
Inspection authority
TÜV NORD CERT GmbH
SIL
2
System
Type B
Input
Pt100
Thermocouple
λ
SD
(rate of detected harmless failures)
1740 FIT
1740 FIT
λ
SU
(rate of undetected harmless failures)
601 FIT
542 FIT
λ
DD
(rate of detected dangerous failures)
741 FIT
742 FIT
λ
DU
(rate of undetected dangerous failures)
127 FIT
105 FIT
(1 FIT = 1 failure / 10
9
h)
Number of harmless failures (SFF)
96 %
97 %
Average probability of failure (PFD
avg
)
3.80 x 10
-3
2.80 x 10
-3
Probability of failure per hour (PFH)
1.27 x 10
-7
1.05 x 10
-7
Duration of use of safety function
10
Years
