DRAFT - 26 March 2015
SM45-55-SAD Rev 3
5
4
Note:
When using the modules that are not loop-powered it is important that the solenoids being driven be
chosen to ensure that the residual field current that flows in the module OFF state does not cause the solenoid
to remain energised. A small field current is used by some module types to determine the line condition and
some low-power solenoids are capable of remaining in their energised state, once energised, with very small
loop currents. If the safe state of the loop is for the solenoid to be OFF then it must be able to drop out, despite
the monitoring of line state.
It is therefore advisable to avoid this risk by selecting module types that do not support line fault monitoring,
unless this function is specifically required by the application.
Similarly, operation of the equipment outside of its environmental ratings induces component stress and in
particular temperatures below -20ºC are to be avoided to ensure required performance.
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Assessment of Functional Safety
4.1
Hardware Safety Integrity
The hardware assessment shows that MTLx52x solenoid/alarm drivers:
• have a hardware fault tolerance of 0
• are classified as Type A devices (“Non-complex” component with well-defined failure modes)
• have no internal diagnostic elements
The definitions for product failure of the modules at an ambient temperature of 45°C were determined as follows:-
Loop-powered modules
Failure mode
Failure rate (FIT)
MTLx521
MTL4521L
MTL4523L
MTL5522
MTL5525
Output stuck ON
0
0
0
0
0
Output stuck OFF (no output)
189
194
191
228
201
Output uncertain (OK or OFF)
17
17
17
24
18
Correct operation but reduced output
voltage when ON
49
50
50
60
50
Correct operation (failures have no
effect)
100
101
73
108
103
Separately-powered modules
Failure mode
Failure rate (FIT)
MTLx523/23x/23VL
MTLx524/4524S
MTL4525
Output stuck ON
8
20
21
Output stuck OFF (no output)
234
227
241
Output uncertain (OK or low) when ON
23
28
30
Correct operation but reduced output
voltage when ON
50
50
51
Correct operation (failures have no
effect)
100
111
116
