SOMMAIRE
E 9
4-20mA signal isolator, signal splitter with 2,3,4 outputs
SIL2 / SIL3
CAL4/100ig CAL4/100igM
Functional analysis
The converter consist of :
an power stage
two analog input stage
two isolation stage (signal transmission)
two output stage (current amplifier)
Dreaded event definition
For the converter
CAL4-100ig,
the dreaded event (i.e the dangerous failure as defined in the previous section) it's to
supply an wrong output current :
Either an erroneous output current of more than 1% compared to the process demand,
either an output current, blocked to a value, such as it is not possible to have a security fallback state : output current
blocked in the range >3.6mA or <21mA. Therefore in the impossibility to transmitting an alarm.
Security fallback definition
The security fallback state is defined with an output current out of the range 3.6mA - 21mA.
Either an output current =< 3.6 mA
Either an output current >= 21mA
The application program of the "Logic safety equipment" shall be configured to detect all current values out of range
(=< 3.6mA and >= 21mA) and considered them as "invalidate".
Thereby, in the FMEA analysis, this state is considered as a not dangerous state.
Study hypotheses
The failure rate of component are considered as constant for the all system life time.
The evaluation of the safety features of a device involves a number of assumptions:
Only the catalectic failures are taken in account : straight, sudden, or unpredictable failures.
Are not considered the failures that could be due to:
- design errors
- batch defect in production
- environment (electrical interference, temperature cycles, vibrations)
- human errors in operation or maintenance
Precautions are taken to avoid them: management of a L.O.F.C (List of manufacturing operation and control)
Only simple fault are handled. Welding defect, which are usually due to a lack of quality detectable at the end of pro-
duction by a specific burn-in, are not taken into account.
All aspects of power-on specific features are not treated.
Failure rate
The simple failure rate for the components of the converter CAL4-100ig are classified on the document:
AMDEC CAL4-100ig rev4.XLS
( internal document not communicated for reasons of design confidentiality)
Created with "ALD MTBF calculator" according to the reliability reports :
- MIL-HDBK-217F Notice 2 Electronic Reliability Prediction et iec-tr-62380.e Reliability data handbook