961 Datasheet, PDF (13/16 Pages) Magnetrol International, Inc. – SIL Safety Manual for Echotel Model 961/962

6.2 FMEDA Report Management Summary
6.2.1 Model 961 Loop-powered Level Switch
The Model 961 ultrasonic level switch is a complex device
classified as Type B according to IEC61508, having a hardware
fault tolerance of 0. This 24 VDC loop-powered unit contains
self diagnostics programmed to output either 3.6 mA or
22 mA during a failure state. Additionally, any output that
exceeds the nominal values of 8 mA and 16 mA by a
tolerance of ±1.6 mA must be detected as a fault by the
logic solver. The FMEDA analysis assures the diagnostic
signal is being transmitted to a logic solver programmed to
detect over-scale and under-scale currents.
The Model 961 failure rates are shown on Table 6.
Table 6
Model 961 IEC 61508 Format Failure Rates
Failure Category λsd
λsu
λdd
λdu
Model 961
0 FIT
96 FIT
288 FIT 36 FIT
SFF
91.4%
These failure rates can be used in a probabilistic model of a
Safety Instrumented Function (SIF) to determine suitability
in part of Safety Instrumented System (SIS) usage in a
particular Safety Integrity Level (SIL). A more complete
listing of failure rates is provided in Section C.4 of the
FMEDA for the 961 Loop-Powered Switch.
6.2.2 Model 961 AD/DC Ultrasonic Level Switch
with Relay Output
The Model 961 ultrasonic level switch is a complex device
and is classified as Type B according to IEC 61508 having a
hardware fault tolerance of 0. The Model 961 ultrasonic
level switch is an 18–32 VDC or 102–265 VAC power
device that provides relay outputs. Model 961 has only one
output failure state (fail-safe), with the relay de-energized.
Model 961 failure rates are shown in Table 7.
Table 7
Model 961 IEC 61508 Format Failure Rates
Failure Category λsd
λsu
λdd
λdu
Model 961
0 FIT 106 FIT 351 FIT 40 FIT
SFF
92.0%
A more complete listing of failure rates is provided in
Section C.4 of the FMEDA for the 961 Line-Powered Switch.
51-650 Echotel SIL Safety Manual
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