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.

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.

A more complete listing of failure rates is provided in

Section C.4 of the FMEDA for the 961 Line-Powered Switch.

13

51-650 Echotel SIL Safety Manual

Failure Category

λsd

λsu

λdd

λdu

SFF

Model 961

0 FIT

96 FIT

288 FIT

36 FIT

91.4%

Table 6

Model 961 IEC 61508 Format Failure Rates

Failure Category

λsd

λsu

λdd

λdu

SFF

Model 961

0 FIT

106 FIT

351 FIT

40 FIT

92.0%

Table 7

Model 961 IEC 61508 Format Failure Rates