6.2.3 Model 962 Loop-powered

Ultrasonic Level Switches

The Model 962 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 assumes the diagnostic

signal is being transmitted to a logic solver programmed to

detect over-scale and under-scale currents.

Model 962 failure rates are shown in Table 8.

A more complete listing of failure rates is provided in Table 1,

of the FMEDA for the 962 Loop-powered Switch.

6.2.4 Model 962 AC/DC Ultrasonic Level Switch

with Relay Output

The Model 962 ultrasonic level switch is a complex device

classified as Type B according to IEC61508, having a hardware

fault tolerance of 0. The Model 962 ultrasonic level switch

is an 18–32 VDC or 102–265 VAC power device that pro-

vides relay outputs. The Model 962 has only one output

failure state (fail-safe) with the relay de-energized.

The Model 962 failure rates are shown in Table 9.

A complete listing of failure rates is provided in Table 1 of

the FMEDA for the 962 Line-Powered Switch.

14

51-650 Echotel SIL Safety Manual

Failure Category

λsd

λsu

λdd

λdu

SFF

Model 961

0 FIT

110 FIT

362 FIT

42 FIT

91.8%

Table 8

Model 962 IEC 61508 Format Failure Rates

Failure Category

λsd

λsu

λdd

λdu

SFF

Model 961

0 FIT

130 FIT

427 FIT

52 FIT

91.5%

Table 9

Model 962 IEC 61508 Format Failure Rates