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Reference Manual
00809-0100-4021, Rev DA
November 2004
Rosemount 3144P
www.rosemount.com
Prior Use SIS
Section 7 Prior Use (PU) Safety
Instrumented System
4–20 mA only
Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . page 7-1
Safe Failure Fraction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . page 7-2
Installation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . page 7-2
OVERVIEW This section details the requirements for using the 3144P in Prior Use (PU)
Safety Instrumented Systems (SIS). Although the 3144P is certified for
functional safety per IEC61508, the non-certified transmitter may also be
used in Safety Applications using the PU. The complete Failure Modes,
Effects, and Diagnosis Analysis (FMEDA) was completed to determine the
safe failure fraction (SFF) when using this device in a SIS application.
FMEDA are the device characteristics that are taken into account when
attempting to achieve functional safety certification per IEC61508 of a device.
From the FMEDA, failure rates are determined for all temperature sensing
device options. Furthermore, the Safe Failure Fraction is calculated for each
of the four different input device configurations.
The non-certified 3144P is an isolated 2-wire 4-20 mA SMART device
classified as Type B according to IEC61508. It contains self-diagnostics and is
programmed to send its output to either a high or low failure state upon
internal detection of a failure.
The analysis shows that the device has a safe failure fraction between 60 and
90% (assuming that the logic solver is programmed to detect over-scale and
under-scale currents). The device has a safe failure fraction of over 90%
when used with a temperature sensing device, such as thermocouple or RTD.
The device can detect open and short circuit failures of these temperature
sensing devices.
Refer to the 3144P SIS FMEDA report for failure rate data.