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Reference Manual
00809-0100-4021, Rev GC
Section 7: Prior Use (PU) Safety Instrumented System
April 2014
187
Prior use (PU) safety instrumented system
Section 7
Prior use (PU) safety instrumented
system
Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . page 187
Safe failure fraction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . page 188
Installation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . page 188
Note
This section applies to 4–20 mA only.
7.1 Overview
This section details the requirements for using the 3144P in Prior Use (PU) Safety Instrumented
Systems (safety-certified). 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 safety-certified 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 greater than 90% (assuming that
the logic solver is programmed to detect over-scale and under-scale currents). The device also
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 safety-certified FMEDA report for failure rate data.
Notes
1. Transmitter output is not Prior Use safety-rated during: configuration changes,
multidrop, simulation, active calibrator mode, and loop test. Alternative means should
be used to ensure process safety during transmitter configuration and maintenance
activities. Loop test, simulation and active calibrator modes are normally disabled
automatically through the user interface. However, cycling power or performing a
Processor Reset is recommended to ensure these functions are disabled. This action is
effective regardless of the Security switch setting.
2. DCS, or safety logic solver, should be configured to match transmitter configuration.
3. Figure 6-1 identifies the Emerson
®
standard alarm and saturation levels. Alarm and
saturation values are user-configurable.
Setting the alarm values is a two step process.
1. With a field communicator, select the alarm and saturation levels.
2. Position the alarm switch to the required HI or LO position.