EC: One Series Safety Transmitter
The One Series Safety Transmitter provides much more than the process variable to the PLC. In addition to the NAMUR standard 4 to 20 mA analog signal, there are switched and diagnostic outputs available. The internal safety relay output (SRO)—rated at 5 A, 250 Vac—can provide a high speed trip directly to a final control element at a programmed value (set point). This feature allows designers to create independent protection layers (IPLs) where the SRO acts directly on the final element (i.e., actuator, relay or a start/stop circuit) to control a valve or pump without processing the trip signal through the PLC. This saves precious time and allows an alarm or shutdown to occur in less than 100 msec, closer to the point of measure. Diagnostic coverage of the SRO is provided by monitoring the final element and determining if current flowing through it is within range. The SRO monitor feature can detect an open or shorted SRO.
Complementing the SRO are two additional fail-safe outputs providing diagnostic and set point status, facilitating the use of voting logic applications. When monitored by a safety PLC, these discrete outputs allow the One Series Safety Transmitter to achieve a high safe failure fraction (SFF) of 98.8%, the highest of any transmitter certified for use in SIL 2 safety systems as a single element (HFT=0).
Unlike process transmitters re-purposed for plant safety, the One Series Safety Transmitter was designed specifically for functional safety applications using the latest available version of the IEC 61508-2010, 2nd edition standard and certified by exida. The meaning of this to owner/operators and their safety designers is that they can have even greater confidence in hardware assessed under the latest IEC 61508 standard.
United Electric Controls, www.ueonline.com