SIL 3-rated relays: The new 'accepted industry practice’

Accepted industry practice for process safety applications has been the use of standard relays in redundant or triplicate configurations to achieve 'safe’ standard relay functionality. This functionality has included: Unfortunately, the use of standard relays has its drawbacks in comparison to the new breed of SIL-rated relays.

04/15/2007


Accepted industry practice for process safety applications has been the use of standard relays in redundant or triplicate configurations to achieve 'safe’ standard relay functionality. This functionality has included:

  • Amplification

  • Isolation

  • Voltage level translation

  • Monitoring emergency stops

  • Monitoring safety guards.

    • Unfortunately, the use of standard relays has its drawbacks in comparison to the new breed of SIL-rated relays. The first and foremost advantage of the SIL-rated relay is the SIL rating itself.

      The SIL-rated product guarantees a certain level of reliability, predictability and traceability that is directly related to failure consequences as defined in IEC 61508. Achieving this rating for a device is a big deal. It’s not as easy as developing a device according to a standard or a set of regulations and then submitting it to a test facility for 'SIL testing.’

      The IEC organization TÜV begins with an audit of the facility, followed by design reviews, test procedure audits and finally, auditing the quality assurance program.

      To begin to grasp why a relay can have an SIL rating, we have to be aware of the required qualifications according to IEC 61508. For sake of simplicity, a device or process that qualifies for SIL receives input, solves logic and provides output based on logic in a reliable, predictable manner (Fig. 1).

      How the SIL-3-rated relay works

      Discrete components create the circuitry required to monitor the input and output and solve logic based on status. The outputs are electromechanical relays and are the heart of the SIL-3-rated relay. These electromechanical relays are force-guided (positively driven) contacts connected in series with the coils for the series contacts controlled in parallel for redundancy. At a minimum, two channels of this series combination are provided to the user.

      Functionality of these force-guided contacts must be described to see the value being presented. The value begins with knowing that the minimum configuration for a force-guided contact is one normally closed contact and one normally open contact that are mechanically linked. It’s not possible that the 'NO’ and 'NC’ contacts are in the same state. If the 'NO’ contact welds, the 'NC’ contact cannot close when the relay is de-energized (Fig. 2).

      In regard to reliability and safety, this functionality becomes apparent when two 'NO’ contacts from force-guided relays are wired in series, and the 'NC’ contact is monitored. Figure 3 shows how to make sense of the monitoring ability when an 'NO’ contact in the series connection has not opened (welded). In this case, the logic must determine that the series connection should not be energized again due to the fact that the redundancy has been defeated.

      The logic in the SIL-3-rated relays monitors the series combination of the force-guided contacts. Based on input from a SIL-rated PLC, DCS or other controller, the logic determines if it is safe to energize the outputs. This ability to receive input and to determine if the output should be energized qualifies this relay as a miniature SIL-rated relay system. If the relay made the decision not to energize, the controllers can base additional decisions on the status contact provided by the SIL-rated relay to determine the next steps.

      In addition to the proper functional aspects, SIL 3-rated relays have physical improvements over standard relays. These physical advantages also lead to increased reliability. The reliability increase is not only desirable for hazardous locations but is desired for industrial machines in general.

      Applications for the SIL 3-rated relay include:

      • As a reliable, predictable relay

      • Use as an interposing relay on 24-V outputs from SIL-rated PLCs, SIL-rated DCSs or other SIL-rated controllers

      • Monitoring shaft rotation

      • Emergency stop or safety door.

        • Even with shortcomings, using series-wired contacts from standard relays has been considered 'accepted’ industry practice. However, 'best’ industry practice should lend itself to the best available approach. The approach has become more versatile with a relay system that can be configured with redundant or double-redundant (quadruplicate) series contacts. Again, the SIL-3-rating itself provides audited and tested reliability/safety.

          Finally, integrating SIL-rated devices into the overall system will lessen the responsibility/liability of in-house safety calculations, which is possibly the most important result of their use. It makes sense to take advantage of the manufacturer’s willingness to accept responsibility for the SIL calculation of their specific device.

          For these reasons, a new 'accepted industry practice’ has been redefined for the integration of relays in systems requiring SIL-ratings.


          <table ID = 'id3008833-0-table' CELLSPACING = '0' CELLPADDING = '2' WIDTH = '100%' BORDER = '0'><tbody ID = 'id3002853-0-tbody'><tr ID = 'id3002855-0-tr'><td ID = 'id3008753-0-td' CLASS = 'table' STYLE = 'background-color: #EEEEEE'> Author Information </td></tr><tr ID = 'id3001571-3-tr'><td ID = 'id3001573-3-td' CLASS = 'table'> Mike Garrick is lead product specialist for the interface product line at Phoenix Contact, Inc., Harrisburg, PA. He is responsible for power supplies, relays and systems cabling. Garrick was formerly a lead engineer in Phoenix Contact’s automation group. </td></tr></tbody></table>


No comments
The Top Plant program honors outstanding manufacturing facilities in North America. View the 2015 Top Plant.
The Product of the Year program recognizes products newly released in the manufacturing industries.
Each year, a panel of Control Engineering and Plant Engineering editors and industry expert judges select the System Integrator of the Year Award winners in three categories.
2017 Lubrication Guide; Software tools; Microgrids and energy strategies; Use robots effectively
Prescriptive maintenance; Hannover Messe 2017 recap; Reduce welding errors
Safety standards and electrical test instruments; Product of the Year winners; Easy and safe electrical design
Infrastructure for natural gas expansion; Artificial lift methods; Disruptive technology and fugitive gas emissions
Mobility as the means to offshore innovation; Preventing another Deepwater Horizon; ROVs as subsea robots; SCADA and the radio spectrum
Future of oil and gas projects; Reservoir models; The importance of SCADA to oil and gas
Diagnostic functions for system safety; Specifying industrial enclosures; Effective decision support for a crisis
Transformers; Electrical system design; Selecting and sizing transformers; Grounded and ungrounded system design, Paralleling generator systems
Natural gas for tomorrow's fleets; Colleges and universities moving to CHP; Power and steam and frozen foods

Annual Salary Survey

Before the calendar turned, 2016 already had the makings of a pivotal year for manufacturing, and for the world.

There were the big events for the year, including the United States as Partner Country at Hannover Messe in April and the 2016 International Manufacturing Technology Show in Chicago in September. There's also the matter of the U.S. presidential elections in November, which promise to shape policy in manufacturing for years to come.

But the year started with global economic turmoil, as a slowdown in Chinese manufacturing triggered a worldwide stock hiccup that sent values plummeting. The continued plunge in world oil prices has resulted in a slowdown in exploration and, by extension, the manufacture of exploration equipment.

Read more: 2015 Salary Survey

Maintenance and reliability tips and best practices from the maintenance and reliability coaches at Allied Reliability Group.
The One Voice for Manufacturing blog reports on federal public policy issues impacting the manufacturing sector. One Voice is a joint effort by the National Tooling and Machining...
The Society for Maintenance and Reliability Professionals an organization devoted...
Join this ongoing discussion of machine guarding topics, including solutions assessments, regulatory compliance, gap analysis...
IMS Research, recently acquired by IHS Inc., is a leading independent supplier of market research and consultancy to the global electronics industry.
Maintenance is not optional in manufacturing. It’s a profit center, driving productivity and uptime while reducing overall repair costs.
The Lachance on CMMS blog is about current maintenance topics. Blogger Paul Lachance is president and chief technology officer for Smartware Group.
The maintenance journey has been a long, slow trek for most manufacturers and has gone from preventive maintenance to predictive maintenance.
Featured articles highlight technologies that enable the Industrial Internet of Things, IIoT-related products and strategies to get data more easily to the user.
This digital report will explore several aspects of how IIoT will transform manufacturing in the coming years.
Maintenance Manager; California Oils Corp.
Associate, Electrical Engineering; Wood Harbinger
Control Systems Engineer; Robert Bosch Corp.
This course focuses on climate analysis, appropriateness of cooling system selection, and combining cooling systems.
This course will help identify and reveal electrical hazards and identify the solutions to implementing and maintaining a safe work environment.
This course explains how maintaining power and communication systems through emergency power-generation systems is critical.
click me