Safety PLCs Protect Critical Operations

EP Editorial Staff | September 1, 2021

In today’s complex, automated systems, safety PLCs provide standard and safety control, simplify system architectures, decrease design time, and reduce electrical-panel footprints.

Modern safety PLCs increase uptime by eliminating the need for hardware- and wiring-intensive safety relays, and by simplifying safety system design, commissioning, and maintenance.

By John D’Silva, Siemens Industry

Once a staple of safe machine operation, traditional safety relays are being increasingly replaced by safety PLCs. Safety relays have been around since the early days of industrial-automation technology, and are still widely used today due to their low cost and simplicity. As safety PLCs, however, become more reliable, cost effective, and easier to manage, many machine builders are revamping their standards and transitioning to this advanced solution.

Hesitant to Upgrade

The comparison between safety relays and safety PLCs can be likened to the relationship between incandescent lightbulbs and LEDs. Technology advancements have greatly improved the light quality, extended the life, and reduced the price of LEDs—but the high cost of initial implementation still deters many from making the switch from outdated and inefficient incandescent bulbs.

In the lightbulb example, LEDs save money in the long run, returning their higher upfront price difference in just one year of energy cost savings, while improving sustainability by reducing demand on the power grid. The situation is similar with safety-control systems. Some manufacturers are hesitant to incorporate what they believe to be the more expensive technology represented in safety PLCs, despite guaranteed and measurable operations and maintenance lifecycle savings. Beyond the cost savings, the technology dramatically speeds troubleshooting, reducing downtime.

Relay Shortcomings

Plant maintenance staff often appreciate the straightforward simplicity of safety relays and contactors. However, these components don’t have the flexibility to meet many of the requirements of a modern automation solution.

The nature of how a safety-relay-based system is wired makes it difficult to modify without compromising the safety of the personnel and equipment it’s protecting. Significant engineering and rewiring time is required to make changes, with lots of opportunity for errors.

When these types of modifications are implemented, electrical enclosures originating as a well-organized collection of control components and cables can become a confusing tangle of crisscrossed jumpers and rewires. Future changes become increasingly difficult, and any issues become more challenging to resolve.

All motors, switches, and other control devices that must be accommodated in the safety system typically require an individually hardwired safety relay, creating many potential points of failure to examine when the need for troubleshooting arises. These electro-mechanical relays are more prone to wear and failure than non-mechanized components.

Additionally, safety-relay protection systems may be mistakenly bypassed by operators in the event of a malfunction, compromising the intended protective function.

Safety relays require significant wiring, making modifications and troubleshooting more difficult.
Figures courtesy Siemens

A Modern Solution

Safety PLCs address these and other concerns. Contrasted with the past, there is a need to monitor many more input/output (I/O) points, including those performing safety-related functions. A single safety PLC—also referred to as a safety controller or fail-safe PLC—can control hundreds of safety I/O points, whereas, with a safety-relay system, each point requires a distinct and dedicated relay device.

For today’s complex control and safety systems, safety PLCs are the most capable and cost-efficient devices for performing required safety tasks and complying with regulations. These controllers can provide standard and safety control, simplify system architectures, decrease design time, and reduce electrical-panel footprints.

With software and program version control built into most modern safety PLCs, system changes are easily implemented and traced. These changes can be made by simply modifying a programming routine, and safety function libraries—pre-certified to meet IEC 61508 and ISO 26262 standards for functional safety management, and automotive functional safety management, respectively—reduce testing time, increase flexibility, and ensure proper functionality.

Using safe-ladder logic as one means of safety programming, safety functionality changes can be easily made through programming, rather than relay device replacement and related rewiring. Modifying an application is as simple as entering appropriate credentials for access to the safety program, updating the program, and then loading the updated program to the safety controller.

Additionally, OEMs can create custom libraries for safety functionalities commonly used in their solutions. This provides a high degree of scalability and easy transfer of safety standards to new and retrofitted applications.

When safety PLCs are used in systems instead of standard PLCs, the need for safety relays is eliminated. Safety PLCs incorporate embedded diagnostics for identifying faults in safety and standard circuits, and alerts and information can be presented by an HMI or web server to inform technicians of equipment status. This helps direct them to a fault’s location, enabling quick resolution and rapid return to production.

Safety PLCs can quickly add fail-safe functionality to applications, with no additional hardware required.


An automotive manufacturer recently modernized their assembly lines with rail-guided carts (RGCs), using fail-safe controllers, along with wireless access points and client modules mounted on the carts.

RGCs are autonomous and cannot be connected to a fieldbus system for the transfer of control signals or warnings. By using the wireless PROFIsafe safety-communications protocol, RGCs can be programmed to fail into a safe mode in the event of signal loss.

RGCs in this facility have a safety bumper, an emergency stop switch, and safety scanners, and implementing their uninhibited maneuverability would not be possible using a hard-wired-only safety system. Additionally, installation using a wireless safety communications protocol was more economical compared with installing safety relays and wired carts, as it reduced deployment time and required labor by 25%.

In another application, a manufacturer of decanter centrifuges replaced its primary controller with a safety PLC for its hardwired fail-safe systems. Safety is of high concern when operating the manufacturer’s horizontally rotating decanters, and even more so for its much faster vertically rotating separators. Due to the heavy intrinsic and process weights, combined with radial acceleration reaching as much as 10,000 g, any faults can have disastrous consequences.

To prevent unauthorized changes of operational parameters, access-control levels are synchronized between the PLC and HMI. This limits modifications to the process, depending on whether an operator, maintenance technician, or programmer is making adjustments—with each group only able to access the functions required for their assigned tasks. For example, on some centrifuge systems, an operator can specify safe-operating limits using the HMI display, but only the responsible programmer may access the safety logic.

Reduce System Complexity

Many OEMs, panel builders, and end users continue to use older safety relay technology because it provides necessary functionality in a simple, affordable, and proven device—but some safety-relay users may employ the technology simply because of familiarity. In critical safety systems, the use of safety PLCs enhances fail-safe reliability and can provide cost benefits, especially in systems with several safety I/O points.

Safety PLCs provide OEMs and their end users with the ability to standardize safety programming and manage software versions. Additionally, their use reduces the number of required devices by combining safety and normal control logic in a single controller, and it simplifies wiring and contactor requirements. Incorporating safety PLCs helps boost overall equipment effectiveness and, more important, provides assurance of safe operation at every stage of a machine’s lifecycle. EP

John D’Silva is a professional engineer with more than 28 years of international industrial-automation experience, including 20 years of functional safety in North America. He also works in close coordination with UL, NFPA, RIA, TUV, and other machine-safety-standards organizations.


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