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The One Where Everyone Wins:

A Mutually Beneficial Contracting Method

As machine tools and process operations become more complex, the shortcomings of traditional safety relays become more obvious.

Shrinking budgets and growing to-do lists are all too common in the world of construction. Owners struggle to meet budgets and timelines while satisfying transparency goals. Traditional procurement models, such as price-based contracts; design, bid, and build; time and materials; and construction manager at-risk, typically don’t emphasize healthy relationships and partnerships between owners and contractors. Job Order Contracting is a great alternative that fosters mutually beneficial relationships while addressing common project challenges.

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Evolving from Safety Relays to Safety PLCs

Want to dive deeper about the benefits and advantages of switching away from traditional safety-control systems to today’s safety PLCs solutions?

Safety Relays

Traditional safety-control systems.

Single-Function Safety Relays

Single-function safety relays ensure proper working of the safety function, keeping both people and equipment well protected. They interface between the control and the actuating device. When you look only at the cost of safety relays, they can be a low-cost solution in limited applications, typically controlling three or fewer safety functions/devices. They are simple to operate and have a clear, predefined structure with fixed functionality.

Typical Applications for Single-Function Safety Relays

  • Stopping Movement in a Controlled and Safe Manner
  • Monitoring the Position of Movable Guards
  • Interrupting a Closing Movement During Access
  • Emergency Off/Stop

Typical Shortcomings of Single-Function Safety Relays

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Difficult troubleshooting due to complex wiring between each device.

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Requires significant engineering for system modifications or upgrades.

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Inability to operate in mixed modes (safety and standard).

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Tedious functionality changes because of each relay’s defined purpose.

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Additional safety functionality requires additional safety relays.


The Logic is Done by Wiring

Despite their limitations, safety relays are widely used, are commonly built into new equipment, and could be the right choice in simple applications. This would be based on the risk assessment for the machine or application. The relays are typically connected to a standard PLC to control the safety application. It’s worth noting that the need for a safety relay is eliminated entirely when safety PLCs are used instead of safety relays.

Learn about the benefits and advantages of switching to safety PLCs.

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Multi-Function Safety Relays

This solution represents a configurable device with a few more inputs and outputs. As a rule of thumb, a programmable safety relay can control between two or three safety functions, representing an incremental flexibility improvement over a single-function safety relay. Additionally, they have a smaller footprint that enables more-compact control panels or cabinets and will reduce and simplify wiring. That simplicity, though, is dependent on the safety functionality required. Multi-function safety relays are available in a number of styles and configurations.

Typical Applications for Multi-Function Safety Relays

  • A simple base unit can handle a variety of inputs.
  • Most require no software for configuration or operation. Typical setup and maintenance is simple (but with no flexibility).
  • Multi-function safety relays are often a choice when single-function relays can’t meet the safety functionality.
  • Suitable for systems with more-complex requirements such as machine tools or robots with many I/O points or a high number of safety-related tasks.

Typical Shortcomings of Multi-Function Safety Relays

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Cost and complexity of hard-wiring control devices.

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Difficulty in expanding or modifying the control system.

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Time-consuming processes to troubleshoot faults.

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Limited safety logic due to predefined safety relay functionality.

Safety PLCs

Safety-control solutions for today’s complex systems.

Today’s Safety PLCs

As machine tools, automation systems, and process facilities have grown increasingly complex, so have the demands on the safety-control systems. Compared to the past, there is often a need today to monitor a much larger number of input and output (I/O) points.

Safety PLCs offer a distinct advantage in comparison to safety relays; they provide both the standard and safety functionality in a single controller. The fact that both the standard and safety-related programs can be executed via a single controller simplifies the system, decreases design time, and considerably reduces the panel size.

Benefits of Safety PLCs

Meets Safety Standards
These PLCs meet the required safety standards, able to achieve up to PL e as per ISO 13849-1 or SIL 3 as per IEC 61508.
Safety-functionality changes can be easily done via programming rather than device replacement and related rewiring.
Safety-Function Libraries
TÜV-certified, safety-function libraries are available, significantly reducing the design time and increasing flexibility.


Pat Raines, Co-Founder, Gate City Brewing Co., Roswell, GA.

Digitalization helps craft breweries establish a competitive edge.

Today’s craft beer producers face many challenges, including aging infrastructures, sourcing raw materials, and navigating government standards and regulations. They must also achieve high productivity and operational flexibility to ensure their new products achieve faster time-to-market to meet ever-changing customer expectations. Product development becomes a key differentiator for successful firms in this market space.

Learn why digitalization technologies matter to brewers.

Make an Informed Decision Regarding the Right Safety Solution

The cost and productivity advantage in many, and probably most, applications of safety PLCs over older technology is clear and measurable. OEMs and panel builders should consider a number of factors when exploring the potential benefits in their applications.

One of the first things to consider is the number of I/O points required. In applications with more than three safety functions, the safety PLC is almost always the right choice. The simple economy of a single, multi-function device rather than a device for each I/O makes the PLC a clear choice in these applications.

Guidelines supporting a transition to safety PLCs:

Panel builders and OEMs facing pressures to bring products to market more quickly, or to deliver products to customers within a tight timeframe, can benefit from shortened system design times. Product engineers are less constrained by selection of parameters because of the flexibility of the safety PLC. This technology offers numerous possibilities to meet safety functionality requirements across a multitude of applications. Rather than sizing components based on specific parameters, they can specify a safety PLC be able to operate within a wide range of application parameters.

End-users also benefit greatly from this advantage of flexibility when modifying or expanding their existing system, or when combining multiple machines.

Operators of some production or process operations can tolerate downtime better than others. Taking a machine off line for several hours to troubleshoot or modify the control system in these applications doesn’t present a critical issue.

In most cases, though, downtime must be avoided at all costs. In these situations, the built-in, advanced diagnostics provided by safety PLCs can be incredibly valuable. Maintenance personnel and operators are instantly guided to the exact location of the fault, enabling rapid restoration and return to productive operations. And the programmability enables much shorter downtimes for system maintenance or modification.

Some safety-control systems are relatively static. Once designed and installed, it’s highly unlikely they will be touched aside from routine maintenance. Other systems, though, are likely to be modified or expanded to incorporate process changes or increased production demands. In these more-dynamic applications, the ability to modify functionality solely via reprogramming the device software makes the case for safety PLCs very strong.

“While each safety switching device has a specific function, the parameters can change frequently; That’s something that wouldn’t be feasible with conventional hard-wired safety system. And the installation process is also much more economical, as it reduces the time and labor required by 25 percent.”

– Dean Colwell, Controls Engineering Manager for Assembly and Welding Systems, Fori Automation.

Cost Considerations

When comparing technologies, cost is always a consideration. The ability of a safety PLC to perform the task of multiple single-function safety relays creates a straightforward cost-benefit analysis. In a basic application incorporating two to three safety relays with the standard PLC, the cost of the safety PLC can be initially about 30% less. Additional savings are realized over time as a result of the increased flexibility and simplicity of the safety PLC. As the number of IOs increase, both the initial and on-going savings multiply.

Safety PLCs can replace multiple separate relays performing various functions, which reduces component costs. Built-in diagnostics reduces the time required to troubleshoot, repair, modify, or upgrade safety PLC-based systems, providing additional ongoing savings and benefits. And safety PLCs can enable wireless-device communications.

OEMs and panel builders interested in providing their customers with more-flexible, lower-cost products would definitely benefit by including safety PLCs in their automation solutions.

The hardware, including basic and advanced safety controllers, was as much as 40 percent less expensive than our other supplier and software was less than half the cost.”

– Bill Taylor, BWI Group Controls Engineering Supervisor.


Siemens Integrator Helps Small Town Modernize Water Supply While Cutting Costs

Learn about how Kinard Control Systems worked with the town of Tega Cay to provide their citizens with a reliable and stable water supply.

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