Compare between safety system by safety relays and safety system by safety PLC

As safety control systems become more common in machine/industrial environments, different solutions are becoming available. This allows for more precise safety control systems in specific applications, but it also makes selecting a safety control system more difficult. At issue are an understanding of what architectures are available, what applications best suit those architectures, and what costs are involved. This article explores each of these criteria to help the reader make more informed decisions when designing a safety control system.

A variety of architectures are available with which to build an appropriate safety control system:

?Component systems

The most basic system consists of a control device and an actuating device (example: an e-stop button that opens a safety relay coil). These systems offer the most rudimentary functionality and are the most cost-effective solutions for low-risk applications.

?Dedicated safety relay systems

So called because they are hardwired, application-specific controls, dedicated safety relay systems are designed to interface with particular safety components. Some are intended for use with e-stops, for example, while others are created to work specifically with light curtains.

Most dedicated safety relay systems perform simple logic functions, like timing and muting, while monitoring the performance of the safety system. They provide basic diagnostics via front panel LEDs and auxiliary contacts connected to a PLC or indicator lamp. Dedicated relay systems typically offer a very small number of I/O points - all of them digital - and are ideal for medium- to high-risk safety applications.

Expandable safety relay systems

This relatively new architecture helps bridge the gap between dedicated safety relay systems and safety PLCs by offering plug and play expandable digital I/O connected to a single base relay module for additional flexibility. Because they are microprocessor based, they also boast enhanced diagnostics and communication functionality. This means that they can deliver output and error status over a fieldbus network to an operator panel or other device.

As with dedicated systems, expandable safety relay systems are used in medium to high-risk safety applications.

Safety PLCs

Safety PLCs offer additional benefits to a safety system architecture. Perhaps one of the biggest benefits is the flexibility afforded by the safety PLC's inherent programmability. Modifying an application is as easy as changing the program on one's laptop and uploading it to the controller. Safety PLCs offer the highest I/O count of all - up to several hundred digital I/O points, if necessary. They are also the only solution if analog I/O is required as part of the system. Safety PLCs are a cost-effective solution for systems requiring large numbers of I/O points, whether they be centralized or distributed (fully distributed safety systems are achievable today through the use of safety networks). Safety PLCs can be applied in a variety of ways, but are most often used in complex, high-risk applications.

Conclusion:

Safety relays and safety programmable logic controllers (PLCs) are two types of safety systems commonly used in industrial automation and control. Both types of systems are designed to ensure the safety of workers, machinery, and equipment by monitoring and controlling various safety-related functions.

Safety relays are electromechanical devices that operate on the principle of opening and closing circuits based on certain conditions. They are simple, cost-effective, and reliable devices that are commonly used in small and medium-sized machines and applications. Safety relays are typically used for basic safety functions such as emergency stop, two-hand control, and light curtains.

On the other hand, safety PLCs are computer-based control systems that use specialized hardware and software to monitor and control safety-related functions. They are more flexible and powerful than safety relays, and can be used to control complex safety functions and processes. Safety PLCs can also integrate with other control systems and devices, making them ideal for large-scale industrial applications.

In terms of performance, safety PLCs offer better performance than safety relays. They have faster response times, more precise control, and can handle more complex safety functions. Safety PLCs also offer advanced diagnostic and troubleshooting features, which can help identify and address safety issues more quickly.

However, safety PLCs are typically more expensive than safety relays, and require specialized skills and knowledge to program and maintain. Safety relays are easier to install and maintain, and are often preferred for small-scale applications and projects with tight budgets.

In summary, safety relays are simple, cost-effective, and reliable safety devices that are suitable for small and medium-sized applications. Safety PLCs, on the other hand, offer advanced features and performance, making them ideal for large-scale industrial applications and complex safety functions. The choice between safety relays and safety PLCs ultimately depends on the specific safety requirements, budget, and technical expertise of the application or project.

Credit(s)

Rockwell Automation and Mostafa Sayed