With a growing focus on safety and an increasing number of safety products installed on machinery to protect personnel, end users are finding a greater number of safety relays in their control panels. There is a great desire to reduce panel space and wiring, improve communications and increase the automation of all control systems — including safety. This has piqued the interest in safety programmable logic controllers (PLCs) in safety-related systems.
Safety PLCs provide all of the same functionality of traditional safety relays, but offer space savings and improved communications, while also providing the safety levels needed for the protection of personnel. Used primarily in large systems, safety PLCs can provide a greater concentration of safety I/O in a smaller footprint than safety relays, saving control panel space and related interwiring. All of the functionality of safety relays, from emergency stops to light curtains to zero speed control, are provided in safety-certified function blocks. While safety relays are typically rated up to category 4 per EN/ISO 954-1 only, safety PLCs generally include this rating, along with ratings up to performance level “e” per EN/ISO 13849-1 and SIL 3 per IEC 61508. These ratings will allow safety PLCs to be used in most safety circuits.
A variety of communications options are available with safety PLCs. Some communicate safety-related information via the backplane of the PLC rack and through the cables connecting the various PLC racks, but external communications are typically not safety rated. Others provide safety communications only between the safety PLC processor and remote I/O via a certified safety communications network, and external communications are also non-safety rated. Still others have communications networks that can carry safety and non-safety information on the same cable at the same time. The latter systems can either be used for safety only, non-safety only, or a combination of safety and non-safety-rated communications simultaneously. This allows the user to choose between using one network for both safety and standard control system communications, or separate networks for safety and control running independently from each other – in short, whichever method is the best fit for their application. All safety PLCs have communications networks available that are not rated for safety but are used for non-safety-rated communications such as diagnostics, allowing them to communicate to other standard PLCs in the system.
This flexibility is important, as many times a user will want to upgrade their safety systems but not disturb the existing control system which is running well. The control system may or may not communicate seamlessly with the safety communications of the safety PLC chosen for the upgrade. They may want to choose a safety PLC that can run an independent safety network amongst all of the safety components and then communicate the data and diagnostics separately to the system to keep the two systems separate. All of these networks can allow improved communications between the safety and control systems, as well as to other supervisory controls. Improved communications along with advanced diagnostics make these safety PLC systems easier to troubleshoot and monitor.
The safety PLCs’ software provides users opportunities as well. Some safety PLCs utilize the same software to program the standard control system as well as the safety-related portions of the control system. Users can appreciate the convenience of being able to program all of the control with the same programming language and software, as there is no new software for technicians to master. The same programming also allows the embedding of safety-related functionality into the rest of the automation and control system. The user does, however, need to make sure the hardware is “non-interfering” and does not have any negative impact on the safety-related components and instructions.
Some safety PLCs have programming software that operates separately from the rest of the standard control system, and for many users and OEMs, this is the preferred method for the safety system. They want to minimize interactivity between the safety system and the rest of the control system, and also want to make sure that if someone gains access to the standard control system and is able to make changes to it, they will be unable to make any changes to the safety system. Once they have designed the safety system, they feel there should not be any changes made to it, and a separate software system with a different software package helps ensure this.
While many are interested in safety automation and safety PLCs, implementing them can present challenges. Some small- to medium-sized systems may have safety circuits that are not large enough to justify a full safety PLC due to the number of I/O and installed cost – for these systems a safety relay or safety controller may be the most appropriate, cost-effective solution. For others, the standard control system is running well, and they cannot justify the cost of replacing all of the existing control with a safety PLC solution that integrates safety and control with the same software package and communications network. The solution here is a separate safety PLC system just for the safety function. Still, there are other PLC automation systems that can run standard control, but to which the safety component can be added at a later time while using the same software and some of the same hardware.
