Go for safety relays if you have simple, low-cost, and fixed-function applications. But upgrading to the ABB AC500-S will be wise in case of high I/O counts, complex logic, and flexible zoning and advanced diagnostics needs.
Today, safety standards like the SIL3/PLe are becoming more common. So, engineers are faced with the task of deciding between traditional hardwired safety relays and modern safety PLCs like the ABB AC500-S.
Safety relays are suitable for simple and localised safety functions. On the other hand, dedicated safety PLCs are great for complex, networked, or scalable systems that require advanced diagnostics, complex zoning, or extensive communication. If you want to learn more about when to choose between dedicated safety PLCs and safety relays, then keep reading.
The Case for Safety Relays: Simple & Localized
Safety relays serve as a dedicated and hardwired solution for monitoring safety devices. They help ensure reliable and fail-safe machine operation. They are ideal for use in simple, standalone machinery that features limited safety functions.
You will find safety relays used mostly in simple, localised applications like one E-stop, one light curtain, or one guard. Here are the advantages and limitations of a traditional safety relay.
| Advantages | Limitations |
| Cost-Effective: You will find it to be a low initial hardware investment for small and simple systems. | Scalability Issues: You will find that complexity exponentially increases with every added sensor. More relays and wiring will be required. |
| No Programming: It operates based on hardwired logic. So, electricians lacking software knowledge will find it intuitive. | Troubleshooting: It can be time-consuming to find issues in hardwired systems, compared to software-based diagnostics available in the ABB AC500-S. |
| Reliable Protection: You will find built-in self-monitoring features for instantly detecting faults like wire breaks or welded contacts. | Inflexibility: If you wish to make changes to safety logic, then physical rewiring will be required. You cannot do that through software updates like in dedicated safety PLCs. |
Table 1: Pros and Cons of Hardwired Safety Relays
The Case for Safety PLC (AC500-S): Complex & Scalable
You can see the ABB AC500-S safety PLC as a premier, scalable solution for managing complex safety architectures, especially in large machines and production lines. Thus, you will get the SIL 3/PLe compliance with dedicated safety PLCs.
Safety PLCs are best suited for advanced functional safety tasks, such as speed monitoring, muting, and complex zone control. You will find it ideal for enhancing safety and productivity in fast-paced, demanding industrial environments. Here are its key advantages:
Scalability
You can effortlessly scale safety PLCs like the ABB AC500-S to handle hundreds of I/O points. You won’t have to go through a massive rewiring process as needed in traditional safety relays.
Reduced Panel Space
A dedicated safety PLC like the ABB AC500-S can replace multiple bulky relay modules with a single compact and intelligent controller.
Advanced Diagnostics
A dedicated safety PLC will provide you with precise diagnostics instantly. So, it will become easier to pinpoint the exact gate switch causing a stop, thus reducing costly system downtimes.
Flexibility
Using software like the ABB Automation Builder, you can make rapid logic changes to a dedicated safety PLC like the ABB AC500-S. So, there will be no need for physical rewiring in the cabinet.
Integration
Safety PLCs feature seamless PROFINET/PROFIsafe integration with standard PLCs like ABB AC500, safety drives, and I/O modules. It is about high-speed communication on a single unified network, thus reducing cabling, simplifying architecture, and enabling advanced safety functions.
Cost-Benefit Analysis: The Rule of Thumb
To decide whether to choose safety relays and a safety PLC, you must evaluate between cost and complexity. So, consider these points.
- The Tipping Point: It is a common rule of thumb that if your system requires more than 3 to 5 safety relays, then it will be more cost-effective to switch to a single safety PLC like the ABB AC500-S.
- Total Cost of Ownership: Safety PLCs come with a higher initial hardware or acquisition cost. However, in the long run, your labour needs, writing time, and troubleshooting complexity will be reduced.
- Application Scenarios: Remember that safety relays are best for simple, single-machine setups. Safety PLCs are ideal in complex process plants or multi-axis robots, where future scalability and advanced logic will justify your investment.
You can go for safety relays if you have minimal I/O, a constrained budget, simple logic, and no need for network diagnostics. But choose the ABB AC500-S if your system has a high number of safety sensors, zone control, and safety-over-fieldbus needs, and rapid troubleshooting requirements.
Ready to Switch to a Safety PLC?
Traditional safety relays are ready for basic tasks. However, you will always find the ABB AC500-S a superior choice in complex industrial applications. This is due to its flexibility, advanced diagnostics capability, and long-term cost-efficiency. Your decisions must be based on consideration of both safety and efficiency.
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