February 21, 2018
The fundamental question when it comes to control systems is what type of PLC is most appropriate for your circumstances: a safety PLC or a regular PLC? How do you choose?
A PLC is the cornerstone of your manufacturing automation. It is the brains recording information from the sensors and telling the subassemblies what to do and when. Occupational safety is paramount with an automated manufacturing line, but there are many different ways to implement proper safety protocol in any given situation. Proper protocol is dictated by standards such as IEC 62061, ISO 13849-1 or IEC 61508, and the resulting safety circuit category that the equipment falls into.
Let’s back up a moment. What does “safety” mean in a control system? In one word, redundancy. The system designer must create a plan to safely shut down the machine in case of unexpected events, such as the failure of a subassembly, unexpected input, fault, etc. This redundancy can be created manually with a regular PLC and wired safety sensors, or in a more streamlined way with a safety PLC.
The decision to choose a regular PLC or safety PLC has three components: safety circuit category, system complexity and cost. When using a safety circuit rather than physical guarding as the mechanism to protect human operators and machinery, the designer must first decide which category is required for the application. There are four categories for safety circuits, detailed here.
As previously mentioned, the safety circuit function can either be accomplished by using safety relays and a regular PLC, or by combining the two into one unit, the safety PLC. The safety PLC differs from a regular PLC in that it contains self-checking mechanisms and redundancy. With a safety PLC, the safety protocol is handled by technology such as Siemens’ PROFIsafe instead of by field wiring. Because of this, field wiring and installation costs tend to be much lower than with a regular PLC.
The safety PLC also treats inputs and outputs differently than would a regular PLC. Inputs are continuously monitored in a safety PLC, and additional circuits exist on the outputs that protect the safety PLC in the event of a failure down the line. Troubleshooting is greatly reduced because the status of each networked safety device is displayed on the same HMI as the automation statuses.
If your system has low complexity and a low-number safety circuit category, using a regular PLC and safety relays may be the most cost effective option. For this category, using a Siemens S71200 Safety PLC instead of a regular PLC offers the benefits of low cost and easy expansion if reconfiguration is required in the future. The scale tips rapidly towards the safety PLC once the automated system complexity or safety circuit category number rises.
Regular PLC + Safety Relays
PROS:
- Lower initial cost
- Best choice for simple systems
CONS:
- No redundancy is built into the device, it must be addressed using relays
- Cost of field wiring depends on complexity, could be significant
- More difficult troubleshooting
- Expansion of the system requires tedious and potentially costly rewiring
Safety PLC
PROS:
- Redundancy is built into the device
- No need for safety relays
- Lower electrical and installation costs
- Safety messages are handled by a safety protocol such as PROFIsafe instead of by field wiring
- Inputs are continuously monitored
- Circuits are in place to protect from output failure down the line
- Troubleshooting is greatly reduced
- Scalable for easy expansion of the system
CONS:
- Higher initial cost
- Potentially unnecessary for low-complexity applications
Siemens is a clear technology leader and Siemens’ safety PLCs are arguably the most advanced safety PLCs available. For a deeper dive into safety PLCs see the article ‘Be Not Afraid — the Time has come to Trust Safety rated PLCs’ written by Tim Palmer, Product Manager at Siemens Industry US. Contact RAVEN’s experts to help guide you through the decision process to best meet your specific application.
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