A safety programmable logic controller (PLC) is same as like a standard PLC. It is mostly used to control and automate pieces of industrial equipment. A safety PLC also supports all the applications that a standard PLC does; however, a safety PLC consist of integrated safety functions that allow it to control safety systems.
The primary objective of safety PLC is: Do not fail, and, if unavoidable, fail only in a predictable safe way. Safety PLC have the ability to achieve these objectives through its redundant microprocessors, eliminating the need for safety relays to create redundancy. It also has inbuilt diagnostics allows continuous input and output monitoring. A safe shut down of the PLC will occur whenever an internal fault or failure be detected.
A PLC to be said as a safety PLC, it must satisfy a set of demanding international standards, specifically IEC 61508 – Functional Safety of Electrical/Electronic/Programmable Electronic Safety-related Systems. The standard design in PLC covers design methods, and the testing of software and hardware of PLC. It also acknowledge the zero-risk can never be achieved, but that non- bearable risks must be reduced as low as reasonably practicable. It is very important that a safety PLC have a high level of diagnostic coverage, it is a most important part of assuring whether a safety PLC is safe.
For a safety PLC to achieve a safety integrity level (SIL) of three, it must be capable of detecting over 99% of potential failures. Some tests are required to determine the diagnostic coverage involves some tests such us data verification and program flow control. Such type of tests ensures the PLC stores critical data and verifies whether the internal functions, that the PLC executes are in the correct manner. In addition to that the safety PLC must undergo software-fault injection testing. This is where the programs are corrupted and downloaded into the PLC to verify that the PLC responds in a safe way.
Advantages of a safety PLC compared to a standard PLC
A safety PLC gives us safety control and standard control whereas a standard PLC only provides the latter. When we uses a safety PLC to control a safety system, time and money can be saved on the field wiring because there is no need for safety relays. So, a safety system designed around a safety PLC is extremely reliable and flexible. It is easy to modify because it requires only changes in programming but does not need to do any wiring changes or additional relays. Once the safety system has been installed, tested and validated, the safety program in the PLC can be locked and password protected to prevent unauthorized manipulations. Some of the models even support integrated motion over Ethernet, and can be used to initiate safe torque-off function on different types of variable frequency drives (VFDs).
However, safety PLCs also have some disadvantages as well. The cost is higher, so this type of PLC would not be ideal for smaller and simpler applications. An example would be a system that only requires one E-stop button and a light curtain. It is easier to simply use a safety relay and associated hard wiring for the safety system. Another important thing is that safety PLCs are still a relatively new product with latest technology. Engineers and maintenance technicians are unfamiliar with these latest technology so additional training may be needed for safety PLCs to be used correctly.
Most of the people don’t fully trust the reliability and have doubts regarding the integrity ofthe safety system. While safety PLCs are still new concept, they have already found their own way into safety applications in industries all over the world if take in kerala,then PLC Automation In Cochin. By doing strict certification processes to ensure security,reliability and to minimize risks. Offering many advantages that can make designing a safety system that is much easier and less complex, they might have drawbacks as well, so it is important to determine whether or not the use of a safety PLC is appropriate for the application.