SAFETY PROGRAMMABLE LOGIC CONTROLLERS (PLCs)

A safety programmable logic controller (PLC) is similar to a standard PLC.  It can be used to control and automate different industrial equipments. A safety PLC supports all the functions/applications that a standard PLC does moreover, a safety PLC contains integrated safety functions that allow it to control safety systems as well. A safety PLC is designed to accomplish two important objectives: Do not fail, and, if unavoidable, fail only in a predictable safe way. It achieves these objectives through its redundant microprocessors, eliminating the need for safety relays to create redundancy. It also has built-in diagnostics allowing it to continuously monitor inputs and outputs. In case of internal fault or failure detection a safe shutdown of PLC occurs preventing any damages.

For a PLC to be considered a safety PLC, it must meet a set of rigorous international standards, specifically IEC 61508 – Functional Safety of Electrical/Electronic/Programmable Electronic Safety-related Systems.

For a safety PLC to achieve a safety integrity level (SIL) of three (3), it must be capable of detecting over 99% of potential failures. Some of the testing required determining the diagnostic coverage involves a series of tests like data verification and program flow control. These tests ensure that the PLC stores critical data and verifies the internal functions the PLC executes are in the correct order. In addition, a safety PLCs must undergo software-fault injection testing (carried out to check the healthiness of PLC).

Advantages of a safety PLC compared to a standard PLC:

·         A safety PLC allows for safety control and standard control while a standard PLC only allows the latter.

·         By using a safety PLC to control a safety system, time and money are saved on field wiring since there is no need for safety relays. As a result, a safety system designed around a safety PLC is extremely flexible.

·         It is easy to modify because it simply requires programming changes but does not require any wiring changes or additional relays.

·         Once the safety system has been tested and validated, the safety program in the PLC can be locked and password protected to prevent unauthorized variations. Certain models even support integrated motion over Ethernet, and can be used to initiate safe torque-off function on various types of variable frequency drives (VFDs).

Disadvantages:

·         The upfront cost is higher, so this type of PLC would not be ideal for smaller, simpler applications.

·         Safety PLCs are still a relatively new product with new technology. Engineers and maintenance technicians are unfamiliar with the newer technology so additional training may be necessary for safety PLCs to be used correctly.

·         Delay time in purchasing from local manufacturers as these PLC modules are not easily available at local vendors.

Safety PLCs are still a relatively new concept; they have already found their way into safety applications in industries around the world.