Safety PLC

 

26 July, 2020.

In this post, we will understand the use of a safety PLC and compare it with a standard PLC. (Click on the images for a more zoomed view)

Hello programmers. We all know about a standard PLC that we use in our automation solutions. But, there are various systems in industrial automation which require critical safety and zero failure with smart redundancy techniques to prevent it and the personnel working around from damage. Every PLC needs to achieve a SIL (safety integrity level) rating for use. When applying any safety technology, people need to understand applicable safety standards. As more applications integrate automation and safety; SIL ratings, long familiar in process controls, are appearing more readily in machine safety applications. Safety system design is all about mapping out everything ahead of time. Half of failures can be attributed to design. A better design significantly improves overall safety.

What is a Safety PLC?

So, what’s a safety PLC? It is almost similar to a standard PLC that we use normally. But, when a safety PLC fails, it shut downs the system in a sequential and safe way without endangering the system and lives. A safety PLC needs to achieve level 3 of SIL; to do this task. Basically, how can you identify that system? A safety PLC would be appropriate for use in applications that could pose harm to the environment, business, equipment, and especially people, including applications and installations that need to meet IEC 61508 and IEC 61511. These are a set of rigorous international standards.

Let’s have a look at the history. Before a standard PLC, there was an abundant use of safety relays and everything was dependent on proper hardwire circuitry. This system was complex and so, day by day, PLC’s evolved and these hardwired systems gradually started to decline for use. With reading inputs and giving outputs through logic written in it’s processor, the system was made simpler, efficient and reliable to use. But, if the number of IO’s and it’s hardware circuitry related to PLC is increased, it can quickly get difficult. You can end up messing the system if the numbers of emergency stops are increased, but the hardware circuit is designed poorly. Also, a standard PLC has some limitations in diagnostics and status indications. This gave a need to design a much harder and safer system above a normal PLC to operate. And, thus, was born safety PLC. Agreed, that a programmer can program with any level of complexity in a normal PLC so that it can detect any kind of failure and put the equipment in a safe and controlled manner. But, a safety PLC has all the extra diagnostic and redundant features inside it; which makes the system more efficient and safe to use.  

Safety PLC vs Standard PLC:

Simply put; the difference between a standard and a safety PLC can be related to diagnosis, internal architecture, software & firmware, program protection and certification for applications where compliance with a certain SIL level is required. Let’s go deeper into this.

When it comes to diagnostics, it can detect any possible internal fault in the hardware or firmware by use of many diagnostic functions, so that a failure in PLC does not cause any untoward incident. This diagnostic coverage is a must for this PLC to distinguish itself from a standard PLC. The onboard IO’s are redundant and continuously monitored by a watchdog circuit; which prevents any failure or malfunction. This also reduces the number of hardware safety relays which would been used if a normal PLC was implemented.  If you have multiple emergency stop zones/chains, then this PLC is the best. The safety PLC has many internal status bits and words (hardware, program execution and operating system) for troubleshooting and programming.

The internal architecture of this PLC is purposely built, tested and certified to meet international safety standards. It has a very high level of software architecture; which makes it eligible for the use of critical safety applications.

Once the safety system has been tested and validated, the program in the PLC can be locked and password protected to prevent unauthorized changes. This makes the overall system reliable to use.

But, the cost of installing a safety PLC is much higher than a standard PLC. It is not ideal for smaller applications. Installing this PLC requires a highly skilled set of technicians and programmers to implement the overall safety circuit.

So, if you have found, almost all these features are available in a standard PLC. A programmer can make complex programs with the help of available IO’s and various safety network controllers / IO blocks / safety relays for a price much less than a safety PLC. Also, a standard PLC too, has it’s SIL rating (lower than a safety PLC) which is acceptable for an application. But, we distinguish it by just one simple word – extra. This PLC is recommended for larger and critical applications like oil and gas, petrochemicals, refineries, critical chemicals, marine, power plants, incineration, machinery, boiler controls and burner management, and high-pressure applications. The initial installation cost may be high, but it will lifetime secure the system efficiently. It will simplify overall design and also provide it’s re-use in a much flexible way. The installation, startup and support time will be reduced to a great extent.

Many of us are not familiar with this PLC. It will require initially, a very good understanding and training of the system. Once you get familiar, you can safely design such type of critical systems.  

I have covered the general theory related to a safety PLC. I have also not attempted to cover every type of features deeply; you can learn it easily in the software help file or internet. I have just given you an insight of this type of PLC. Learn the basics and explore a new type of study in this type of automation.

 

Thank you guys; I hope you enjoyed reading the practices normally used for this type of study in industrial automation.