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A safety programmable logic controller is a dedicated controller specially built to meet certain requirements such as the ISO 13849-1, IEC 62061 and IEC 61508 functional directives that define the Integrity Level or the risk reduction level enabled by a particular function. There are several important differences at the hardware level between the physical controller and the logic controller. The main difference between the two is the presence of self-checking and redundancy mechanisms in the programmable logic controllers that are not found in the standard programmable logic controller.

Another difference is in the input and output stage. In the case of the former, a safety programmable logic controller continuously keeps track of the input status to identify and malfunctions or failures that could indicate a problem in the machine itself or sometime in the future. In the latter case, there is additional safety circuitry between a device and the output meant to limit the damage to the external device in the event of any fault of malfunction.

So how can it be decided if an organisation needs a regular programmable logic controller or a safety programmable logic controller? Typically, the machine safety requirements are understood in one of two ways; by the safety regulations governing machines in the country where it is going to be used or on the basis of the machine type designed. For e.g.- there are substantial machine safety requirement differences between the U.S. and the E.U. or between the E.U. and China. Even machines deployed in the food and beverage industry, a stamping press controller, or an oil refinery controller have different safety requirements that must be taken into account.

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The four main categories available in the safety programmable logic controller are –

  • Category 1 – Lowest category safety circuit. Single channel relay to safety
  • Category 2 – Two channel safety circuit. The first channel is hardwired to an e-stop chain while the second channel can be either the PLC monitoring on safety switch auxiliary contact or another hardwire
  • Category 3 – Two hardwired channels in a safety circuit. Those drives that have a ‘safe off’ feature are included in this category
  • Category 4 – Two hardwired channels for a safety circuit with a safety relay that is self-monitoring. Category 4 is the toughest category and is the most popular in the automobile industry.

Safety Programmable Logic Controller Market: Advantages and disadvantages of safety programmable logic controllers

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The main advantages of a safety programmable logic controller are –

  • Major savings in the field wiring, particularly in safety categories 3 and 4
  • Highly flexible and scalable
  • Remote I/O capability with an added element of safety
  • Standard and safety controls in one unit
  • Adequate diagnostic capability
  • Perfect for many e-stop zone applications

There few disadvantages of safety programmable logic controller are –

  • Higher upfront cost
  • Requires a certain degree of programming
  • Slightly complex for some applications
  • Requires maintenance experts with specialized skills



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