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