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Standard PLC or Safety control system?

Controlling a safety switch with the standard PLC or a safety control system?

Selecting the right safety switch and interlock for machinery and equipment is essential to ensure the safety of operators. The method of controlling the interlock is often discussed, with the question of whether it should be done from a standard PLC or with a safety system like a safety PLC or safety relay. In this text, we discuss the role of the risk assessment in selecting the appropriate safety switch, the necessity of a separate safety function for the interlock, and how the required safety level ultimately determines the control method.

The role of the Risk Assessment

Standard PLC or Safety Control System?

Controlling a safety switch by the PLC or a safety control system. How to control the interlock of a safety switch is frequently discussed. Can this be done from a standard PLC or should it be done with a safety system like a safety PLC or a safety relay?

Risk Assessment
When selecting the right safety switch, one of the first questions you should ask is: Is an interlock necessary? This depends on the risk assessment, so it is important to establish this first. There can be various reasons why a safety interlock is necessary. For example, when an operator has the possibility of being in danger before the danger can be eliminated. In this situation, a safety switch with interlock can be used to stop the movements of, for example, a mixer, saw blades or hydraulic presses before the operator is in danger.

Even if the risk assessment shows that an interlock is not necessary, you can still choose to have an interlock. This can, for example, be done to promote the work process. In this case, the interlock does not need to be fail-safe, and the machine can be stopped at a desired moment with a process interlock, so that it can be quickly restarted after maintenance or other operations without disruptions or the loss of materials in the production process.

Standard PLC Controller has a low safety level
But now we come back to the question; is it better to control the interlock from the standard PLC or with a safety system? The problem with controlling from the standard PLC is that a standard PLC has a very low safety level Performance Level (PL: ISO 13849-1) or Safety Integrity Level (SIL: IEC 62061).

2 Safety Functions = 2 Safety Levels
According to ISO 14119:2013 article 8.4, the interlock in the switch is a separate safety function. To guarantee a higher safety level, the interlock must also be assigned a safety level in addition to the safety switch.

The moment the switch has an interlock, it has two functions: One, opening the door whereby the machine is switched off and two, locking the door after the request to let the operator wait until the machine is stopped.

The safety level determines the control unit
The control unit is ultimately determined by the required safety level. In many cases, the safety level of the interlock will be lower than the safety level of the switch, because the likelihood of both the safety switch and the interlock failing simultaneously is extremely low.

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