Machine standards PL & SIL
Safety products often have to meet certain requirements. To test the safety level of these products, people use Safety Integrity Level (SIL) or Performance Level (PL), for example. From the Machinery Directive (2006/42/EC), these two standards came, among others, EN IEC 62061 and ISO 13849-1. From these, PL and SIL emerged. But when do you use which standard?
Performance Level
Safety Integrity Level
PL also known as performance Level, is used to classify electrical, mechanical, pneumatic, and hydraulic safety solutions. There are five levels at PL ‘a’ to ‘e’. In terms of PFHD (probability of a dangerous failure per hour) values, the five PL levels (a-e) correspond to certain ranges. These indicate the probability of a hazardous failure occurring in the course of an hour. For example, PLe products will have the fewest accidents per hour. Note this is a simplification, more is needed to calculate PL. For example, you need to look at diagnostic coverage of the system, the Mean Time To Dangerous Failure of the component, and the organisation of the system (category B, 1-4).
The PFHD value can be easily determined via this chart:
Severity of effects: S1 = Recoverable, mild S2 = Irreparable, severe or death Frequency and/or exposure to hazard F1 = Rare to less frequent and/or exposure time is short F2 = Frequent to continuous and/or exposure time is long Possibility of preventing or reducing harm P1 = Possible under certain conditions P2 = Barely possible
SIL, Safety Intergrity Level, is used in electrical and electronic systems, also we look not only at the frequency of accidents, but also at the severity of accidents. When determining the SIL level, you can use a risk matrix (LOPA).
- The scenario for loss of containment should be established. This usually comes from the HAZOP study*.
- The cause (the initial event) and consequence of a scenario should be clear. These usually also come from the HAZOP study.
- It is important to establish the frequency of the initial event, which is the trigger.
- It is necessary to establish the acceptable incidence frequency (tolerable risk). This often depends on how serious the expected outcome might be. A risk matrix is often used to write down or record the acceptable risk.
- Current layers of protection (LOPs) are examined. Sometimes the term independent layer of protection (IPL) is used. The LOPs should be sufficiently separated from each other and from the initiating event of the scenario. The LOPs should also respond and perform well. The scenario should be able to be avoided independently by each LOP.
- Conditional modifiers are variables that express how likely a particular outcome is to occur. The analysis can consider factors such as the presence of people, the risk of ignition, the risk of specific injuries, the risk of catastrophic failure, etc.
- The ‘frequency of occurrence’ for the scenario can be determined from specific data. This frequency should be lower than the maximum allowable incident frequency. The calculation can be used to assess whether additional risk mitigation is needed and how large this additional risk mitigation should be.
This will result in:
SIL 4
10-5≥ PofD** <10-4
SIL 3
10-4≥ PofD <10-3
SIL 2
10-3≥ PofD < 10-2
SIL 1
10-2≥ PofD < 10-1
So, what is the difference between PL and SIL?
The approach to both standards is different. The purpose of SIL and PL is identical. The main differences are that standard 13849-1 (PL) provides a broader framework and includes pneumatic and hydraulic systems. Standard ISO 13849-1 (PL) focuses mainly on mechanical engineering, while standard IEC 62061 (SIL) focuses more on electrical and electronic systems.
Examples of SIL and PL compliant products.
Fortress’ tGard range complies with PLd and SIL2, this compact access and control system combines all key interlocks, electrical safety interlocks, push buttons and selector switches together into 1 robust configuration.
The mGard, amGard Pro, amGard Network, amGard S40, Alfred Ex, Atom, Louis, Steute Ex, and Proton safety switch all comply with PLe and SIL3.
Our Fluidsentry products for pneumatic and hydraulic solutions are also marked PLe.
*HAZOP study = Hazard and Operability study, method to identify and evaluate process deviations.
**PoFD = Probability of Failure on Demand.
