Normative Basis
Several standards (generic safety standards and technical safety standards; type A and type B standards) are available for determining the safety level achieved by the safety-related section of a control system. EN ISO 13849-1 is generally applied in the engineering sector. For many machines, the safety level to be achieved can be taken from the respective machinery safety standards (type C standards, e.g. presses ➔ EN 692, EN 693;
robots ➔ EN ISO 10218-1, packaging machinery ➔ EN 415). If there are no C standards for a product, the requirements can be taken from the A and B standards.
Safe Stop Function
The safety function “E-STOP when light curtain is interrupted” is addressed here by the example below; it illustrates a safe stop function for a motor driven axis. The methodology described below is based on EN ISO 13849-1 and as such can only be applied if all the safety function sub-components have their own performance level. Using the terminology of the standard, it is a series alignment of safety-related parts of a control system (SRP/CS).
This example uses a light curtain, a configurable safety control system and a servo amplifier with integrated safety functions. A servo motor with feedback system is connected to the servo amplifier.
The risk analysis permits a stop category 1 for the axis.
Determination of the performance level for the overall circuit:
Note: The values calculated for this look-up table are based on reliability values at the mid-point for each PL.
In the example of the safe stop function, all three components involved have performance level e. As a result, the lowest performance level of a safety-related subcircuit (SRP/CS) is also PL e. Using the standard's terminology, therefore, we have:
If you apply this information to Table 11 of the standard, the result for the example is an overall classification of PL e.
Several standards (generic safety standards and technical safety standards; type A and type B standards) are available for determining the safety level achieved by the safety-related section of a control system. EN ISO 13849-1 is generally applied in the engineering sector. For many machines, the safety level to be achieved can be taken from the respective machinery safety standards (type C standards, e.g. presses ➔ EN 692, EN 693;
robots ➔ EN ISO 10218-1, packaging machinery ➔ EN 415). If there are no C standards for a product, the requirements can be taken from the A and B standards.
Safe Stop Function
The safety function “E-STOP when light curtain is interrupted” is addressed here by the example below; it illustrates a safe stop function for a motor driven axis. The methodology described below is based on EN ISO 13849-1 and as such can only be applied if all the safety function sub-components have their own performance level. Using the terminology of the standard, it is a series alignment of safety-related parts of a control system (SRP/CS).
This example uses a light curtain, a configurable safety control system and a servo amplifier with integrated safety functions. A servo motor with feedback system is connected to the servo amplifier.
The risk analysis permits a stop category 1 for the axis.
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| Structure of Safety Function |
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| The block diagram shows the logical structure of the safety function, comprising the series alignment of the safety-related sub-circuits. |
Determination of the performance level for the overall circuit:
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| EN ISO 13849-1: Table 11 – Calculation of PL for series alignment of SRP/CS |
Note: The values calculated for this look-up table are based on reliability values at the mid-point for each PL.
In the example of the safe stop function, all three components involved have performance level e. As a result, the lowest performance level of a safety-related subcircuit (SRP/CS) is also PL e. Using the standard's terminology, therefore, we have:
- 3 x SRP/CS each with PL e
- The lowest performance level of the 3 subcircuits (SRP/CS) = PL e and is assigned the parameter PLlow
- The lowest performance level occurs in 3 subcircuits and so the parameter Nlow = 3
If you apply this information to Table 11 of the standard, the result for the example is an overall classification of PL e.
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