Function Manual
01/2017
191
Simple sub-systems (e.g. sensors and actuators) in electromechanical components can, in turn, comprise sub-system
elements (devices) interconnected in different ways with the characteristic quantities required for determining the relevant
PFHD value of the sub-system.
Safety-related characteristic quantities for subsystem elements (devices):
● λ: Failure rate
● B10 value: For elements that are subject to wear
● T1: Lifetime
For electromechanical devices, a manufacturer specifies a failure rate λ with reference to the number of operating cycles.
The failure rate per unit time and the lifetime must be determined using the switching frequency for the particular application.
Parameters for the sub-system, which comprises sub-system elements, that must be defined during the design phase:
● T2: Diagnostic test interval
● β: Susceptibility to common cause failure
● DC: Diagnostic coverage
The PFHD value of the safety-related controller is determined by adding the individual PFHD values for subsystems.
The user has the following options when setting up a safety-related controller:
● Use devices and sub-systems that already comply with EN ISO 13849-1, IEC/EN 61508, or IEC/EN 62061. The standard
provides information specifying how qualified devices can be integrated when safety functions are implemented.
● Develop own subsystems:
– Programmable, electronic systems and complex systems: Application of EN 61508 or EN 61800-5-2.
– Simple devices and subsystems: Application of EN 62061.
EN 62061 does not include information about non-electric systems. The standard provides detailed information on
implementing safety-related electrical, electronic, and programmable electronic control systems. EN ISO 13849-1 must be
applied for non-electric systems.
Note
Function examples
Details of simple sub
-systems that have been implemented and integrated are now available as "functional examples".
Note
EN 62061 and machinery directive
IEC 62061 has been ratified as EN 62061 in Eur
ope and harmonized as part of the Machinery Directive.
Series of standards EN 61508 (VDE 0803)
This series of standards describes the current state of the art.
EN 61508 is not harmonized in line with any EU directives, which means that an automatic presumption of conformity for
fulfilling the protective requirements of a directive is not implied. The manufacturer of a safety-related product, however, can
also use EN 61508 to fulfill basic requirements of European directives in accordance with the latest conceptual design, for
example, in the following cases:
● If no harmonized standard exists for the application in question. In this case, the manufacturer can use EN 61508,
although no presumption of conformity exists here.
● A harmonized European standard (e.g. EN 62061, EN ISO 13849, EN 60204-1) references EN 61508. This ensures that
the appropriate requirements of the directives are fulfilled ("standard that is also applicable"). When manufacturers apply
EN 61508 properly and responsibly in accordance with this reference, they can use the presumption of conformity of the
referencing standard.
EN 61508 covers all the aspects that must be taken into account when E/E/PES systems (electrical, electronic, and
programmable electronic System) are used in order to execute safety functions and/or to ensure the appropriate level of
functional safety. Other hazards (e.g. electric shock) are, as in EN ISO 13849, not part of the standard.
EN 61508 has recently been declared the "International Basic Safety Publication", which makes it a framework for other,
sector-specific standards (e.g. EN 62061). As a result, this standard is now accepted worldwide, particularly in North
America and in the automotive industry. Today, many regulatory bodies already stipulate it (e.g. as a basis for NRTL listing).
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