Introduction to Safety Standard
Risk Assesment
The Idea of Performance Level
ISO 13849-1 Revisions
PL (Performance Level)
PL Parameters
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ISO 13849-1 Revisions

Background of Revision

ISO 13849-1 (Safety of machinery - Safety-related parts of control systems - General principles for design), an international standard, was revised in 2006. As the background of the revision, semiconductor parts such as transistors and MOS-FETs have been put to use in the safety machinery that composes the safety-related parts of control systems, which represents a change in control methods from control by way of hard wiring to control by way of software. In the conventional way of thinking about categories, safety was determined according to system architectures (structures) that used mechanical safety devices and relays with forcibly guided contacts, so it could not be said that sufficient thought was given to safety attributable to the reliability of parts.
Under these circumstances, attempts were made to regulate mechanical safety according to functions and reliability from around the year 2000.
This way of thinking is called “functional safety.” ISO 13849-1:2006 is a standard that revises ISO 13849-1:1999, which was based on the conventional standard EN 954-1, by adding details from IEC 61508 (IEC 62061), which defined functional safety.

Background of Revision

Overview of Revision

ISO 13849-1:1999
ISO 13849-1:1999
Safety “Category” was defined in ISO 13849-1: 1999. It is classification of the safety-related parts of a control system in respect of their resistance to faults and their subsequent behavior in the fault condition, and which is achieved by the structural arrangement of the parts and/or by their reliability.
ISO 13849-1:2006
ISO 13849-1:2006
Performance level (PL) was introduced in ISO 13849-1: 2006, which is quantitatively expressed as the reliability of safety-related parts of a control system, including diagnostic coverage or failure rate.
Reference: Requirements before Revisions

ISO 13849-1: 1999 specified the determination method of category and its requirements.

Symbol Symbol details Parameter Short explanation on the parameter
S Severity of injury S1 Slight (normally reversible) injury
S2 Serious (normally irreversible) injury including death
F Frequency and/or exposure time to the hazard F1 Seldom to quite often exposure time is short
F2 Frequent to continuous exposure time is long
P Possibility of avoiding the hazard P1 Possible
P2 Scarcely possible

Explanation of symbols
The following is a criteria for each parameter according to AISI/RIA R15.06.

  • S: In a case where it requires more than first-aid, including absence from work, S2 would be assigned.
  • F: In a case where typical exposure to the hazard more than one per hour, F2 would be assigned.
  • P: In a case where robot speed is greater than 250 mm/s, P2 would be assigned.
Category requirements defined by ISO 13849-1
Category Summary of requirements System behavior
B Safety-related parts of control systems and their protective equipment shall be designed, constructed, selected, assembled, and combined in accordance with relevant standards so that they can withstand the expected influence. The occurrence of a fault can lead to the loss of the safety function.
1 Requirements of B shall apply.
Well-tried components and well-tried safety principles shall be used.*
The occurrence of a fault can lead to the loss of the safety function, but the probability of occurrence is lower than for category B.
2 Requirements of B and the use of well-tried safety principles shall apply.
The safety function shall be checked at suitable intervals by the machine control system.
The occurrence of a fault can lead to the loss of the safety function between the checks. The loss of the safety function is detected by the check.
3 Requirements of B and the use of well-tried safety principles shall apply.
Safety-related parts shall be designed so that
- a single fault in any of these parts does not lead to the loss of the safety function and
- whenever reasonably practicable, the single fault is detected.
When a single fault occurs, the safety function is always performed.Some, but not all, faults will be detected.
Accumulation of undetected faults can lead to the loss of the safety function.
4 Requirements of B and the use of well-tried safety principles shall apply.
Safety-related parts shall be designed so that
- a single fault in any of these parts does not lead to the loss of the safety function and
- single faults are detected at or before the next demand upon the safety function, but that if this detection is not possible, an accumulation of undetected faults shall not lead to the loss of the safety function.
When the faults occur, the safety function is always performed.The faults will be detected in time to prevent the loss of the safety function.

* Well-tried safety principles are, for example, 1) avoidance of certain faults (ex. avoidance short-circuit by separation), 2) reducing the probability of faults (ex. over-dimensioning or underrating of components), 3) by orientating the mode of fault (ex. by ensuring an open circuit in the event of fault), 4) detect faults very early, and 5) restrict the consequences of a fault (ex. earthing of the equipment).

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