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Acronym Definition Reference standard Description
β (Beta) Common cause
failure factor
IEC 62061 Degree of operational independence of channels of a multi-channel system.
Ranging from 0.1 to 0.01 depending on CCF attained.
λ (Lambda) Failure rate IEC 62061 Random failure frequency. The time-random failure frequency of a component is usually known as Failure Rate, described as number of failures per unit of hour. Its inverse is known as Mean Time Between Failures (MTBF), expressed in hours.
Random failures are the result of sudden stress accumulation above maximum design strength of a component. May occur at random intervals and entirely unexpectedly. Frequency of failure over sufficiently long periods is virtually constant. PFHd calculation methods given in both Standards refer
only to the assessment of random failures. The unit of measure for failure rate is FIT (Failure In Time) equivalent to one failure per billion of operating hours (F=1 means one failure every 109 hours)
λs Safe failure rate IEC 62061 Failure rate for non-dangerous failures. Non-dangerous failures which have no adverse safety-related effect on control system. The control system continues to ensure protection.
λd Dangerous failure rate IEC 62061 Failure rate of failures which may involve dangerous operation. Dangerous failures prevent the control system from continuing to provide protection.
λdd Dangerous detected
failure rate
IEC 62061 Failure rate for detectable dangerous failures. Detectable dangerous failures may be detected by automatic self-diagnostic systems.
λdu Dangerous undetected
failure rate
IEC 62061 Failure rate for undetectable dangerous failures. Undetectable dangerous failures cannot be detected by internal automatic self-diagnostic systems.
They determine the value of PFHd and, consequently, the value of SIL or PL.L.
Cat. Category ISO 13849-1 The Category is the main parameter to consider to attain a given PL.
Describes the SRP/CS performance in relation to its ability to resist failure and resulting performance in failure conditions.
Five Categories are envisaged depending on structural positioning of components.
CCF Common Cause Failure ISO 13849-1
IEC 62061
Failure resulting from common causes.
Failure resulting from one or more events causing simultaneous malfunction of channels of a multi-channel system.
Provides a measure of the degree of independence of redundant channel operation.
Assessed by assigning marks. Maximum possible score is 100.
DC Diagnostic Coverage ISO 13849-1
IEC 62061
Reduced probability of dangerous hardware failure due to automatic selfdiagnostic system operation. A measure of system effectiveness in promptly detecting its own possible malfunction.
Expressed as 60% to 99%.
MTTFd Mean Time
to dangerous Failures
ISO 13849-1 Average operating time, expressed in years, to potentially dangerous random failure (not generic failure). May refer to a single component, or to a single channel, or to the entire safety-related system.
PFHd Probability of
dangerous Failure/Hour
IEC 62061 Average probability of dangerous failure per hour.
Quantitative representation of risk reduction factor provided by the safetyrelated control system.
PL Performance Level ISO 13849-1 Level of performance. In ISO 13849-1, the extent to which failures are controlled is assessed using the Performance Level concept (PL).
Represents SRP/CS ability to perform a safety-related function within predictable operating conditions. There are 5 levels, PLa to PLe.
PLe represents the highest level of risk reduction, PLa the lowest level.
PLr Performance Level
ISO 13849-1 Level of performance required.
Represents the contribution to risk reduction by each safety-related part implemented in SRP/CS. PLr is obtained using the risk curve.
SIL Safety Integrity Level IEC 62061 Level of integrity of a safety-related function. Discrete level (one of three) used to describe the ability of a safety-related control system to resist failure as per IEC 62061, where level 3 assures the highest protection and level 1 the lowest.
SILCL IN SIL CLaim IEC 62061 Max. SIL attainable by a subsystem in relation to architecture and ability to detect failure.
SRP/CS Safety Related Parts
of Control Systems
ISO 13849-1 Part of machine control system able to maintain or achieve machine safety status in relation to the status of certain safety-related sensors.
SRECS Safety-Related Electrical,
electronic and programmable
electronic Control
IEC 62061 Electrical, electronic and programmable electronic control system the failure of which immediately increases the risk factor associated with machine operation.
T1 Proof test interval IEC 62061 Interval of proof test. The Proof Test is an external manual inspection for detecting component failure and performance decay, undetectable by internal self-diagnostic systems. The unit of measure is time (months or, more usually, years).
T2 Diagnostic test interval IEC 62061 Test interval of self-diagnostic functions. Time elapsed between one test for the detection of possible internal failure and the next. Tests are carried out in automatic mode by dedicated circuitry which may be internal to the SRECS in question or may belong to other SRECSs.
The unit of measure is time (milliseconds to hours).
SFF Safe Failure Fraction IEC 62061 Fraction of overall failure rate which does not involve dangerous failure.
Represents the percentage of non-dangerous failures relative to total number of failures of the safety-related control system.


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