|β (Beta)||Common cause
|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.
|IEC 62061||Failure rate for detectable dangerous failures. Detectable dangerous failures
may be detected by automatic self-diagnostic systems.
|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
|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
Assessed by assigning marks. Maximum possible score is 100.
|DC||Diagnostic Coverage||ISO 13849-1
|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%.
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.
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.
|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||SIL CLaim||IEC 62061||Max. SIL attainable by a subsystem in relation to architecture and ability to
|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
|IEC 62061||Electrical, electronic and programmable electronic control system the failure
of which immediately increases the risk factor associated with machine
|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
|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.