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1.1.6 Special Abbreviation

Short

Unit

Value Description

FIT

8515

(

): Dangerous failure rate detected by diagnostics. Meaning: These failures could cause a dangerous state on the

machine, but are detected and reacted upon safely. (

): Safe failure rate. Meaning: These failures do not cause a dan-

gerous state on the machine.

FIT

932

(

): Rate of dangerous failures not detected by diagnostics. Meaning: These failures cause a dangerous state on the

machine.

HFT

Hardware Fault Tolerance (HFT): HFT = n means, that n+1 faults could cause a loss of the safety function. Example:
HFT=1 means, the required function is still performed in the presence of 1 arbitrary fault of the safety device.

PFD

4.1e-3 The Probability of Failure of Demand (PFD) specifies the average probability of a failure to perform the safety function on

demand. In the low demand mode the frequency of demands for operation made on a safety related system is not greater
than one per year and no greater than twice the proof-test frequency. The PFD, equivalent to the unavailability of a system
at the time of a process demand. Calculated on the basis of an FMEDA. Calculated for test interval one year

Abbreviation for ”out of”

PFH

-1

The Probability of dangerous failure per hour (PFH) specifies the failure rate (e.g. per hour) to perform the safety function
continuously. This value shall be considered if the safety device is operated in high demand (more often than once per
year) or continuous mode of operation, where the frequency of demands for operation made on a safety-related system
is greater than one per year or greater than twice the proof-test frequency. The PFH, may approximately be calculated
to PFD/8760h under the assumption that the safety function is needed less than once per year and a diagnostic test is
performed more than once per year. If the safety function is used more than once per year, or continuously, then PFH is
equal to

for a 1oo1D system.

SFF

Safe Failure Fraction (SFF): Percentage part of safe failures and dangerous detected failures of a safety function or a
subsystem related to all failures.

SIL

1-4

Safety Integrity Level (SIL) is defined as a relative level of risk-reduction provided by a safety function, or to specify a
target level of risk reduction. Four SIL levels are defined, with SIL4 being the most dependable and SIL1 being the least.
A SIL is determined based on a number of quantitative factors in combination with qualitative factors such as development
process and safety life cycle management. The requirements for a given SIL are not consistent among all of the functional
safety standards.

FIT

1E-9/
hour

Failure In Time: 1E-9 failures/hour

SIL2

One of the four safety integrity levels defined by IEC 61508. See also table below.

ISO
13849

Cat 2. The standard ISO 13849 deals with “shutdown” systems, which perform a shut-down of the hazardous motion to achieve

a safe state when a fault occurs. For this reason, what are termed “designated architectures” have been created, in
accordance to the categories B, 1, 2, 3, and 4. In the case of categories B and 1, the architectures are identical, but the
requirements of the safety principles differ; the same applies to categories 3 and 4. Here, the requirements for fault
detection, in particular, differ. The categories are mainly characterised by the structure. So the first parameter that has
to be evaluated is the fault-tolerance. If the control system is tolerant to any single fault, category 3 or 4 can be claimed.
In the other case, only category B, 1 or 2 is possible.

FMEDA

Failure Modes, Effects, and Diagnostic Analysis

1oo1D
system

The architecture consists of a single channel for the safety function (HFT=0)

Safe failure rate (per hour)

Safe undetectable failures

Safe

detectable

failures

Undetectable dangerous failure rate (per hour)

Detectable dangerous failure rate (per hour)

tot