Technical information

Functional safety:
Safety-relevant temperature measurement per IEC 61508
WIKA data sheet IN 00.19

Introduction
Under certain conditions, electrical thermometers canbe used in a safety-related system in accordance withIEC 61508. The version of the electrical thermometer as
resistance thermometer or thermocouple as well as thetechnical features of the used temperature transmitter haveto be taken into account for the evaluation of the safetyrelated system.
This technical information describes the basics of functionalsafety in accordance with IEC 61508 and offers advice on thesafety-related design of a temperature measuring point.

Need for risk reduction
Due to rising expectations of society on the safety oftechnical plants, the risks presented from technical systems
have been ever more reduced over time. Guidelines andstandards have been created to help every plant operator
to operate his or her plant to the highest levels of safety.
Conducting accident analyses and risk assessments is thebasis for this. The aim is to reduce the risk presented by atechnical system to an acceptable risk in line with society´svalues by means of safety measures.
To prevent a failure to danger in a plant, electrical/electronic/
programmable electronic systems (E/E/PE systems) are
employed. The totality of all required safety functions which
serve towards maintaining the safe state of a plant is referred
to as a safety instrumented system SIS or safety-related
system.
An example of such a safety system is a temperaturemonitoring system that, when the temperature limits are
exceeded, reliably shuts down the power supply of a plant,
placing it in the safe state and thus preventing a hazardous
event.

WIKA data sheet IN 00.19 ∙ 10/2014

Page 1 of 13

Architecture of a safety-related system
An electrical/electronic/programmable electronic system
basically consists of the elements of sensor, controller
and actuator. In this case one refers to a single-channelarchitecture of the safety system (1oo1 system). The
architecture describes the specific configuration of
hardware and software elements in a system. A 1oo1
system indicates that the system is made up of one channel,
which must operate safely so that the safety function canbe performed (1 out of 1). For safety systems with multichannel architecture, hardware or software elements are
implemented with redundancy (see “Redundant systems”).

Example of a single-channel architecture for a safety instrumented system
Sensor subsystem

Logic subsystemelectrical thermometer withtemperature transmitter

Actuator subsystem

programmable logic controller

Valve

Scope of responsibility of the system installer/plant operator
An electrical thermometer with models T32.1S temperaturetransmitter (head mounting version) and T32.3S (rail
mounting version) can be used by the plant operator as asensor subsystem of a safety instrumented system.

Temperature transmitter, model T32.xS

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WIKA data sheet IN 00.19 ∙ 10/2014

Legislative basis
The IEC 61508 series of standards “Functional safety ofelectrical/electronic/programmable electronic safety-related
systems” is referred to as a fundamental safety standard. It
describes the measures for the prevention and containmentof failures in instruments and plants and can be usedirrespective of the industry sector.
IEC 61508 should be used in particular whenWith the certification of the model T32.xS temperaturetransmitter, the temperature transmitter has been considered
in connection with an electrical thermometer. In the safety
manual “Information on functional safety for temperaturetransmitter model T32.xS”, safety-relevant characteristic
values for the temperature transmitter, the connected
temperature sensors and the entire assembly are specified.

■■ the safety function is implemented through an

For the evaluation, the sensor subsystem is divided into theelements “electrical thermometer (temperature sensor)”and “temperature transmitter”. The temperature sensors areclassified as type A components (elementary component)
and the temperature transmitter as type B components(complex component).

IEC 61508 represents the state of the art with respect tothe design of safety instrumented systems. With the designof safety systems, the best available technology, and thus
IEC 61508, absolutely must be followed.

Sensor subsystem consisting of temperaturetransmitter and temperature sensor

E/E/PE system■■ a failure of the safety instrumented system will lead to ahazard to people and the environment■■ no application-specific standard exists for the design ofsafety systems

For planners, contractors and operators of the safetysystem, there are also application-specific standards. These
are, for example, IEC 61511 “Functional safety - safety
instrumented systems for the process industry sector” forthe process industry and EN 62061 “Safety of machinery Functional safety of safety-related electrical, electronic
and programmable electronic control systems” for machinebuilding.
An electrical thermometer can be used in a safety
instrumented system in accordance with the IEC 61508
standard when the thermometer is used in conjunction
with a temperature transmitter certified for safety-relevant
applications. The model T32.xS temperature transmitter from
WIKA has been developed with reference to IEC 61508 foruse in the process industry and certified by TÜV Rheinland
for this application.

Thermocouple orresistance thermometer

Temperature transmittermodel T32.xS

An electrical thermometer without a temperature transmitter,for example a resistance thermometer or a thermocouple, is
not covered by IEC 61508, since (for example) a measuringresistor is a simple electrical component that cannot perform
any self-diagnostics nor detect errors.
For electrical thermometers without a temperature transmitter
certified to IEC 61508, only failure rates can be specified.
This is because it always depends on the operator's
evaluation instrument as to what failure types can bedetected and safely recognised in the electrical thermometer.

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Evaluation of safety-related systems
The probability that a safety function on demand is carriedout (i.e. when a system fault occurs) is defined by thesafety integrity. To obtain a measure of the requirements forsafety integrity, these are divided into four Safety Integrity
Levels (SIL). If SIL 4 is achieved, the probability that thesafety function is executed is at its maximum, and thus the
maximum possible risk reduction is ensured.

The safety integrity level always refers to the entire safetysystem. An element has no SIL, but may still be suitable for a
SIL application. For example, the model T32.xS temperaturetransmitter alone does not form a safety-related system.
The operator is responsible for defining and maintaining
the required safety integrity level as well as the entire safetysystem and the individual elements!

Levels of safety integrity

WIKA, as a manufacturer of electrical thermometers,
supports the user in this. On the one hand, by confirmingthat the requirements of IEC 61508 have been met, such as
during the development of the T32.xS. On the other hand, theoperator can be provided with the appropriate safety-relatedcharacteristic data for the plant design and the evaluation of
the safety function.

SIL1
low low

SIL2

SIL3

safety integrity
safetyintegrity

SIL4
high
high

The term “SIL” is thus an important parameter of the safetysystem, but is often used as a synonym for “Functional
Safety”.

Requirements on a safety system
In order to design a temperature measuring point which is
optimised for a safety-related system, the following aspects
must be considered:■■ The safe state of the plant and the safety function of eachelement must be defined by the plant operator.
■■ The required safety integrity level must be determinedby the operator of the safety system through a risk
assessment, e.g. with risk graphs.
■■ The thermometer's operating conditions (process
medium, environmental influences) should be sufficiently
specified so that the temperature measuring point can be
designed optimally in cooperation with WIKA.
■■ The instructions in the WIKA documentation about thethermometer used must be observed.
■■ Ensure that wetted parts are suitable for the measuring
medium.
Fundamental for optimal safety at the temperature measuring
point is the correct design of the electrical thermometer,
corresponding to the requirements of the process. The next
step is the selection of a temperature transmitter suitable forsafety systems, that detects as many fault types as possibleof the electrical thermometer and of the transmitter itself.

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WIKA data sheet IN 00.19 ∙ 10/2014

Determination of the maximum achievable safety integrity level with the example ofthe temperature transmitter model T32.xS
To determine the safety integrity level of a safety-related
system, both the requirements for systematic safety integrity
and the hardware safety integrity must be determined.

Systematic safety integrity

Hardware safety integrity

To fulfil the requirements for systematic safety integrity,
systematic failures must be taken into account. Systematic
failures are design faults, manufacturing faults or operating
faults. To reduce these, IEC 61508 specifies safety measuresthat must be maintained throughout the full service life
(product lifecycle) of a technical system. The safety life cycleof safety systems begins with the conception and ends withthe decommissioning. As part of the safety management
during the development of the T32.xS, systematic failures
were prevented, for example, by means of validation andverification activities as well as planning and thoroughdocumentation. Thus the software of model T32.xS evenfulfils the criteria for SIL 3 concerning safety integrity.

Random faults
To evaluate the hardware safety integrity, random faults must
be observed. These are caused by random changes of acomponent's behaviour, e. g. open circuit, short circuit or
random change in value of a capacitor in an electrical circuit.
Random faults cannot be avoided. Only the probability of theoccurrence of such a fault can be calculated. The failure rate
is given in the unit FIT (Failures in Time).
It is defined as:
The totality of all failures in a time interval at a constant rateof failure is referred to as the base failure rate λB. The basefailure rate is composed of hazardous faults λD = dangerous,and non-hazardous faults λS = safe, that have an impact on
the safety function.
Depending on whether a fault, for example, can be detected
through a diagnostic function of the electronics in thesafety system or remains undetected, the hazardous andnon-hazardous faults are further divided.
Sub-division of failure rates

λSU =safe, undetectable

λDD =dangerous, detectableλSD =safe, detectable

λDU =dangerous, undetectable

WIKA data sheet IN 00.19 ∙ 10/2014

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Failure types in an electrical thermometer
The following failures can occur in an electrical thermometer:■■ Open circuit - the measuring circuit is interrupted■■ Short circuit - two connecting cables are connected

unintentionally■■ Drift due to changes in the resistor material or drift in thethermoelectric voltage■■ Change in the lead resistance, e.g. through temperaturechanges

Depending on the fault detection functions of the temperaturetransmitter used, the type of failure (λSD, λSU, λDD, λDU) for
different faults in the electrical thermometer must be defined.
Table 1: Fault detection through the temperature transmitter model T32.xS
Possible failure cases inelectrical thermometers
Open circuit
Short circuit
Drift
Change in the leadresistanceResistance thermometer,
2-wire connection
λDD
λDD
λDU
λDU

Resistance thermometer,
3-wire connection

λDD
λDD
λDU
λDD 1)

Resistance thermometer,
4-wire connection

λDD
λDD
λDU
λDD

Thermocouple

λDD
λDU
λDU
λDD

1) A change of the lead resistance in a 3-wire connection can only be detected based on the understanding that the connecting cables between the measuring resistor and transmitter are
the same length and have the same conductor cross-section.

In the literature, the failure rates for thermocouples andresistance thermometers are given in different applications
and configurations. The failure rates are based on the “worst
case” of a thermometer failure and serve as guidance forthe design of safety instrumented systems. The failure rates
should be used taking into account the operating conditions
and the connecting cable between the measuring point andthe transmitter. They are differentiated in accordance withthe vibration requirements at the site of operation (low stress/
high stress) and on the type of connection between themeasuring point and temperature transmitter (close-coupled/
extension wire) (see “Definitions and abbreviations”).
Table 2: Failure rates for thermocouples without temperature transmitter 2)
Type of fault
Open circuit
Short circuit
Drift

Close coupled

Low stress
95 FIT
4 FIT
1 FIT

High stress1,900 FIT
80 FIT
20 FIT

Extension wire
Low stress
900 FIT
50 FIT
50 FIT

High stress
18,000 FIT
1,000 FIT
1,000 FIT

2) The stated failure rates are based on calculations by WIKA using key basic data from exida.com L.L.C. (see page 12 “Literature and sources”,“Exida”)

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Table 3: Failure rates for resistance thermometers with 4-wire connection without temperature transmitter 2)
Type of fault
Open circuit
Short circuit
Drift

Close coupled

Low stress
42 FIT
3 FIT
6 FIT

High stress
830 FIT
50 FIT
120 FIT

Extension wire
Low stress
410 FIT
20 FIT
70 FIT

High stress
8,200 FIT
400 FIT
1,400 FIT

Table 4: Failure rates for resistance thermometers with 2- or 3-wire connection without temperature transmitter 2)
Type of fault
Open circuit
Short circuit
Drift

Close coupled

Low stress

38 FIT
1 FIT
9 FIT

High stress

758 FIT
29 FIT
173 FIT

Extension wire

Low stress

371 FIT
10 FIT
95 FIT

High stress

7,410 FIT
190 FIT
1,900 FIT

2) The stated failure rates are based on calculations by WIKA using key basic data from exida.com L.L.C. (see page 12 “Literature and sources”, “Exida”)

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Limitation of the safety integrity level of an element
The maximum achievable SIL of an element of the safety system is limited by the following factors:■■ Proportion of safe failures of a hardware element (SafeFailure Fraction, SFF)■■ Hardware Fault Tolerance (HFT)

The hardware fault tolerance represents a measure of
the degree of redundancy of the safety system. With ahardware fault tolerance of N, N+1 is the minimum numberof errors that could lead to the loss of a safety function.
A safety instrumented system with single-channelarchitecture has a hardware fault tolerance of 0.

■■ Complexity of the components (type A and B components)- Type A components are primary components whose
failure performance is fully defined and whose
malfunction is identified. Type A components are,for example, resistance temperature sensors andthermocouples.
- For complex type B components, the failure performanceof at least one component is not defined, or not fully
defined. A type B component is, for example, anelectronic circuit containing a microprocessor. The
T32.xS temperature transmitter is defined as a type B
component (see table 5).

In order to calculate the SFF value of resistance temperature sensors and thermocouples which are connected to the T32.xS
temperature transmitter, the failure rates of the temperature sensors should be subdivided into the categories (λS, λDD, λDU),
taking into account the diagnostic function of the transmitter. Consequently, the SFF value can be calculated according to thefollowing formula:Thus temperature sensors defined as type A components in a single-channel architecture (HFT = 0) should be used in safety
instrumented systems up to SIL 2, and an SFF ≥ 60 % is maintained in accordance with table 5. For the same application, for
the T32.xS temperature transmitter as type B components, an SFF ≥ 90 % is required.

Table 5: Maximum safety integrity level of a component dependent on the hardware fault tolerance, the complexity of thecomponents and the safe failure fraction
SFF< 60 %
60 … < 90 %90 … < 99 %≥ 99 %

Hardware fault tolerance
0Type A

SIL 1
SIL 2
SIL 3
SIL 3

Type B

not allowed
SIL 1
SIL 2
SIL 3

1
Type A

SIL 2
SIL 3
SIL 4
SIL 4

Type B

SIL 1
SIL 2
SIL 3
SIL 4

2
Type A

SIL 3
SIL 4
SIL 4
SIL 4

Type B

SIL 2
SIL 3
SIL 4
SIL 4

Only if the SFF value of both the temperature transmitterand also the temperature sensor meet the specified limit arethese elements permissible for safety instrumented systems
with the corresponding SIL. In addition, the PFD value of theentire safety function must satisfy the requirements of table 6.

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WIKA data sheet IN 00.19 ∙ 10/2014

Limitation of the SIL of the entire safety system
The IEC 61508 standard specifies values ​​that limit the safety integrity level of the entire safety system. Depending on
how often the safety system is required, two characteristic values are differentiated:■■ PFH (probability of dangerous failure per hour

Thus this combination, with respect to the requirements on the
PFDavg value, is suitable for safety systems with increased
safety integrity level to SIL 2, however, due to the singlechannel structure (see “Limitation of the safety integrity level ofan element”) and the SFF, it is limited to SIL 2.

■■ PFDavg (probability of failure on demand)

The formula described above is derived from IEC 61508. It is
assumed that the time period of 8 hours, which is required forthe renovation of the system is negligibly small in comparison
with the proof test interval of 8,760 h.

Tproof indicates the interval of the repeat testing. After thisinterval, through a suitable test (proof test), the system isbrought to an almost “as new” state within the stipulated
service life. With this test, dangerous, undetectable faults canalso be detected. For an electrical thermometer, it is ensured
by regular calibration that the measured value still lies within
the required accuracy. With this, an unacceptably high drift isalso excluded.

The PFDavg value conforms almost linearly to the proof test
interval, Tproof. The shorter the proof test interval, the betterthe PFDavg value achievable. Likewise, the proof test interval
can be increased if the PFDavg value of the entire system islower than the permissible limit value. If the proof test interval
is shortened to 0.5 years, the PFDavg value is halved, and if it
is extended to 2 years, it is doubled.

Average frequency of a dangerous failure of the safetyfunction for an operating mode with high or continuous
demand rates (high demand). These modes are particularly
relevant for machine building.
Average probability of dangerous failure on demand of asafety function for an operating mode with low demand rate
(low demand).

With a proof test interval of one year (Tproof = 8,760 h) the
following PFDavg values result for a resistance thermometerwith 4-wire connection and a model T32.xS temperaturetransmitter connected:- Ambient condition: low stress- Connection between measuring point and transmitter: close
coupled- Failure rate λDU = 16 FIT 3)

The smaller the PFDavg or PFH value, the greater theachievable SIL of the entire system. In table 6 the PFDavg
or PFH characteristic values are assigned a safety integrity
level.

Table 6: Limitation of the SIL of the safety system by PFDavg and PFH values
Safety Integrity
Level (SIL)

Average probability of a dangerous failure on
demand of a safety function (PFDavg)

Average frequency of a dangerous failure
per hour (PFH)

4
3
2
1

≥ 10-5 to < 10-4
≥ 10-4 to < 10-3
≥ 10-3 to < 10-2
≥ 10-2 to < 10-1

≥ 10-9 to < 10-8 h-1≥ 10-8 to < 10-7 h-1≥ 10-7 to < 10-6 h-1≥ 10-6 to < 10-5 h-1

3) see page 12 “Literature and sources” and page 18-20 in the safety manual “Information on functional safety for temperature transmitter model T32.xS”

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For the operator of the system, it is always the PFDavg
value of the entire safety system and not the value of asingle element that is relevant. For evaluation, the following
distribution of the PFDavg values for the safety system has
been established as a guideline:
Distribution of sensor, controller, actuator in the total
PFD value of the SIS

Actuator
50 %A different distribution of the components can be specified by
the plant operator.
If the sensor uses less than 35 % of the maximum allowable
PFDavg value of the safety system, such as for an electricalthermometer with a model T32.xS temperature transmitter,then the operator can use a controller and an actuator withcorrespondingly poorer PFDavg values.

Sensor(electricalthermometer)
35 %Controlsystem15 %Structural limitations
Structural characteristics of the safety instrumented system
may limit the maximum achievable SIL. In a single-channelarchitecture, the maximum SIL is determined by the weakest
link. In the safety system illustrated, the “sensor” and “logic”
subsystems are suitable for SIL 2, while the “actuator”
subsystem is only suitable for SIL 1. The entire safety system
can therefore only achieve a maximum of SIL 1.
Components of a safety-related system

Sensor subsystem
SIL 2Page 10 of 13

Logic subsystem
SIL 2Actuator subsystem
SIL 1

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Redundant systems
If two electrical thermometers with model T32.xS temperaturetransmitter are assembled in parallel, common cause
failures must be considered. Common cause failures canoccur, for example, when environmental conditions or EMC
interferences influence several channels simultaneously.
These faults affect all channels of a redundant system at thesame time.
Reliability block diagram: electrical thermometer in redundant configuration
SensorChannel 1
Channel 2

electrical thermometer 1(with temperature transmitter)electrical thermometer 2(with temperature transmitter)

Common cause
failureThe electrical thermometers from the previous figure
represent, in this case, a two-channel architecture (1oo2)
system. Such a structure is referred to as MooN system.
A MooN system (M out of N) consists of N independent
channels, of which M channels must function safely in orderthat the entire system can perform the safety function.

Operationally proven field instruments in a two-channelarchitecture achieve, as a maximum, the SIL of the individual
instrument. Unlike the model T32.xS temperature transmitter,systematic faults in these instruments are not prevented or
reduced in the first place, e.g. during the development of the
instrument.

The occurrence of common cause failures is less likely if thetwo electrical thermometers with temperature transmittersused are as diverse as possible with respect to construction,measuring principle and software. Thus, for example, aresistance thermometer can be used for one channel anda thermocouple may be used for the other channel. For
measuring, one thermowell can be used for the resistancethermometer and another for the thermocouple, or a single
thermowell can be used for both. When using a single
thermowell, the common cause failures are correspondingly
more likely. A higher diversity is additionally achieved
when the temperature transmitters used are from different
manufacturers and differ in their construction as well as theirsoftware.

To account for the effect of common cause failure, a “β factor”
is needed to calculate the PFD value of redundant systems.
The β factor refers to the proportion of undetected common
cause failures. In accordance with IEC 61508-6 and taking
into account that the period of 8 h, which is needed for therenovation of the system, is negligibly small compared tothe proof test interval of 8,760 h, the PFD value for a 1oo2
structure is calculated using the following simplified formula:

In particular, the WIKA model T32.xS temperature transmitter
has the advantage that it can be used in homogeneous
redundant systems up to SIL 3. This means that an electricalthermometer with a model T32.xS temperature transmitteris connected in parallel with a second thermometer witha structurally identical transmitter. In a single-channelarchitecture, the transmitter is suitable up to SIL 2. Dueto the complete development and certification of themodel T32.xS temperature transmitter to all elements of
the IEC 61508 standard (Full-Assessment Development),
the transmitter is also suitable in a homogeneous
redundant assembly for SIL 3 applications. Even during
the development, the measures for fault avoidance in thesoftware have been designed for use in SIL 3 applications.
Thus, the model T32.xS temperature transmitter differs fromoperationally proven instruments that are only suitable for SIL
applications on the basis of earlier use.

WIKA data sheet IN 00.19 ∙ 10/2014

To determine the β factor, measures must first be defined that
reduce the occurrence of common cause failures. Throughengineering assessment it must be defined, in cooperation
with WIKA, the extent to which each measure reduces theoccurrence of common cause failures.

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Summary of recommendations

Abbreviations and definitions

For the best possible design of a temperature measuring
point for safety-related applications, the requirements in thechapter “Requirements for a safety system” must be followed.

Abbreviation Definition

Furthermore, in safety applications, it is recommended thatthe model T32.xS temperature transmitter (head-mounted orrail-mounted version) is used in conjunction with a resistancethermometer in 4-wire connection or with a thermocouple.
Through the extensive diagnostic features of the T32.xS
and the benefits of a 4-wire connection, a high safety in thetemperature measurement is guaranteed.

DC
Extensionwire

To protect the measuring insert from the process medium
and to enable a quick and easy calibration of the electricalthermometer, protective thermometer fittings withexchangeable measuring inserts should be used. It is
important to pay particular attention to the proper design ofthe thermowell in accordance with the requirements of theprocess.

Close
coupledFIT

HFTHigh Stress
Low stress
PFDavgPFH
RTD
SFF

Literature and sources
1.) IEC 61508:2010:
Functional safety of safety-related electrical/electronic/
programmable electronic systems
Beuth Verlag GmbH, 10772 Berlin

SIS
TC
TR

The temperature transmitter is locatedin the connection head of the electricalthermometer (head-mounted).
Diagnostic coverage
The temperature transmitter is locatedoutside of the connection head of theelectrical thermometer, and is located,for example, in a cabinet distant from themeasuring point (remote-mounted).
Failures in time

Hardware Fault Tolerance
Applications with vibration (≥ 67 % of
the maximum vibration resistance of theelectrical thermometer)Low vibration (< 67 % of the maximum
vibration resistance of the electricalthermometer)
Average probability of a dangerous failure
on demand of the safety function
Average frequency of a dangerous failure of
the safety function“Resistance temperature detector”;
resistance thermometer
Safe Failure Fraction of a hardware element
Safety Instrumented System

Thermocouple“Temperature Resistance”; resistancethermometer

2.) Exida:
Safety Equipment Reliability Handbook - 3rd Edition, 2012,
exida.com L.L.C.
3.) WIKA Alexander Wiegand SE & Co. KG:
Safety manual “Information on functional safety fortemperature transmitter model T32.xS” (from firmware
revision 2.2.3)

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Impact of the re-evaluation of the temperature transmitter model T32.xS(from firmware revision 2.2.3) on the safety-relevant characteristic values
Within the scope of the re-evaluation, no safety-relatedchanges were carried out on the temperature transmitter. The
diagnostic coverage of the transmitter remains unchanged.
Only the new assessment approach led to a change in thesafety-relevant characteristic values.
New edition of the IEC 61508 standard
Since the initial assessment of the model T32.xS
temperature transmitter, the base standard for functionalsafety, IEC 61508 “Functional Safety of Electrical/Electronic/
Programmable Electronic Safety-related Systems” has been
updated to the revision IEC 61508:2010. From firmware
revision 2.2.3, the T32.xS will be evaluated against thisedition of the standard.
Updated failure rates
In this context, the FMEDA (Failure Modes, Effects andDiagnostic Analysis) was also repeated with current
component failure rates. The calculations were based oncomponent failure rates in accordance with SN29500. For
the temperature resistance sensors and thermocouples
connected to the temperature transmitter, the failure rates
determined by exida.com LLC were used.

Application-specific failure rates
With the re-evaluation of the T32.xS the failure rates aredefined specifically for the application, depending on thevibration levels at the point of installation of the electricalthermometer and dependent on the connection of thethermometer to the transmitter. Furthermore, failure rates forthe “stand alone” temperature transmitter are calculated for
different configurations.
Rather improved failure rates
The failure rates of the T32.xS transmitter with connected
thermocouple or resistance sensor have shown a trend of
improvement. In particular, for the conditions of “low stress,
close coupled”, the failure rate for hazardous, non-detectable
failures has decreased.
Effects on the PFDavg value
Especially for the application condition “low stress, close
coupled”, the PFDavg value has improved. This allows
the user, if required, to use logic or actuating subsystems
with correspondingly larger PFDavg values ​​in the safety
instrumented system or to extend the proof testing interval.

Elemental analysis of the “sensor” subsystem
With the introduction of the term “element” into IEC 615084:2010 Section 3.4.5, the interconnection of the temperaturetransmitter and electrical thermometer as a “sensor”
subsystem was considered and evaluated as follows:
Element 1
Electrical thermometer
without transmitter(thermocouple orresistance thermometer)

Element 2
Model T32.xS temperaturetransmitter (without
thermocouple orresistance thermometer)

Type A / SFF ≥ 60 % for
HFT = 0 and SIL 2

Type B / SFF ≥ 90 % for
HFT = 0 and SIL 2This separate consideration affects the assessment of the
SFF value. For example, the required SIL 2 SFF value forthermocouples or resistance thermometers drops to 60 %.

10/2014 GB© 2013 WIKA Alexander Wiegand SE & Co. KG, all rights reserved.
The specifications given in this document represent the state of engineering at the time of publishing.
We reserve the right to make modifications to the specifications and materials.

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WIKA Alexander Wiegand SE & Co. KG
Alexander-Wiegand-Straße 30
63911 Klingenberg/GermanyTel.
+49 9372 132-0
Fax+49 9372 132-406
info@wika.de
www.wika.de