ISO 23552-1:2026

International
Standard
ISO 23552-1
Second edition
Control and protective devices
2026-01
for gaseous and liquid fuels —
Particular requirements —
Part 1:
Electronic fuel/air ratio control
systems, including associated
sensors and mechanical actuators
Dispositifs de commande et de protection pour combustibles
gazeux et liquides — Exigences particulières —
Partie 1: Systèmes électroniques de commande des dispositifs de
régulation du rapport air y compris les capteurs associés et les
actionneurs mécaniques
Reference number
© ISO 2026
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Published in Switzerland
ii
Contents Page
Foreword .v
Introduction .vi
1 Scope . 1
2 Normative references . 2
3 Terms and definitions . 2
3.1 Types of electronic fuel/air ratio systems .2
4 Classification . 7
4.1 Classes of controls .7
4.2 Groups of controls .7
4.3 Types of DC supplied controls .7
4.4 Classes of control functions .7
5 Test conditions and tolerances . 8
5.1 Test conditions .8
5.2 Tolerances .8
6 Construction . 8
6.1 General .8
6.2 Construction requirements.9
6.3 Materials .9
6.4 Connections .10
6.5 Gas controls employing with electrical components in the gas way .10
7 Performance . 10
7.1 General .10
7.2 Leak-tightness .10
7.3 Torsion and bending .10
7.4 Rated flow rate .10
7.5 Durability .10
7.6 Functional requirements . .10
7.6.1 General .10
7.6.2 Interface to the automatic burner control system .11
7.6.3 Start-up sequence .11
7.6.4 Range of operation . .11
7.6.5 Restart from defined state . 12
7.6.6 Pre-set/predefined range. 12
7.6.7 Accuracy requirements . 12
7.6.8 Sensor(s) and actuators . . 13
7.6.9 Repeatability . 13
7.6.10 Requirements for the adaptive control function (ACF) . 13
7.7 Endurance . 13
7.7.1 Performance requirements . . 13
7.7.2 Test conditions .14
7.7.3 Thermal stress test .14
7.7.4 Long-term performance test . 15
7.8 Vibration test .16
7.9 Data exchange .16
7.10 Performance test for electronic controls .16
7.10.1 At ambient temperature .16
7.10.2 At lower temperature .16
7.10.3 At high temperature .16
7.11 Protection against internal faults for the purpose of functional safety .16
7.11.1 Design and construction requirements .16
7.11.2 Circuit and construction evaluation .17
7.11.3 Information to be supplied by the manufacturer .17

iii
7.11.4 Information .17
7.11.5 Compliance .18
8 Electrical equipment .18
8.1 General .18
8.2 Requirements .18
8.3 Test .18
8.4 Protection by enclosure .18
9 Electromagnetic compatibility (EMC) . .18
9.1 Protection against environmental influences .18
9.1.1 General .18
9.1.2 Requirements .19
9.2 Harmonics and interharmonics including mains signalling at AC power port, low
frequency immunity . 20
9.3 Voltage dips, voltage interruptions and voltage variations in the power supply network . 20
9.4 Test of influence of voltage unbalance . 20
9.5 Surge immunity test . 20
9.6 Electrical fast transient/burst . 20
9.7 Ring wave immunity .21
9.8 Electrostatic discharge .21
9.9 Radio-frequency electromagnetic field immunity .21
9.9.1 Immunity to conducted disturbances .21
9.9.2 Immunity to radiated disturbances .21
9.10 Influence of supply frequency variations .21
9.11 Power frequency magnetic field immunity .21
9.12 Evaluation of compliance . 22
10 Marking, installation and operating instructions .22
10.1 Marking . 22
10.2 Installation and operating instructions . 22
10.2.1 General . 22
10.2.2 Technical information for the safety after integration in the appliance . 23
10.3 Warning notice .24
Annex A (informative) Leak-tightness test – Volumetric method .25
Annex B (informative) Leak-tightness test – Pressure-loss method .26
Annex C (normative) Conversion of pressure loss into leakage rate .27
Annex D (normative) Gas quick connector (GQC).28
Annex E (informative) Elastomers/requirements resistance to lubricants and gas .29
Annex F (informative) Specific regional requirements in European countries.30
Annex G (normative) Specific regional requirements in Canada and USA .31
Annex H (normative) Specific regional requirements in Japan .32
Annex I (normative) Manufacturer's declaration for sensors, actuators and repeatability .33
Annex J (normative) Special requirements for single position feedback potentiometers in
electromechanical actuators .37
Annex K (informative) Overall fuel/air ratio accuracy .38
Bibliography .40

iv
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards
bodies (ISO member bodies). The work of preparing International Standards is normally carried out through
ISO technical committees. Each member body interested in a subject for which a technical committee
has been established has the right to be represented on that committee. International organizations,
governmental and non-governmental, in liaison with ISO, also take part in the work. ISO collaborates closely
with the International Electrotechnical Commission (IEC) on all matters of electrotechnical standardization.
The procedures used to develop this document and those intended for its further maintenance are described
in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the different types
of ISO document should be noted. This document was drafted in accordance with the editorial rules of the
ISO/IEC Directives, Part 2 (see www.iso.org/directives).
ISO draws attention to the possibility that the implementation of this document may involve the use of (a)
patent(s). ISO takes no position concerning the evidence, validity or applicability of any claimed patent
rights in respect thereof. As of the date of publication of this document, ISO had not received notice of (a)
patent(s) which may be required to implement this document. However, implementers are cautioned that
this may not represent the latest information, which may be obtained from the patent database available at
www.iso.org/patents. ISO shall not be held responsible for identifying any or all such patent rights.
Any trade name used in this document is information given for the convenience of users and does not
constitute an endorsement.
For an explanation of the voluntary nature of standards, the meaning of ISO specific terms and expressions
related to conformity assessment, as well as information about ISO's adherence to the World Trade
Organization (WTO) principles in the Technical Barriers to Trade (TBT), see www.iso.org/iso/foreword.html.
This document was prepared by Technical Committee ISO/TC 161, Controls and protective devices for gaseous
and liquid fuels.
This second edition cancels and replaces the first edition (ISO 23552-1:2007), which has been technically
revised. It also incorporates the Amendment ISO 23552-1:2007/Amd 1:2010.
The main changes are as follows:
— the title and scope have been modified;
— the document has been aligned with ISO 23550 and IEC 60730-1;
— the requirements for fuel/air ratio using oil as the fuel have been integrated;
— the control types ERS and ERT have been added;
— the terms and definitions clause has been expanded and updated;
— the accuracy requirements have been updated;
— Clause 7 “Endurance” has been updated;
— subclause 7.9 “Data exchange” has been added;
— update 7.11 “Protection against internal faults for the purpose of functional safety”;
— update Clause 9 “Electromagnetic compatibility”.
A list of all parts in the ISO 23552 series can be found on the ISO website.
Any feedback or questions on this document should be directed to the user’s national standards body. A
complete listing of these bodies can be found at www.iso.org/members.html.

v
Introduction
This document is designed to be used in combination with ISO 23550. Together with ISO 23550, this
document establishes the full requirements as they apply to the product covered by this document.
Where needed, this document adapts ISO 23550 by stating in the corresponding clause:
— “with the following modification”;
— “with the following addition”;
— “is replaced by the following”;
— “is not applicable”.
In order to identify specific requirements that are particular to this document and that are not already
covered by ISO 23550, this document contains clauses or subclauses that are additional to the structure of
ISO 23550. These subclauses are indicated by the introductory sentence: “Subclause (or Annex) specific to
this document.”
To ensure the global relevance of this document, the differing requirements resulting from practical
experience and installation practices in various regions of the world have been taken into account.
The variations in basic design associated with gas controls and appliances have also been recognized,
some of which are addressed in Annexes F, G and H. This document intends to provide a basic framework of
requirements that recognize these differences.
For electronic fuel/air ratio control/supervision systems, there are numerous solutions for specific
applications in the market. For this reason, ISO/TC 161, Controls and protective devices for gaseous and liquid
fuels, decided to draft an International Standard for type testing for electronic fuel/air ratio control systems
(ERC), electronic fuel/air ratio supervision systems (ERS) and electronic fuel/air ratio trim systems (ERT)
intended for use with burners and appliances burning gaseous or liquid fuels.
NOTE Trim function is a special design of electronic fuel/air ratio control function.
This document does not override requirements of relevant appliance standards. It is the intention of this
document that the safety of the appliance will not be reduced by any normal or abnormal operation of the
ERC, ERS or ERT.
In this document, there is no classification, either by heat input or by applications.
The accuracy of actual fuel/air ratio is not specified as a fixed value.
This document specifies which parameters are declared in the instruction and under what conditions this
declaration is considered fulfilled. These parameters relate to the fuel/air ratio control/supervision systems
rather than the combustion process.
This document does not include a standard test rig. However, the purpose of the tests is to verify the
manufacturer's declaration under the conditions required in this document.

vi
International Standard ISO 23552-1:2026(en)
Control and protective devices for gaseous and liquid fuels —
Particular requirements —
Part 1:
Electronic fuel/air ratio control systems, including associated
sensors and mechanical actuators
1 Scope
This document specifies safety, design, construction and performance requirements and testing for
electronic fuel/air ratio control (FARC) systems. Systems can include sensors, mechanical actuators and
motors/blowers that make up the complete system. Systems can be described as:
— closed loop fuel/air ratio control systems (ERC), see 3.1.1;
— closed loop fuel/air ratio trim systems (ERT), see 3.1.2;
— fuel/air ratio supervision systems (ERS), see 3.2.
NOTE 1 Throughout this document, the word “system” means “FARC control and system components”.
These systems are intended for use with burners and appliances that burn gaseous or liquid fuels. This
document describes the procedures for evaluating these requirements and specifies information necessary
for installation and use.
This document applies to the inherent safety of the system, to the declared operating values, operating times
and operating sequences if they are associated with FARC safety and to the testing of FARC systems used in,
on or in association with appliances.
NOTE 2 The documents for burners, appliances or processes which use ERC, ERS or ERT can override the
requirements of this document.
This document prescribes testing requirements in addition to the relevant standards for integrated devices
or components (e.g. actuators) to ensure safe operation of the FARC system.
If no part 2 standards in ISO/TC 161, Controls and protective devices for gaseous and liquid fuels, or IEC/TC 72,
Textile machinery and accessories, exist, then IEC 60730-1 is considered.
NOTE 3 Supplemental or modified requirements and tests and evaluations can be needed for ERC, ERS or ERT
systems for conformance with relevant burner, appliance or process standards. Conformance of an ERC, ERS or ERT
with this document does not imply that such systems or components thereof are acceptable for use on an appliance
without supplemental tests of the control system applied to the particular appliance to meet the requirements of the
relevant burner, appliance or process standard(s).
This document applies to electronic fuel/air ratio control systems that can be tested independently or as a
system or as part of an appliance or as part of a burner.
This document does not cover burner control function like e.g. burner ignition, flame supervision and fuel
shutoff.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content constitutes
requirements of this document. For dated references, only the edition cited applies. For undated references,
the latest edition of the referenced document (including any amendments) applies.
ISO 23550:2018, Safety and control devices for gas and/or oil burners and appliances — General requirements
IEC 60730-1:2022, Automatic electrical controls — Part 1: General requirements
IEC 60730-2-5:2021, Automatic electrical controls — Part 2-5: Particular requirements for automatic electrical
burner control systems
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 23550 and the following apply.
ISO and IEC maintain terminology databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at https:// www .electropedia .org/
3.1 Types of electronic fuel/air ratio systems
3.1.1
electronic fuel/air ratio control system
ERC
closed-loop system consisting of the electronic control unit, actuating elements for the fuel flow and the air
flow as a minimum, and allocated feedback signal(s) that are used to control fuel/air ratio of a burner
Note 1 to entry: Figure 1 shows an example of different feedback alternatives. For details, see also Table 1.
Key
1 electronic control unit (ECU) 8 sensor flame (for FARC function)
2 combustion process 9 sensor flue gas
3 actuator air 10 actuator air feedback
4 actuator fuel 11 actuator fuel feedback
5 sensor air 12 automatic burner control system

6 sensor fuel
7 sensor fuel/air mixture scope of ERC
a
From the ERC point of view, 12 acts in case a defined state is requested by 1 to de-energize the safety shut-off valves.
Figure 1 — Scope of the electronic fuel/air ratio control system (ERC)
3.1.2
electronic fuel/air ratio trim system
ERT
closed loop system consisting of the electronic control unit, actuating element(s) or control output(s) to
influence the fuel/air ratio controlled by other means (see Figure 2, item 13), and the allocated feedback
signal(s) to the electronic control unit
Note 1 to entry: Figure 2 shows an example of different feedback alternatives. For details, see also Table 1.
Key
1 electronic control unit (ECU) 9 sensor flue gas
2 combustion process 10 actuator air feedback
3 actuator air 11 actuator fuel feedback
4 actuator fuel 12 automatic burner control system
5 sensor air 13 arbitrary control
6 sensor fuel 14 trim signal air
7 sensor fuel/air mixture 15 trim signal fuel
8 sensor flame 16 state signal to 13, e.g. release signal
scope of ERT
a
From the ERT point of view, 12 acts in case a defined state is requested by 1 to de-energize the safety shut-off valves.
Figure 2 — Scope of the electronic fuel/air ratio trim system (ERT)
3.2
electronic fuel/air ratio supervision system
ERS
supervision system consisting of electronic control unit and sensor(s), providing at least one output signal
to indicate if the fuel/air ratio is outside of the safe area of the application
Note 1 to entry: Figure 3 shows an example of different feedback alternatives. For details, see also Table 1.

Note 2 to entry: Electronic control unit is not intended to control actuators.
Key
1 electronic control unit (ECU) 8 sensor flame
2 combustion process 9 sensor flue gas
3 actuator air 10 actuator air feedback
4 actuator fuel 11 actuator fuel feedback
5 sensor air 12 automatic burner control system
6 sensor fuel 13 arbitrary control
7 sensor fuel/air mixture scope of ERC
a
From the ERS point of view, 12 acts in case a defined state is requested by 1 to de-energize the safety shut-off valves.
Figure 3 — Scope of the electronic fuel/air ratio supervision system (ERS)
3.3
electronic control unit
ECU
electronic main control module incorporating all inputs and outputs of the ERC, ERS or ERT function
3.4
actuator
device for controlling the amount of fuel and/or air
Note 1 to entry: A variable speed fan is an example of an air actuator.
3.5
sensor
device which responds to a physical activating quantity on the sensing element that then generates a signal
related to the value of that activating quantity
3.6
combustion process
chemical reaction between fuel and air to produce heat

3.7
defined state
state assumed by system to ensure a safe situation under all circumstances
Note 1 to entry: For example, the system can execute a protective action to the burner control system causing it to
safety shut-down and lock out or the system remains in operation, continuing to satisfy all safety related functional
requirements.
3.8
fault tolerating time
FTT
time between the occurrence of a fault and reaching a defined state, which is tolerated by the application
without resulting in a hazardous situation
Note 1 to entry: For illustration of the fault tolerating time, see Figure 4 or Figure 5.
[SOURCE: IEC 60730-1:2022, H.3.22.2, modified — The part of the sentence “the shut-down of the controlled
equipment” has been replaced by “reaching a defined state”. Note 1 to entry has been replaced.]
3.9
fault reaction time control
FRTc
time that the ERC, ERS or ERT needs from the moment the fault condition reaches the sensor to the moment
of an initialization to reach a defined state
Note 1 to entry: For illustration of the fault reaction time of control for ERC and ERS, see Figure 4.
Note 2 to entry: For illustration of the fault reaction time of control for ERT, see Figure 5.
Key
1 fuel/air ratio moves outside of the safe area of the application
2 effect of fault condition reaches the sensor
3 sensor e.g. position sensors, flue gas sensors outputs effect of fault
4 sensor output is amplified
5 amplified signal passes filter
6 effect of fault condition is flagged by limit switch
7 flag is processed by automatic burner control
8 defined state is reached
Figure 4 — Relationship between fault tolerating time and fault reaction time of control for ERC and
ERS
Key
1 fuel/air ratio moves outside of the safe area of the application
2 effect of fault condition reaches the sensor
3 sensor e.g. position sensors, flue gas sensors outputs effect of fault
4 sensor output is amplified
5 amplified signal passes filter
6 effect of fault condition is flagged by limit switch
a
7 flag may be processed by optional control system to initiate a defined state
8 shut-off element or actuating element influences the fuel/air ratio to reach a defined state
a
Examples for optional control are key 12 or key 13 in Figure 2. An optional control can only be used to reach
a defined state if the control has the required safety class for this function.
Figure 5 — Relationship between fault tolerating time and fault reaction time of control for ERT
3.10
safety shut-down
de-energization of the main fuel flow means as the result of the action of a limiter, a cut-out or the detection
of an internal fault of the system
Note 1 to entry: Safety shut-down may include additional actions by the system.
[SOURCE: IEC 60730-2-5:2021, 2.3.32]
3.11
lock-out
process in which the system goes into either volatile lock-out (3.11.2) or non-volatile lock-out (3.11.1)
following safety shut-down
[SOURCE: IEC 60730-2-5:2021, 2.3.112]
3.11.1
non-volatile lock-out
condition such that a restart can only be accomplished by a manual reset of the system and by no other cause
[SOURCE: IEC 60730-2-5:2021, 2.3.112.1]
3.11.2
volatile lock-out
condition such that a restart can be accomplished by either a manual reset of the system or by an interruption
of the power supply and its subsequent restoration
[SOURCE: IEC 60730-2-5:2021, 2.3.112.2]

3.12
normal operation
operation of the ERC, ERS or ERT control system within its specification including the effect of influences
which can occur during intended operation
Note 1 to entry: Examples of influences on sensors are given in Table I.1.
3.13
abnormal operation
operation of the appliance under the effect of internal failures of the ERC, ERS or ERT control system
3.14
form closure construction
mechanical construction ensuring non-slippage of a connection between at least two components
3.15
accuracy
ability to provide an indicated value close to the true value
[SOURCE: IEC 60050-311:2001+A1: 2015+ A2: 2016+ A3: 2017+ A4: 2020, 311-06-08, modified — The term
“measuring instrument” and expression “of the measurand” have been deleted. Note 1 to entry and Note 2 to
entry have been deleted.]
3.16
adaptive control function
ACF
control function, intended to maintain λ constant in a range Δλ by adapting the flow of fuel, and/or the flow
of air and/or other physical quantity to compensate changes in input parameters relevant for the combustion
process
Note 1 to entry: Changes in input parameters can be the composition of the fuel or the combustion air temperature.
4 Classification
4.1 Classes of controls
Shall be according to ISO 23550:2018, 4.1.
4.2 Groups of controls
ISO 23550:2018, 4.2 is not applicable.
4.3 Types of DC supplied controls
Shall be according to ISO 23550:2018, 4.3.
4.4 Classes of control functions
Shall be according to ISO 23550:2018, 4.4 with the following addition:
The class of control function shall be determined based on a risk assessment that has been agreed upon with
a certification body. If no risk assessment is available the ERC, ERT, ERS shall be a class C control function.
If the control functions are combined with each other or with other fuel/air ratio control system technologies
(e.g. pneumatic, mechanical), then they may have a lower class of control function if the entire ERC, ERS or
ERT complies with determined class of control function.

5 Test conditions and tolerances
5.1 Test conditions
For the purposes of this document, ISO 23550:2018, 5.1 is replaced by the following:
Unless otherwise stated, the ERC, ERS or ERT shall be tested in the relevant situations such as standby,
start-up, normal operation and lock-out (volatile or non-volatile).
NOTE A lock-out situation can also be a lock-out demand signal to the automatic burner control system.
Unless otherwise stated, the practical assessment shall be carried out under the following conditions:
— ambient temperature: 20 °C ± 5 °C;
— relative humidity: between 40 % and 80 %;
— at rated supply voltage and frequency.
Testing shall always be performed according to a test plan, which shall be included in the test report. The
relevant assessment criteria should be part of this test plan.
Where possible, the tests already covered by other standards (e.g. the relevant parts of IEC 60730-1) can be
combined with tests presented in this document.
5.2 Tolerances
Shall be according to ISO 23550:2018, 5.2.
6 Construction
6.1 General
Shall be according to ISO 23550:2018, 6.1 with the following addition.
The electrical components shall be designed for their intended use.
The quality of the materials and the design and structure of the components used shall be such that the
control will operate safely and in accordance with the requirements of this document for its designed
lifetime as stated in the instruction. That shall continue under the mechanical, chemical, thermal and
environmental conditions normally expected, even in the event of careless behaviour as can occur in normal
use. Compliance is checked by carrying out the tests specified in this document.
The ERC, ERS or ERT shall be designed such that changes in critical component values (e.g. those affecting
timing, sequence and accuracy) within the worst-case specifications of the components, including the long-
term stability, shall result in the control continuing to function in accordance with this document.
The ERC shall consist of at least two actuators and at least one sensor; see Figure 1 and Table 1, which shall
be considered together.
The ERS shall consist of at least one sensor and one output to initiate reaching a defined state; see Figure 3
and Table 1, which shall be considered together.
The ERT shall consist of at least one output to trim the fuel/air ratio and at least one sensor; see Figure 2 and
Table 1, which shall be considered together.
The ERC, ERS or ERT shall have a self-checking function. When the ERC, ERS or ERT is not active (e.g. in
stand-by), any fault in its control function shall move to or remain in a defined state.
An ERC, ERS or ERT shall use a feedback system which ensures the fuel/air ratio meets the requirements of
7.6.6.
When analysing the system safety, the complete system shall be taken into account. This shall include the
peripheral elements, e.g. servo motors, actuators, positional devices, sensors, variable-speed controls for
combustion air fans and combustion analysis feedback systems.
Table 1 — Acceptable feedback types
Actuator output
Actuator feedback Process feedback
feedback
Fuel/ air
b
Position Speed Flow/Δp p Flame Flue gas
ratio
Valve /
X X X
Actuator
damper
Air
a, d
Fan X X X
Valve / X X X
X X X
damper
d, e
Actuator
Pump X X X
Fuel
Pressure / X X
flow regu-
c c
At least 2 feedbacks (1 fuel, 1 air) required At least 1 feedback required
lator
a
If a fan speed signal is used to control the air flow, proof of air flow cannot rely on fan rotation only. A possible additional
signal can be derived from an independent air proving device at least checked during start up.
b
A feedback signal which is directly related to the mechanical part of the actuator.
c
The two actuator feedbacks and the process feedback are alternatives.
d
A feedback signal from a variable speed control (e.g. a frequency converter) can be used as long as the feedback signal is
derived through a class C control function representing the actual speed.
e
For pumps used on oil burners.
6.2 Construction requirements
Shall be according to ISO 23550:2018, 6.2 with the following addition.
6.2.11 Special requirements for electromechanical actuators with position feedback sensors
Subclause specific to this document.
The actuator feedback sensor shall always represent the actual position of the controlling element. At least
the following aspects shall be considered:
— The mechanical connection between the actuator and the actuator feedback sensor shall be of form
closure construction or of adequate design to ensure that no slippage occur.
— These requirements shall include the complete connection up to the shaft of the control element.
Compliance shall be verified by design assessment.
If an actuator consists of a separate actuating and controlling element, the non-slippage requirement shall
be fulfilled. Screws, pins and other components that are necessary to ensure no slippage shall be secured,
e.g. by a locking compound.
6.3 Materials
Shall be according to ISO 23550:2018, 6.3 with the following addition in 6.3.1.2:
6.3.1.2 Requirements
Shall be according to ISO 23550:2018, 6.3.1.2 with the following addition.

Components constructed of non-metallic material shall be considered with respect to the mechanical,
chemical, thermal and environmental conditions of the intended use. The following shall apply:
— dimensional stability;
— flow characteristics of the material;
— possible heat shrin
...