IEC TS 61400-31:2023

Wind energy generation systems - Part 31: Siting risk assessment

IEC TS 61400-31:2023 establishes a guideline for the assessment of the risks which a wind turbine may pose to the general public.
This document aims to facilitate a uniform scope and a uniform use of methods in wind turbine risk assessments. This document covers risk due to internal or external causes, such as technical failures, human errors, extreme wind conditions, turbine icing, lightning strikes, earthquakes, flooding, landslides or fire.This document covers only onshore wind turbines with a horizontal axis and a swept area greater than 200 m2. Substations and other external structures are excluded.
This document describes risks during operation of the wind turbine including maintenance, idling and standstill. It does not describe risks during construction, civil works, crane operations, assembly or decommissioning.
Risks according to this document are assessed by prescriptive and/or risk-based methods.
This document covers risk reduction measures that might be necessary to reduce risk to a tolerable level.

General Information

• Status: Published
• Publication Date: 14-Nov-2023
• ICS: 27.180 - Wind turbine energy systems
• Technical Committee: TC 88 - Wind energy generation systems
• Drafting Committee: PT 61400-31 - TC 88/PT 61400-31
• Current Stage: PPUB - Publication issued
• Start Date: 09-Aug-2023
• Completion Date: 15-Nov-2023

Overview

IEC TS 61400-31:2023 - Wind energy generation systems, Part 31: Siting risk assessment - is an IEC Technical Specification (Edition 1.0, 2023-11) that provides guidelines for assessing the risks a wind turbine may pose to the general public. The standard focuses on onshore, horizontal-axis wind turbines with a swept area greater than 200 m² and covers risks during operation (including maintenance, idling and standstill). Substations and external structures are excluded. The document supports a uniform scope and consistent methods for wind turbine risk assessment, addressing internal and external causes such as technical failures, human error, extreme wind, icing, lightning, earthquakes, flooding, landslides and fire.

Key Topics

• Risk assessment process: documented workflow, stakeholder involvement and lifecycle risk management.
• Hazard identification: turbine failure modes (tower collapse, hub/nacelle shedding, blade failure), ice fall/throw, fire and domino effects.
• Risk estimation methods: prescriptive, qualitative, semi‑quantitative and quantitative approaches; selection flowcharts for method fidelity.
• Risk acceptance criteria: individual and societal risk regions, f–N plots and policy factors - options for applying prescriptive or risk‑based thresholds.
• Risk treatment and reduction: selection and documentation of measures (e.g., exclusion zones, ice detection/rotor heating systems), plus examples and guidance.
• Uncertainty and reporting: guidance on documenting assumptions, models and uncertainty sources.
• Informative annexes: failure frequency summaries, international risk-criteria overview and blade/fragment trajectory modeling.

Applications

IEC TS 61400-31 is practical for activities that require documented, repeatable siting risk assessment for onshore wind projects:

• Project developers and site assessors: informing site selection, layout and setback distances to meet tolerable risk targets.
• OEMs and safety engineers: evaluating design-related failure consequences and mitigation needs.
• Consultants and risk analysts: conducting hazard identification, quantitative risk modelling and reporting to regulators.
• Regulators, permitting authorities and insurers: using standardized risk assessments for approvals, compliance checks and underwriting decisions.
• Operations and maintenance teams: integrating operational risk controls and emergency response planning.

Related standards

IEC TS 61400‑31 complements other IEC 61400 series parts (wind turbine design, testing and safety) and aligns with general risk-management guidance (ISO/IEC guides referenced in the document). For practical compliance, users typically combine this Technical Specification with site‑specific meteorological, geotechnical and land‑use regulations.

Keywords: IEC TS 61400-31:2023, siting risk assessment, wind turbine risk assessment, onshore wind turbines, risk-based methods, prescriptive risk criteria, ice throw, blade failure, tower collapse.

Standards Content (Sample)

IEC TS 61400-31 ®
Edition 1.0 2023-11
TECHNICAL
SPECIFICATION
colour
inside
Wind energy generation systems –
Part 31: Siting risk assessment

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IEC TS 61400-31 ®
Edition 1.0 2023-11
TECHNICAL
SPECIFICATION
colour
inside
Wind energy generation systems –

Part 31: Siting risk assessment

INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
ICS  27.180 ISBN 978-2-8322-7744-7

– 2 – IEC TS 61400-31:2023 © IEC 2023
CONTENTS
FOREWORD . 5
1 Scope . 7
2 Normative references . 8
3 Terms, definitions and symbols. 8
3.1 Terms and definitions . 8
3.2 Symbols used in this document . 12
3.3 Abbreviated terms . 13
4 Risk assessment process . 13
4.1 Overview. 13
4.2 Documentation requirements in the risk assessment process . 14
4.3 Involvement of stakeholders. 14
5 Risk management throughout service life . 15
5.1 Overview. 15
5.2 Reviewing, documenting and reporting of the risk management process . 15
6 Harm to people . 16
6.1 Overview. 16
6.2 Direct harm . 16
6.3 Indirect harm. 16
6.4 Domino effect . 17
6.5 Consequences of impacts of objects . 17
7 Risk assessment approaches and associated acceptance criteria . 17
7.1 Risk assessment approaches . 17
7.2 Risk acceptance criteria . 19
7.3 Risk regions . 20
7.4 Types of risk criteria . 21
7.5 Prescriptive risk acceptance criteria . 21
7.6 Qualitative risk acceptance criteria . 22
7.7 Semi-quantitative risk acceptance criteria . 22
7.8 Quantitative risk acceptance criteria . 23
7.8.1 General . 23
7.8.2 Quantitative risk criteria for individuals . 24
7.8.3 Quantitative societal risk criteria . 26
8 Hazard identification . 30
8.1 General . 30
8.2 General principles of hazard identification . 30
8.3 Wind turbine failure modes . 30
8.3.1 General . 30
8.3.2 Tower collapse . 30
8.3.3 Shedding of hub or nacelle . 30
8.3.4 Rotor blade failure . 31
8.4 Ice fall and ice throw . 31
8.5 Fire . 32
8.6 Occupancy . 32
8.7 Project relevant hazards . 33
9 Estimation of the risk . 33
9.1 General . 33

9.2 Wind turbine failures – tower collapse, shedding of hub or nacelle and rotor
blade failure . 33
9.2.1 General . 33
9.2.2 Input information . 34
9.2.3 Additional assumptions/models . 34
9.2.4 Tower collapse . 35
9.2.5 Shedding of hub or nacelle . 35
9.2.6 Blade breakage . 35
9.2.7 Summation of impact probabilities and risks . 36
9.3 Ice fall and ice throw . 36
9.3.1 Input information . 36
9.3.2 Additional assumptions/models . 37
9.3.3 Calculation of trajectories of ice pieces . 37
9.4 Wind turbine fire . 38
9.5 Calculation of the risk . 38
9.5.1 General . 38
9.5.2 Effective cross-section for people and cars . 39
9.6 Analysis of domino effects . 39
10 Risk evaluation . 40
11 Risk treatment . 40
11.1 General . 40
11.2 Selection of risk reduction measures . 40
11.3 Examples of risk reduction measures . 40
11.4 Ice detection systems and rotor blade heating systems . 41
12 Uncertainties in risk assessments . 42
Annex A (informative) Summary of failure frequencies published by the Dutch RIVM . 44
Annex B (informative) Overview of used risk criteria in different countries . 45
Annex C (informative) Introduction to trajectory models for blades and blade
fragments . 49
Bibliography . 54

Figure 1 – Flow chart of the risk assessment process (Modified from
ISO/IEC Guide 51 [3]) . 13
Figure 2 – The risk assessment process . 14
Figure 3 – Flow chart of the selection of risk assessment methods with different levels
of fidelity . 19
Figure 4 – Risk regions . 20
Figure 5 – Example tables for a semi-quantitative risk assessment . 23
Figure 6 – Combination of hazards and impacted persons. . 27
Figure 7 – Example of an f-N plot . 28
Figure 8 – Example of societal risk criteria . 29
Figure C.1 – Blade-fixed and inertial reference frames. . 50

– 4 – IEC TS 61400-31:2023 © IEC 2023
Table 1 – Examples of risk acceptance criteria for different risk assessment
approaches . 21
Table 2 – Policy factor according to [11] . 26
Table 3 – Examples for hazardous installations that could be affected by domino
effects triggered by wind turbine failures . 39
Table A.1 – Failure frequencies from [13] in units of failures per turbine and year. . 44
Table B.1 – Overview of used risk criteria in different countries . 45

INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
WIND ENERGY GENERATION SYSTEMS –

Part 31: Siting risk assessment

FOREWORD
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IEC TS 61400-31 has been prepared by subcommittee PT 61400-31: Wind energy generation
systems – Part 31: Siting Risk Assessment, of IEC technical committee 88: Wind energy
generation systems. It is a Technical Specification.
The text of this Technical Specification is based on the following documents:
Draft Report on voting
88/936/DTS 88/956/RVDTS
Full information on the voting for its approval can be found in the report on voting indicated in
the above table.
– 6 – IEC TS 61400-31:2023 © IEC 2023
A list of all parts in the IEC 61400 series, published under the general title Wind energy
generation systems, can be found on the IEC website.
The language used for the development of this Technical Specification is English.
This document was drafted in accordance with ISO/IEC Directives, Part 2, and developed in
accordance with ISO/IEC Directives, Part 1 and ISO/IEC Directives, IEC Supplement, available
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WIND ENERGY GENERATION SYSTEMS –

Part 31: Siting risk assessment

1 Scope
This part of IEC 61400, which is a Technical Specifiation, establishes a guideline for the
assessment of the risks which a wind turbine may pose to the general public.
Incidents in wind farms causing harm to the general public are very rare events. However, there
are requirements to cover this topic in the permitting procedures of several countries. This
document aims to facilitate a uniform scope and a uniform use of methods in wind turbine risk
assessments.
This document covers harm to the general public. It does not cover occupational exposure, e.g.
of personnel involved in the operation and maintenance of the turbine, since occupational risks
are usually dealt with in occupational health and safety regulations. The risk of damage to
structures or other objects is also not part of this document unless such damage in turn poses
a risk to the public.
Harm according to this document can be direct harm or indirect harm via damage to buildings
or infrastructure, e.g. gas pipelines, nuclear facilities, dykes, rail infrastructure or roads.
This document covers risk due to internal or external causes, such as technical failures, human
errors, extreme wind conditions, turbine icing, lightning strikes, earthquakes, flooding,
landslides or fire. However, the specific cause of an incident (e.g. an incident such as a turbine
collapse) is irrelevant to the assessment of the consequences. The only relevant factor is the
expected probability of occurrence for the incident considered.
In terms of transmission of the hazard to the people affected, this document describes tower
collapses, shedding of the nacelle, blade failures, falling or throwing of ice pieces and fire
spread.
This document does not cover risks from visual distraction and environmental risk such as noise
or shadow flicker.
Wind turbines may pose a hazard to aviation through incidents such as collisions with aircrafts
or disturbance of air traffic control radar. These hazards are not covered in this document. In
order to mitigate the hazard of aircrafts colliding with wind turbines, aviation lights are installed
on wind turbines as covered in IEC 61400-29[1] .
Risks connected to terrorist attacks and other malicious actions are not covered by this
document.
___________
Numbers in square brackets refer to the Bibliography.

– 8 – IEC TS 61400-31:2023 © IEC 2023
This document covers only onshore wind turbines with a horizontal axis and a swept area
greater than 200 m . Substations and other external structures are excluded. Other tall
structures associated with a wind farm or wind turbine (e.g. temporary or permanent
meteorological masts) also introduce risks related to their possible collapse or failure. Such
structures are not covered by this document. Guidance on the risks can be inferred from the
reliability classes of the tall structure as determined with reference to EN 1993 Eurocode 3:
Design of steel structures [2], including the national annexes where local design requirements
are specified.
As to the extent of the harm, this document is limited to the immediate, potentially lethal,
physical harm. Non-lethal harm is indirectly covered as described in Clause 6.
This document describes risks during operation of the wind turbine including maintenance,
idling and standstill. It does not describe risks during construction, civil works, crane operations,
assembly or decommissioning.
Risks according to this document are assessed by prescriptive and/or risk-based methods.
In evaluating risk, the risk is first expressed as a localized risk. Along with the probability of
people being present at the location, a risk of lethal harm per year will be used to quantify the
risk of harm to people.
This document covers risk reduction measures that might be necessary to reduce risk to a
tolerable level.
2 Normative references
There are no normative references in this document.
3 Terms, definitions and symbols
For the purposes of this document, the following terms, definitions and symbols apply.
ISO and IEC maintain terminological databases for use in standardization at the following
addresses:
• IEC Electropedia: available at http://www.electropedia.org/
• ISO Online browsing platform: available at http://www.iso.org/obp
3.1 Terms and definitions
3.1.1
risk
combination of the probability of occurrence of harm and the severity of that harm
[SOURCE: IEC 60050-903:2013, 903-01-07]
3.1.2
harm
physical injury or damage to persons, property, and livestock
Note 1 to entry: Harm to property and to livestock is excluded from the scope of this document.
[SOURCE: IEC 60050-903:2013, 903-01-01, modified – addition of Note 1 to entry]

3.1.3
hazard
potential source of harm
Note 1 to entry: In English, the term "hazard" can be qualified in order to define the origin of the hazard or the
nature of the expected harm (e.g. "electric shock hazard", "crushing hazard", "cutting hazard", "toxic hazard", "fire
hazard", "drowning hazard").
[SOURCE: IEC 60050-903:2013, 903-01-02, modified – deletion of Notes 2 and 3 to entry]
3.1.4
risk assessment
overall process comprising a risk analysis and a risk evaluation
[SOURCE: IEC 60050-903:2013, 903-01-10]
3.1.5
quantitative risk assessment (QRA)
techniques which allow the risk associated with a particular activity to be estimated in absolute
quantitative terms rather than in relative terms such as high or low
[SOURCE: ISO/TS 16901:2015, 3.24, modified – deletion of Note 1 to entry]
3.1.6
risk management
coordinated activities to direct and control an organization with regard to risk
[SOURCE: ISO 31000:2018, 3.2]
3.1.7
risk analysis
systematic use of available information to identify hazards and to estimate the risk
[SOURCE: IEC 60050-903:2013, 903-01-08]
3.1.8
risk evaluation
procedure based on the risk analysis to determine whether the tolerable risk has been achieved
[SOURCE: IEC 60050-903:2013, 903-01-09]
3.1.9
risk treatment
process of selection and implementation of measures to mitigate risk
[SOURCE: ISO 13824:2020, 3.17]
3.1.10
tolerable risk
risk which is accepted in a given context based on the current values of society
[SOURCE: IEC 60050-903:2013, 903-01-12]

– 10 – IEC TS 61400-31:2023 © IEC 2023
3.1.11
ALARP (as low as reasonably practicable)
reducing a risk to a level that represents the point, objectively assessed, at which the time,
trouble, difficulty, and cost of further reduction becomes unreasonably disproportionate to the
additional risk reduction obtained
[SOURCE: ISO/TR 17177:2015]
3.1.12
safe life
prescribed service life with a declared probability of catastrophic failure
[SOURCE: IEC 60050-415:1999, 415-02-08]
3.1.13
residual risk
risk remaining after protective measures have been taken
[SOURCE: IEC 60050-903:2013, 903-01-11]
3.1.14
stakeholder
individual, group or organization that has an interest in an organization or activity
Note 1 to entry: Usually a stakeholder can affect or is affected by the organization or the activity.
[SOURCE: IEC 60050-904:2014, 904-01-10]
3.1.15
risk reduction measure
protective measure
measure intended to achieve adequate risk reduction, implemented:
• by the designer (inherent design, safeguarding and complementary protective measures,
information for use) and
• by the user (organization: safe working procedures, supervision, training; permit-to-work
systems; provision and use of additional safeguards; use of personal protective equipment)
[SOURCE: IEC 60050-904:2014, 903-01-17, modified – risk reduction measure has been
specified as a synonym]
3.1.16
hub height
height of the centre of the wind turbine rotor above the terrain surface
[SOURCE: IEC 60050-904:2014, 415-05-06, modified – deletion of part of definition referring to
vertical axis wind turbines as these are out of scope for this document]
3.1.17
hazard log
document that records or references the identified hazards, the decisions made, the adopted
solutions and the status of implementation
3.1.18
prescriptive approach
method for controlling risks by prescribing rules reflecting industry experience, engineering
judgement, and conservative assumptions

3.1.19
qualitative risk assessment method
method for assessing risk where experts decide on the acceptability of identified risks based
subjective judgement
Note 1 to entry: The outcome of a qualitative risk assessment method is a binary result acceptable/not acceptable.
3.1.20
semi-quantitative risk assessment method
method for assessing risk that involves a categorization of the likelihood of hazardous events
and a categorization of the associated consequences and the assignment of defined risk levels
to each combination of these two categories
3.1.21
quantitative risk measures
standard unit used to express the degree of risk
Note 1 to entry: The risk measures used in quantitative risk assessment relate to 2 objectives:
• The protection of an individual person.
• The protection of a group of people.
3.1.22
individual risk
risk measure for an individual person calculated for the (usually hypothetical) person with the
highest exposure to the hazard
Note 1 to entry: This is an equity-based measure, which starts with the premise that all individuals have
unconditional rights to certain levels of protection. This leads to standards, applicable to all, held to be usually
acceptable in normal life, or which refer to some other premise held to establish an expectation of protection. In
practice, this often converts into fixing a limit to represent the maximum level of risk to which an individual may be
exposed. If the risk estimate derived from the risk assessment is above the limit and further control measures cannot
be introduced to reduce the risk, the risk is held to be unacceptable whatever the benefits.
The measure is defined for a hypothetical most-critical individual, who is exposed to the risk. This hypothetical person
describes an individual, who is in some fixed relation to the hazard and for whom it is assumed, that he/she is most
exposed to it. In this way his/her characteristics (time of exposure, place, etc.) make the risk to which she/he is
subjected over-arching for each possible realistic individual exposure. By using this approach, it will be guaranteed
that each individual person will be protected according to an acceptance framework.
For individual risk, the unit of risk is the loss of life per year.
3.1.23
IRPA (individual risk per annum)
probability that a specific or hypothetical individual will be killed due to exposure to the hazards
or activities during one year
Note 1 to entry: To be able to determine this value, quantitative knowledge is needed about the use of the hazardous
area by individuals. In the population that uses the area, a hypothetical individual is defined such that the associated
risk is conservative for all individuals in the exposed group. This selected hypothetical person is called the critical
individual. When the critical individual is defined as a person who is permanently localized at a given point his IRPA
equals the LIRA-value at this location. This can thus be written as
-1 -1
IRPA [fatalities yr ] = LIRA [fatalities yr ] · probability of occupancy [-].
3.1.24
LIRA (localized individual risk per annum)
probability that an average unprotected person, permanently present at a specified location, is
killed during one year due to a hazardous event
Note 1 to entry: This value is independent of any specific individual and is defined locally. This probability is defined
for the locality in which the exposure is present. Since this value is determined for a specific location, it is possible
to draw contours of equal LIRA-values (LIRA-contours) on a map. These LIRA contours are expressed in units of
-1 -2
[fatalities yr m ].
– 12 – IEC TS 61400-31:2023 © IEC 2023
3.1.25
societal risk
collective risk measure for a group of people exposed to the hazard
Note 1 to entry: Societal risk reflects the society’s point of view. In this perspective, very unlikely hazards with
widespread consequence become important. For societal risk, the unit of risk is the loss of life per year. Societal risk
is generally expressed by f-N curves
3.1.26
domino effect
cumulative effect produced when one primary undesired event sequentially or simultaneously
triggers one or more secondary undesired events in nearby installations
Note 1 to entry: The domino effect can lead to overall consequences that are much more severe than the
consequences of the initial incident. Due to the domino effect, the overall risk of close-by installations can be greater
than the combined risk of each of the installations individually. The word domino in this context refers to the triggering
of secondary incidents, not to a long chain of subsequent events.
3.1.27
f-N curve
relationship between frequency and the number of people suffering from a specified level of
harm in a given population from the realization of specified hazards
Note 1 to entry: An f-N curve plots the annual frequency f of events which cause at least N fatalities against the
number N on log-log scales.
3.1.28
ice throw
ice detaching from the rotor with an initial velocity
Note 1 to entry: The motion of the rotor blade of an operating wind turbine will impart a significant initial velocity to
ice pieces shedding from the blade surface. This initial velocity affects the ice pieces’ trajectory and their impact
position on the ground.
3.1.29
ice fall
ice detaching from a turbine with no or very little initial velocity
Note 1 to entry: Ice shedding from the rotor blades of a stopped or idling wind turbine will have no or little initial
velocity. The ice pieces’ trajectories thus result solely from gravity and wind drift.
3.1.30
effect distance
maximum distance around a wind turbine at which a certain hazard originating from the wind
turbine can be expected to have a relevant effect
3.1.31
tip height
hub height plus half of the rotor diameter
3.2 Symbols used in this document
-1
R Minimum endogenous mortality (MEM) rate [yr ]
m
-1
R Maximum allowable additional mortality rate for hazard i [yr ]
i
β Policy factor for voluntariness and direct benefit [-]
-1
f Cumulative frequency of N or more fatalities in an f-N plot [yr ]
N Number of fatalities in an f-N plot [-]
a Risk version factor; slope in an f-N plot [-]
-1
k Constant factor in a f-N plot [yr ]

-1
P Probability of lethal consequences from hazard i [yr ]
death,i
-1
S Survival probability for hazard I, S = 1 − P [yr ]
i i death,i
-1
P Total probability of lethal consequences from several hazards [yr ]
death,tot
m Mass of an ice piece [kg]
A Frontal area of an ice piece [m ]
-3
ρ Ice density of an ice piece [kg m ]
3.3 Abbreviated terms
EMR endogenous mortality rate
MEM minimum endogenous mortality
4 Risk assessment process
4.1 Overview
A flow chart of the risk assessment, modified from ISO/IEC Guide 51 [3], is illustrated in
Figure 1.
Figure 1 – Flow chart of the risk assessment process
(Modified from ISO/IEC Guide 51 [3])

– 14 – IEC TS 61400-31:2023 © IEC 2023
Some more details regarding the activities in the risk assessment process are given in Figure 2.
In this illustration, motivated by the Norwegian standard NS 5814 [4], the risk assessment
process is structured in four main steps.

Figure 2 – The risk assessment process
4.2 Documentation requirements in the risk assessment process
A risk assessment process shall as a minimum cover all four steps as shown in Figure 2.
Depending on the results and information obtained throughout the process, it might be
necessary to reiterate one or more steps in the risk assessment process until the risk is within
acceptable limits.
The result of the risk assessment process shall be documented in a risk assessment report.
The report shall at least cover these four steps and make it possible to follow the reasoning in
the risk assessment. The choices made in the process shall be accounted for. Any need for
further work shall be pointed out.
Any deviations from the requirements in this document shall be described and justified.
If other documents and literature are referenced, these shall be stated.
4.3 Involvement of stakeholders
The scope and purpose of the assessment determine the stakeholders that need to be involved
in the siting risk assessment. Examples of stakeholders in the risk assessment of a wind farm
include developers, investors, original equipment manufacturers, landowners, operators,
constructors, neighbors, communities and authorities. This list is non-exhaustive and the
relevant stakeholders can be very specific to a particular site.

To perform a siting risk assessment, one should strive to involve all relevant stakeholders in
the process of managing risk from a wind farm. The purpose is to ensure that all relevant
stakeholders understand the risk and the decisions that are made regarding risk mitigation.
This communication shall support an adequate understanding of all relevant risks and how the
risk is mitigated and controlled.
5 Risk management throughout service life
5.1 Overview
In order to maintain the objectives of the risk assessment during the lifetime of the project,
regularly scheduled activities might be needed. Such activities can include upholding of certain
risk reduction measures, monitoring for changes regarding the occupation and usage in the
environment of the wind farm, and the identification of newly emerging hazardous situations.
Therefore, a risk management process shall be in place throughout the lifetime of the wind farm
that at minimum executes the risk management tasks identified in the specific project.
Risk management is the identification, evaluation, prioritization, and treatment of risks (as
defined in ISO 31000) followed by coordination of resources to minimize, monitor, and control
the probability or impact of unfortunate events. ISO 31000 identifies key risk concepts, roles
and responsibilities that should be considered when developing risk management guidelines.
For a wind turbine project, the risk management process runs through all of the various project
phases:
• Pre-construction, planning and wind farm layout design phase
• Building phase
• Operation phase
• Life extension and upgrading phase
• Decommissioning phase.
The preparation of risk assessments can be a part or a step within a risk management process.
Methods, definitions and goals vary widely according to whether the risk management method
is in the context of project management, security, engineering, industrial processes, financial
portfolios, actuarial assessments, or public health and safety.
Strategies to manage risks typically include avoiding the risks, reducing the negative
consequences or probability of occurrence for a risk, or accepting some or all of the potential
or actual consequences of a particular risk. This document covers potentially lethal physical
harm to people. Risk transfer to another party (e.g. insurance) is therefore not an option.
5.2 Reviewing, documenting and reporting of the risk management process
The risk management process should be reviewed regularly to ensure that the risk management
process lives up to its purpose to maintain the risk objectives and to keep up with changing
conditions.
The risk management process shall be documented.
Policies and procedures for managing risk shall be updated when operational data, reporting of
incidents or near misses at facilities suggest a need for doing so.

– 16 – IEC TS 61400-31:2023 © IEC 2023
A dedicated system, often described as the hazard log, can be used for the documentation and
follow-up of hazards and thus form the basis for the on-going risk management process. The
log represents a tool to list hazards, how they are handled and followed up. Typical contents of
a hazard log are:
• Description of hazard including cause(s), likely consequence, and frequency of occurrence.
• The risks related to the hazard.
• Risk reduction measures implemented or planned to be implemented.
• Date for implementation and responsibility for implementation. This can include regularly
scheduled actions.
6 Harm to people
6.1 Overview
Different wind turbine failure modes, determined by external conditions and factors of influence,
can cause immediate potentially lethal injury to people or cause damage to structures, which in
consequence can also cause potentially lethal harm to people.
Whether the risk is acceptable shall be determined by a risk assessment based on an evaluation
of the risk of death. This document does not cover non-lethal risks and near-misses. Note that
injuries are not ignored but are covered in a conservative way as described below.
The most severe harm to a person is death. Because of its overwhelming and irrecoverable
impact, the limits for the acceptable risk of death are the most stringent ones. The acceptable
limits for lesser harm such as injuries are, by comparison, much higher (see EN 50126-2,
Annex A.4 [5]). Thus, even though the probability of occurrence for incidents causing lesser
harm will in general be higher than for fatalities, this is more than compensated for by the much
higher societally accepted limits for injuries. In the scope of this document, the risk of death is
therefore adopted as the relevant variable for risk assessment.
If there are good reasons to assume that a particular case deviates from this general
observation, EN 50126-2, Annex A.4 [5] can be used to derive acceptable risk limits for non-
lethal harm.
6.2 Direct harm
Direct harm is indicated as an immediate effect of a hazardous incident caused by a wind turbine
in operating or maintenance mode to one or more people. Direct harm includes all potentially
fatal injuries and excludes non-lethal and mental consequences or near misses.
An example of direct harm is a detached blade tip that directly strikes a person who is walking
near the wind turbine on a public path.
Direct harm shall be taken into account in the risk assessment.
6.3 Indirect harm
Indirect harm refers to incidences where turbine failure, collapse or falling debris cause damage
to an enclosure or infrastructure, such as cars, buildings, dykes, nuclear plants, overhead power
lines, fuel lines, or fuel storage that then subsequently causes harm to one or several persons.
Long-term effects and environmental consequences such as pollution are not in the scope of
this document.
An example of indirect harm is a detached blade tip that hits a car on a public road and, while
not directly harming the occupants, distracts the driver causing him to veer off the road leading
to a traffic accident.
Indirect harm shall be taken into account in the risk assessment, if the indirect harm is
potentially lethal.
6.4 Domino effect
A domino effect is a scenario in which an incident in a wind farm (e.g. the breakage of a rotor
blade) causes the failure of another hazardous installation (e.g. a chemical storage tank) in its
vicinity, resulting in a major accident (e.g. the release of hazardous substances). The distinction
from indirect harm is that the secondarily affected installation is itself a source of significant
hazards. Usually, this hazardous installation will thus be subject to a specific risk assessment.
Because of the severe consequences they might cause, domino effect scenarios can be
relevant for siting risk assessments, even if they
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