IEC 60721-3-9:2024

IEC 60721-3-9 ®
Edition 2.0 2024-04
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
Classification of environmental conditions –
Part 3-9: Classification of groups of environmental parameters and their
severities – Microclimates inside products

Classification des conditions d'environnement –
Partie 3-9: Classification des groupements des agents d'environnement et de
leurs sévérités – Microclimats à l'intérieur des produits
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IEC 60721-3-9 ®
Edition 2.0 2024-04
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
Classification of environmental conditions –

Part 3-9: Classification of groups of environmental parameters and their

severities – Microclimates inside products

Classification des conditions d'environnement –

Partie 3-9: Classification des groupements des agents d'environnement et de

leurs sévérités – Microclimats à l'intérieur des produits

INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
COMMISSION
ELECTROTECHNIQUE
INTERNATIONALE
ICS 19.040  ISBN 978-2-8322-8626-5

– 2 – IEC 60721-3-9:2024 © IEC 2024
CONTENTS
FOREWORD . 3
1 Scope . 5
2 Normative references . 5
3 Terms and definitions . 5
4 General . 6
5 Classification of microclimatic conditions . 6
6 Types and marking of microclimatic classes . 7
Annex A (informative) Graphical representation and preferred microclimatic classes . 8
A.1 Graphical representation of the microclimatic classes . 8
A.2 Tables of preferred microclimatic classes . 9
Annex B (informative) Constitutional diagram for humid air . 14
B.1 General . 14
B.2 Application . 14
B.3 Description . 14
Bibliography . 17

Figure A.1 – Example of a climatogram for a microclimate: Microclimatic class
3K22/X2/Y1 . 9
Figure B.1 – Constitutional diagram for humid air . 16

Table 1 – Classification of microclimatic conditions . 7
Table A.1 – Characteristic parameters and severities of microclimatic classes –
Weatherprotected locations: Corner points A’, B’, C’, D’, E’ and F’ . 10
Table A.2 – Characteristic parameters and severities of microclimatic classes –
Weatherprotected locations: Corner points B to B and C to C . 11
Y1 Y4 Y1 Y4
Table A.3 – Characteristic parameters and severities of microclimatic classes –
Non-weatherprotected locations: Corner points A’, B’, C’, D’, E’ and F’ . 12
Table A.4 – Characteristic parameters and severities of microclimatic classes –
Non-weatherprotected locations: Corner points B to B and C to C . 13
Y1 Y4 Y1 Y4
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
CLASSIFICATION OF ENVIRONMENTAL CONDITIONS –

Part 3-9: Classification of groups of environmental parameters
and their severities – Microclimates inside products

FOREWORD
1) The International Electrotechnical Commission (IEC) is a worldwide organization for standardization comprising
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8) Attention is drawn to the Normative references cited in this publication. Use of the referenced publications is
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shall not be held responsible for identifying any or all such patent rights.
IEC 60721-3-9 has been prepared by IEC technical committee 104: Environmental conditions,
classification and methods of test. It is an International Standard.
This second edition cancels and replaces the first edition published in 1993, Amendment 1:1994
and Corrigendum 1:1995. This edition constitutes a technical revision.
This edition includes the following significant technical changes with respect to the previous
edition:
a) Clause 2 has been updated;
b) Clause 4 has been re-edited and simplified;
c) Annex A has been revised and updated;

– 4 – IEC 60721-3-9:2024 © IEC 2024
d) a new Annex B has been added and gives the origin of the constitutional diagram for humid
air, which is the basis of drawing the climatogram for a microclimate.
The text of this International Standard is based on the following documents:
Draft Report on voting
104/1041/FDIS 104/1050/RVD
Full information on the voting for its approval can be found in the report on voting indicated in
the above table.
The language used for the development of this International Standard 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
at www.iec.ch/members_experts/refdocs. The main document types developed by IEC are
described in greater detail at www.iec.ch/publications.
A list of all parts in the IEC 60721 series, published under the general title Classification of
environmental conditions, can be found on the IEC website.
The committee has decided that the contents of this document will remain unchanged until the
stability date indicated on the IEC website under webstore.iec.ch in the data related to the
specific document. At this date, the document will be
• reconfirmed,
• withdrawn, or
• revised.
IMPORTANT – The "colour inside" logo on the cover page of this document indicates
that it contains colours which are considered to be useful for the correct understanding
of its contents. Users should therefore print this document using a colour printer.

CLASSIFICATION OF ENVIRONMENTAL CONDITIONS –

Part 3-9: Classification of groups of environmental parameters
and their severities – Microclimates inside products

1 Scope
This part of IEC 60721 classifies groups of microclimatic conditions, to which components
(basic parts, assemblies, built-in units) can be subjected inside products, which are used under
the climatic conditions as classified in IEC 60721-3-3 and IEC 60721-3-4.
Characteristic parameters for the microclimates are high air temperature and high relative air
humidity. Further parameters of the climatic classes, for example low temperature, can affect
the components additionally, but have not been considered here.
A limited number of microclimatic classes is specified taking into consideration typical limiting
high air temperatures of components.
NOTE The term "microclimate" can, for example in meteorology or buildings, have a different meaning than those
discussed in this document.
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.
IEC 60721-1, Classification of environmental conditions − Part 1: Environmental parameters
and their severities
IEC 60721-3-0:2020, Classification of environmental conditions – Part 3-0: Classification of
groups of environmental parameters and their severities – Introduction
IEC 60721-3-3:2019, Classification of environmental conditions – Part 3-3: Classification of
groups of environmental parameters and their severities – Stationary use at weatherprotected
locations
IEC 60721-3-4:2019, Classification of environmental conditions – Part 3-4: Classification of
groups of environmental parameters and their severities – Stationary use at non-
weatherprotected locations
3 Terms and definitions
For the purposes of this document, the terms and definitions given in IEC 60721-1 and the
following apply.
ISO and IEC maintain terminology databases for use in standardization at the following
addresses:
• IEC Electropedia: available at https://www.electropedia.org/
• ISO Online browsing platform: available at https://www.iso.org/obp

– 6 – IEC 60721-3-9:2024 © IEC 2024
3.1
microclimate
climatic condition at the place where a component is installed in the product
Note 1 to entry: Only air temperature and air humidity are taken into account.
3.2
microclimatic class
classified microclimate designated by:
a) the climatic class as specified in IEC 60721-3-3:2019 or IEC 60721-3-4:2019;
b) the class of high air temperature;
c) the optional class of limited relative air humidity, in relation to the climatic class severity
Note 1 to entry: For items b) and c), see Table 1.
4 General
Microclimates at the place where the components are installed in a product can differ
significantly from the climatic conditions to which the product is subjected.
The microclimates describe the climatic conditions at the place where the component is installed
in a product, for example inside an enclosure. These are essentially the climatic classes
specified in IEC 60721-3-3:2019 or IEC 60721-3-4:2019 but with the addition of severities of
high air temperature and limited relative air humidity to account for external or appreciable self-
generated heating during operation. Microclimates can also be used to designate the
operational conditions for components.
When temperatures in excess of those of the environment itself occur inside a product, the
relative air humidity and, therefore, also the humidity stress on the components is reduced.
Even in an environment with a relative air humidity as high as 100 %, the relative air humidity
inside the product is reduced below 60 % by an excess temperature of 10 K. Below this humidity
level, the corrosion effect of chemically active substances is low.
The microclimates described concern the case of placing of components in enclosures with
comparatively free access to the surrounding atmosphere. Difficult access of air (e.g. presence
of non-tight seal of the enclosure) can produce more severe conditions as a result of suction of
moisture into the enclosure and subsequent accumulation of water. This can result from
frequent switching on and off of the product or rapid changes of temperature inside the product
due to external climatic conditions, for example rain, irradiation.
Components in products without external heating or self-generated heating or in the
non-operational state for a significant period of time are affected directly by the conditions of
the climatic class appropriate for a location.
When there is a change in the climatic conditions, with and without external heating or self-
generated heating, the time for which the component is subjected to the climatic conditions
shall be taken into consideration. Details on duration and frequency of occurrence shall be
referred to IEC 60721-3-0:2020, and details on change of climatic conditions, such as the rate
of change of temperature, shall be referred to IEC 60721-3-3:2019 and IEC 60721-3-4:2019.
Further guidance shall be referred to IEC 60721-3-0:2020.
5 Classification of microclimatic conditions
The severities of high air temperature and of limited relative air humidity for a number of
microclimatic classes are specified in Table 1.

Table 1 – Classification of microclimatic conditions
Environmental parameter Class Unit Severity
X1 55
X2 70
X3 85
a) High air temperature X4 °C 100
X5 125
X6 155
X7 200
Y1 65
Y2 75
b) Limited relative air humidity %
Y3 85
Y4 95
Examples for the designation and marking of microclimatic classes are given in Clause 6.
Exceptional conditions may call for severities different from those of the classes; these should
be selected from the values specified in IEC 60721-1.
The graphical representation of a microclimatic class is given in Clause A.1 from which six
characteristic corner points can be determined. For a selection of preferred microclimatic
classes, paired values of air temperature and relative air humidity are stated in Table A.1 and
Table A.2.
The method described in Annex A applies to the period of time when microclimatic conditions
have reached a steady state.
6 Types and marking of microclimatic classes
A microclimatic class is marked with the appropriate class designation of the climatic class of
the product and the appropriate class designation of Table 1.
EXAMPLE 1 – Increased air temperature only

EXAMPLE 2 – Increased air temperature and limited relative air humidity

– 8 – IEC 60721-3-9:2024 © IEC 2024
Annex A
(informative)
Graphical representation and preferred microclimatic classes
A.1 Graphical representation of the microclimatic classes
Figure A.1 is an example of the climatogram of microclimatic class 3K22/X2/Y1.
The characteristic points and lines of the climatogram are obtained in the following manner:
– Draw the climatogram of climatic class 3K22.
– Mark the corner points by A, B, C, D, E and F:
• A is the high air temperature at high absolute air humidity;
• B is the high relative air humidity at high absolute air humidity;
• C is the low air temperature at high relative air humidity;
• D is the low air temperature at low absolute air humidity;
• E is the low relative air humidity at low absolute air humidity;
• F is the high air temperature at low relative air humidity.
– Determine the difference between the air temperature of the microclimate (70 °C) and the
high air temperature of the climatic class (40 °C) i.e. 30 °C.
– Shift the corner points A to F by the value of this difference between the high air
temperatures of the microclimate and the climatic class on the lines of constant absolute air
humidity.
– Mark the obtained corner points by A’ to F’.
– Draw the boundary line for the limitation of the relative air humidity at 65 % with high/low
absolute air humidity, marking the intersection with B and C .
65 65
– Draw the resulting climatogram A’, B , C , D, E’ and F’.
65 65
NOTE This method of transformation on lines of constant absolute air humidity is physically correct only for closed
systems. For open systems, the transformation on the lines of constant partial water vapour pressure can be used.
Since the error in the given temperature range is not significant, for the transformation, the lines of constant absolute
air humidity as specified in Annex B were used also for open systems.
The climatogram of the microclimatic class 3K22/X2/Y1 is the envelope line A’, B , C , D, E’
65 65
and F’.
The microclimate during permanent external or self-generated heating complies with the area
within the line A’, B’, C’, D’, E’ and F’. The two corner points A’ and B’ of a climatogram such
as Figure A.1 represent:
– A' is the high air temperature and related high absolute air humidity of the microclimate with
external heating or self-generated heating;
– B' is the high relative air humidity and related high air temperature of the microclimate with
external heating or self-generated heating;
These corner points indicate the range of maximum stress on components by relative air
humidity during external heating or self-generated heating. They can be suitably described
by pairing values of air temperature and relative air humidity for the points in question from
climatograms. The paired values for the remaining corner points of a climatogram are
generally of minor importance.

Figure A.1 – Example of a climatogram for a microclimate:
Microclimatic class 3K22/X2/Y1
A.2 Tables of preferred microclimatic classes
Table A.1 and Table A.2 give paired values for air temperature/relative air humidity at the corner
points of the climatograms (such as Figure A.1) of the most usual microclimatic classes with
weatherprotected locations. Table A.1 gives the value of corner points A’, B’, C’, D’, E’ and F’,
and Table A.2 gives the value of corner points B to B and C to C .
Y1 Y4 Y1 Y4
NOTE The corner points A’, B’, C’, D’, E’ and F’ are not affected by the change of limited relative air humidity; they
are determined by the high air temperature and the climatic class. The corner points B to B and C to C are
Y1 Y4 Y1 Y4
not affected by the change of high air temperature; they are determined by the limited relative air humidity and the
climatic class.
– 10 – IEC 60721-3-9:2024 © IEC 2024
Table A.1 – Characteristic parameters and severities of microclimatic classes –
Weatherprotected locations: Corner points A’, B’, C’, D’, E’ and F’
Class of
Limited
high air Corner
relative air
Parameter Unit Corner point severities for
tempera- point
humidity
ture
3K20 3K21 3K22 3K23 3K24
A' 55,0 / 14,4 55,0 / 21,2 55,0 / 24,0 55,0 / 27,9 55,0 / 27,9
B' 52,5 / 16,1 52,4 / 23,8 44,4 / 39,4 29,2 / 100 29,2 / 100
X1 C' 50,0 / 15,7 38,0 / 20,8 20,0 / 33,4 −5,0 / 100 −25,0 / 100
(55 °C) D' 50,0 / 4,8 38,0 / 4,3 20,0 / 5,8 −5,0 / 15,4 −25,0 / 90,6
E' 52,5 / 4,3 45,5 / 3,0 37,5 / 2,2 0,5 / 10,0 11,0 / 5,0
F' 55,0 / 4,4 55,0 / 3,2 55,0 / 2,5 55,0 / 10,0 55,0 / 5,0
A' 70,0 / 7,6 70,0 / 11,2 70,0 / 12,7 70,0 / 14,7 70,0 / 14,7
B' 67,5 / 8,4 67,4 / 12,4 59,4 / 19,8 44,1 / 46,2 44,2 / 46,2
C' 65,0 / 8,1 53,0 / 10,1 35,0 / 14,6 10,0 / 34,5 −10,0 / 25,8
X2
(70 °C) D' 65,0 / 2,5 53,0 / 2,1 35,0 / 2,5 10,0 / 5,3 −10,0 / 23,4
E' 67,5 / 2,2 60,5 / 1,5 52,5 / 1,1 15,5 / 3,8 26,0 / 2,1
F' 70,0 / 2,3 70,0 / 1,7 70,0 / 1,3 70,0 / 5,3 70,0 / 2,6
A' 85,0 / 4,3 85,0 / 6,3 85,0 / 7,1 85,0 / 8,3 85,0 / 8,3
B' 82,5 / 4,7 82,4 / 6,9 74,4 / 10,7 59,2 / 23,2 59,2 / 23,2
C' 80,0 / 4,5 68,0 / 5,3 50,0 / 7,0 25,0 / 14,1 5,0 / 8,2
X3
(85 °C) D' 80,0 / 1,4 68,0 / 1,1 50,0 / 1,2 25,0 / 2,2 5,0 / 7,4
E' 82,5 / 1,3 75,5 / 0,8 67,5 / 0,6 30,5 / 1,6 41,0 / 0,9
F' 85,0 / 1,3 85,0 / 1,0 85,0 / 0,7 85,0 / 3,0 85,0 / 1,5
A' 100,0 / 2,5 100,0 / 3,7 100,0 / 4,2 100,0 / 4,9 100,0 / 4,9
B' 97,5 / 2,8 97,4 / 4,1 89,4 / 6,1 74,2 / 12,5 74,2 / 12,5
Air
Y1 to Y4 X4 C' 95,0 / 2,6 83,0 / 3,0 65,0 / 3,6 40,0 / 6,4 20,0 / 3,2
temperature/
°C / %
relative air
(65 % to 95 %) (100 °C) D' 95,0 / 0,8 83,0 / 0,6 65,0 / 0,6 40,0 / 1,0 20,0 / 2,9
humidity
E' 97,5 / 0,7 90,5 / 0,5 82,5 / 0,3 45,5 / 0,7 56,0 / 0,5
F' 100,0 / 0,8 100,0 / 0,6 100,0 / 0,4 100,0 / 1,8 100,0 / 0,9
A' 125,0 / 1,2 125,0 / 1,7 125,0 / 2,0 125,0 / 2,3 125,0 / 2,3
B' 122,5 / 1,3 122,4 / 1,9 114,4 / 2,7 99,2 / 5,1 99,2 / 5,1
X5 C' 120,0 / 1,2 108,0 / 1,3 90,0 / 1,4 65,0 / 2,0 45,0 / 0,8
(125 °C)
D' 120,0 / 0,4 108,0 / 0,3 90,0 / 0,2 65,0 / 0,3 45,0 / 0,8
E' 122,5 / 0,3 115,5 / 0,2 107,5 / 0,1 70,5 / 0,2 81,0 / 0,2
F' 125,0 / 0,4 125,0 / 0,3 125,0 / 0,2 125,0 / 0,8 125,0 / 0,4
A' 155,0 / 0,5 155,0 / 0,8 155,0 / 0,9 155,0 / 1,1 155,0 / 1,1
B' 152,5 / 0,6 152,4 / 0,9 144,4 / 1,2 129,2 / 2,0 129,2 / 2,0
X6 C' 150,0 / 0,5 138,0 / 0,5 120,0 / 0,5 95,0 / 0,7 75,0 / 0,2
(155 °C) D' 150,0 / 0,2 138,0 / 0,1 120,0 / 0,1 95,0 / 0,1 75,0 / 0,2
E' 152,5 / 0,2 145,5 / 0,1 137,5 / 0,1 100,5 / 0,1 111,0 / 0,1
F' 155,0 / 0,2 155,0 / 0,1 155,0 / 0,1 155,0 / 0,4 155,0 / 0,2
A' 200,0 / 0,2 200,0 / 0,3 200,0 / 0,3 200,0 / 0,4 200,0 / 0,4
B' 197,5 / 0,2 197,4 / 0,3 189,4 / 0,4 174,2 / 0,7 174,2 / 0,7
X7 C' 195,0 / 0,2 183,0 / 0,2 165,0 / 0,2 140,0 / 0,2 120,0 / 0,1
(200 °C) D' 195,0 / 0,1 183,0 / 0,0 165,0 / 0,0 140,0 / 0,0 120,0 / 0,0
E' 197,5 / 0,1 190,5 / 0,0 182,5 / 0,0 145,5 / 0,0 156,0 / 0,0
F' 200,0 / 0,1 200,0 / 0,0 200,0 / 0,0 200,0 / 0,1 200,0 / 0,1

Table A.2 – Characteristic parameters and severities of microclimatic classes –
Weatherprotected locations: Corner points B to B and C to C
Y1 Y4 Y1 Y4
Limited
Class of high
relative Corner
air Parameter Unit Corner point severities for
air point
temperature
humidity
3K20 3K21 3K22 3K23 3K24
B
25,1 / 65,0 32,0 / 65,0 34,5 / 65,0 37,3 / 65,0 37,3 / 65,0
Y1 65
(65 %)
C
20,0 / 65,0 15,0 / 65,0 5,0 / 65,0 −5,0 / 65,0 −21,5 / 65,0
B
22,5 / 75,0 29,4 / 75,0 31,8 / 75,0 34,6 / 75,0 34,6 / 75,0
Y2 75
Air
(75 %) C
20,0 / 75,0 15,0 / 75,0 5,0 / 75,0 −5,0 / 75,0 −23,0 / 75,0
X1 to X7 75
temperature/
°C / %
relative air
(55 °C to 200 °C)
B
20,4 / 85,0 27,1 / 85,0 29,4 / 85,0 32,2 / 85,0 32,2 / 85,0
Y3 85
humidity
(85 %) C
20,0 / 85,0 15,0 / 85,0 5,0 / 85,0 −5,0 / 85,0 −24,3 / 85,0
B
— 25,1 / 95,0 27,4 / 95,0 30,1 / 95,0 30,1 / 95,0
Y4 95
(95 %)
C
15,0 / 95,0 5,0 / 95,0 −5,0 / 95,0 −25,0 / 95,0
Table A.3 and Table A.4 give paired values for air temperature/relative air humidity at the corner
points of the climatograms of the most usual microclimatic classes with non-weatherprotected
locations. Table A.3 gives the value of corner points A’, B’, C’, D’, E’ and F’, and Table A.4
gives the value of corner points B to B and C to C .
Y1 Y4 Y1 Y4
– 12 – IEC 60721-3-9:2024 © IEC 2024
Table A.3 – Characteristic parameters and severities of microclimatic classes –
Non-weatherprotected locations: Corner points A’, B’, C’, D’, E’ and F’
Limited Class of
Corner
relative air high air Parameter Unit Corner point severities for
point
humidity temperature
4K23 4K24 4K25 4K26 4K27
A' − − 55,0 / 33,7 55,0 / 28,9 55,00 / 24,0

B' 42,7 / 60,0 34,8 / 76,7 36,48 / 58,6
−15,0 /
C' 15,0 / 53,0 −40,00 / 32,0
X1
63,7
(55 °C)
D' 15,0 / 46,8 −15,0 / 3,6 −40,00 / 3,4

E' 32,5 / 17,3 −11,2 / 2,6 −39,61 / 3,2

F' 55,0 / 18,8 55,0 / 3,2 55,00 / 6,3
A' 70,0 / 17,8 70,0 / 10,2 70,0 / 17,8 70,0 / 15,2 70,0 / 12,7
B' 32,7 / 100,0 22,5 / 100,0 57,7 / 29,9 49,8 / 36,6 51,5 / 28,2
C' −45,0 / 100,0 −50,0 / 100,0 30,0 / 22,4 0,0 / 18,2 −25,0 / 6,9
X2
(70 °C) D' −45,0 / 73,1 −50,0 / 10,5 30,0 / 19,8 0,0 / 1,0 −25,0 / 0,7
E' −16,2 / 4,0 −41,6 / 4,0 47,5 / 8,1 3,8 / 0,8 −24,6 / 0,7
F' 70,0 / 4,0 70,0 / 4,0 70,0 / 10,0 70,0 / 1,7 70,0 / 3,3
A' 85,0 / 10,0 85,0 / 5,7 85,0 / 10,0 85,0 / 8,6 85,0 / 7,1
B' 47,7 / 47,2 37,5 / 44,4 72,7 / 16,0 64,8 / 18,9 66,5 / 14,7
C' −30,0 / 20,2 −35,0 / 18,8 45,0 / 10,4 15,0 / 6,9 −10,0 / 1,8
X3
(85 °C) D' −30,0 / 14,8 −35,0 / 2,0 45,0 / 9,2 15,0 / 0,4 −10,0 / 0,2
E' −1,2 / 1,1 −26,6 / 0,8 62,5 / 4,2 18,8 / 0,3 −9,6 / 0,2
F' 85,0 / 2,3 85,0 / 2,3 85,0 / 5,6 85,0 / 0,9 85,0 / 1,9
A' 100,0 / 5,9 100,0 / 3,4 100,0 / 5,9 100,0 / 5,1 100,0 / 4,2
Air
B' 62,7 / 24,1 52,5 / 21,5 87,7 / 9,1 79,8 / 10,4 81,5 / 8,1
Y1 to Y4
temperature/
X4 C' −15,0 / 4,9 −20,0 / 4,3 60,0 / 5,2 30,0 / 2,9 5,0 / 0,6
°C / %
relative air
(65 % to 95 %)
(100 °C) D' −15,0 / 3,6 −20,0 / 0,5 60,0 / 4,6 30,0 / 0,2 5,0 / 0,1
humidity
E' 13,8 / 0,4 −11,6 / 0,2 77,5 / 2,3 33,8 / 0,1 5,4 / 0,1
F' 100,0 / 1,3 100,0 / 1,3 100,0 / 3,3 100,0 / 0,6 100,0 / 1,1
A' 125,0 / 2,8 125,0 / 1,6 125,0 / 2,8 125,0 / 2,4 125,0 / 2,0
B' 87,7 / 9,1 77,5 / 7,6 112,7 / 4,0 104,8 / 4,4 106,5 / 3,4
X5 C' 10,0 / 0,7 5,0 / 0,6 85,0 / 1,9 55,0 / 0,8 30,0 / 0,1
(125 °C)
D' 10,0 / 0,5 5,0 / 0,1 85,0 / 1,7 55,0 / 0,0 30,0 / 0,0
E' 38,8 / 0,1 13,4 / 0,0 102,5 / 0,9 58,8 / 0,0 30,4 / 0,0
F' 125,0 / 0,6 125,0 / 0,6 125,0 / 1,6 125,0 / 0,3 125,0 / 0,5
A' 155,0 /1,3 155,0 / 0,7 155,0 / 1,3 155,0 / 1,1 155,0 / 0,9
B' 117,7 / 3,4 107,5 / 2,7 142,7 / 1,7 134,8 / 1,8 136,5 / 1,4
X6 C' 40,0 / 0,1 35,0 / 0,1 115,0 / 0,7 85,0 / 0,3 60,0 / 0,0
(155 °C) D' 40,0 / 0,1 35,0 / 0,0 115,0 / 0,6 85,0 / 0,0 60,0 / 0,0
E' 68,8 / 0,0 43,4 / 0,0 132,5 / 0,4 88,8 / 0,0 60,4 / 0,0
F' 155,0 / 0,3 155,0 / 0,3 155,0 / 0,7 155,0 / 0,1 155,0 / 0,2
A' 200,0 / 0,5 200,0 / 0,3 200,0 / 0,5 200,0 / 0,4 200,0 / 0,3
B' 162,7 / 1,1 152,5 / 0,8 187,7 / 0,6 179,8 / 0,6 181,5 / 0,5
X7 C' 85,0 / 0,0 80,0 / 0,0 160,0 / 0,2 130,0 / 0,1 105,0 / 0,0
(200 °C) D' 85,0 / 0,0 80,0 / 0,0 160,0 / 0,2 130,0 / 0,0 105,0 / 0,0
E' 113,8 / 0,0 88,4 / 0,0 177,5 / 0,1 133,8 / 0,0 105,4 / 0,0
F' 200,0 / 0,1 200,0 / 0,1 200,0 / 0,3 200,0 / 0,0 200,0 / 0,1

Table A.4 – Characteristic parameters and severities of microclimatic classes –
Non-weatherprotected locations: Corner points B to B and C to C
Y1 Y4 Y1 Y4
Limited
Class of high
relative Corner
air Parameter Unit Corner point severities for
air point
temperature
humidity
4K23 4K24 4K25 4K26 4K27
B
41,1 / 65,0 30,3 / 65,0 41,1 / 65,0 38,0 / 65,0 34,5 / 65,0
Y1 65
(65 %)
C
−44,0 / 65,0 −50,0 / 65,0 9,7 / 65,0 −20,0 / 65,0 −50,0 / 65,0
B
38,2 / 75,0 27,6 / 75,0 38,2 / 75,0 35,2 / 75,0 31,8 / 75,0
Y2 75
Air
(75 %) C
−45,0 / 75,0 −50,0 / 75,0 7,5 / 75,0 −20,0 / 75,0 −50,0 / 75,0
X1 to X7 75
temperature/
°C / %
relative air
(55 °C to 200 °C) B
35,8 / 85,0 25,4 / 85,0 35,8 / 85,0 32,8 / 85,0 29,4 / 85,0
Y3 85
humidity
(85 %) C
−45,0 / 85,0 −50,0 / 85,0 5,6 / 85,0 −20,0 / 85,0 −50,0 / 85,0
B
33,7 / 95,0 23,4 / 95,0 33,7 / 95,0 30,8 / 95,0 27,4 / 95,0
Y4 95
(95 %) C
−45,0 / 95,0 −50,0 / 95,0 5,0 / 95,0 −20,0 / 95,0 −50,0 / 95,0
– 14 – IEC 60721-3-9:2024 © IEC 2024
Annex B
(informative)
Constitutional diagram for humid air
B.1 General
This Annex B describes the constitutional diagram as applied in the publications of IEC
technical committee 104. The diagram shows the relationship between temperature and
humidity at normal atmospheric pressure (Figure B.1) and is based on the set of formulae given
in Clause B.3. It should be noted that the set of Formulae (B.2) and (B.3) is one of several sets
of approximate formulae for this relationship.
It is recommended that the diagram be applied as far as possible in work with classification or
evaluation of environmental conditions.
B.2 Application
The constitutional diagram can be used to specify the climatic environment, for example when
establishing climatograms. It can also be used for determining the absolute air humidity of air
at a certain temperature and relative air humidity, or for determining the dew point at a drop of
temperature. For exact values, reference should be made to more detailed diagrams or
mathematical calculations.
B.3 Description
The diagram (Figure B.1) has three scales as follows:
a) absolute air humidity in grams per cubic metre is given on the scale along the top horizontal
axis;
b) relative air humidity in percent is given on the scale along the left vertical axis;
c) air temperature in degrees Celsius is given on the scale along the bottom horizontal axis.
This scale also applies when determining the dew point, which is the temperature at a
relative air humidity of 100 % at which condensation occurs.
The relationship between relative air humidity in percent, temperature in degrees Celsius and
absolute air humidity (water content) in grams per cubic metre are given by the curves of the
diagram.
Theses curves are based on Formulae (B.1).
The absolute air humidity (water content) p is a linear function of the density of water vapour at
0 °C and 1 Pa, of the temperature and of the water vapour pressure:
(B.1)
p t, φφ0,007932⋅273,15/ t+273,15⋅⋅е/100
( ) ( ( ))
or simplified:
p t, φ=2,167 е⋅φ /100 / t+ 273,15
( ) (( ) ) ( )
=
where
e is the saturation water vapour pressure, in pascals;
φ is the relative air humidity, in percent;
t is the temperature, in degrees Celsius.
The saturation water vapour pressure, e, has been calculated using Formulae (B.2) and (B.3),
modified by a factor of 100 to account for the use of the pressure unit pascal in place of
millibar. To simplify the formulae, the absolute temperature T in kelvins, where T = t + 273,15,
has been introduced.
– with liquid water, e , for temperatures above 0 °C,
w
n (B.2)
eT 100⋅10
( )
w
where
T

11,344 1−
373,16 373,16 ( )
   
−7 373,16

n=−7,90298 −+1 5,02808⋅log −1,3816×10 10 −1
  10 
TT
    

373,16

−−3,49149 1
( )
−3 T
+8,1328×10 10 −+1 log 1013,246
( )


– with ice, e , for temperatures equal to or below 0 °C,
i
m
(B.3)
eT 100⋅10
( )
i
where
273,15 273,15 T
    
m=−9,09718 −−1 3,56654⋅log + 0,876793 1− + log (6,10714)
  10 10
 
TT 273,15
    
=
=
– 16 – IEC 60721-3-9:2024 © IEC 2024

Figure B.1 – Constitutional diagram for humid air

Bibliography
IEC 60721-2-1:2013, Classification of environmental conditions – Part 2-1: Environmental
conditions appearing in nature – Temperature and humidity

___________
– 18 – IEC 60721-3-9:2024 © IEC 2024
SOMMAIRE
AVANT-PROPOS . 19
1 Domaine d'application . 21
2 Références normatives . 21
3 Termes et définitions . 22
4 Généralités . 22
5 Classification des conditions microclimatiques . 23
6 Types et marquage des classes de microclimats . 24
Annexe A (informative) Représentation graphique et classes de microclimats
préférentielles . 25
A.1 Représentation graphique des classes de microclimats . 25
A.2 Tableaux des classes de microclimats préférentielles . 26
Annexe B (informative) Diagramme constitutionnel pour l'humidité de l'air . 31
B.1 Généralités . 31
B.2 Application . 31
B.3 Description . 31
Bibliographie . 34

Figure A.1 – Exemple de climatogramme pour un microclimat: Classe de microclimats
3K22/X2/Y1 . 26
Figure B.1 – Diagramme constitutionnel pour l'humidité de l'air . 33

Tableau 1 – Classification des conditions microclimatiques . 23
Tableau A.1 – Agents caractéristiques et sévérités des classes de microclimats –
Endroits protégés contre les intempéries: Points extrêmes A', B', C', D', E' et F’ . 27
Tableau A.2 – Agents caractéristiques et sévérités des classes de microclimats –
Endroits protégés contre les intempéries: Points extrêmes B à B et C à C . 28
Y1 Y4 Y1 Y4
Tableau A.3 – Agents caractéristiques et sévérités des classes de microclimats –
Endroits non protégés contre les intempéries: Points extrêmes A', B', C', D', E' et F’ . 29
Tableau A.4 – Agents caractéristiques et sévérités des classes de microclimats –
Endroits non protégés contre les intempéries: Points extrêmes B à B et C à C . 30
Y1 Y4 Y1 Y4
COMMISSION ÉLECTROTECHNIQUE INTERNATIONALE
____________
CLASSIFICATION DES CONDITIONS D'ENVIRONNEMENT –

Partie 3-9: Classification des groupements des agents d’environnement
et de leurs sévérités – Microclimats à l'intérieur des produits

AVANT-PROPOS
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...