IEC TS 60695-2-20:2024

IEC TS 60695-2-20 ®
Edition 4.0 2024-11
REDLINE VERSION
TECHNICAL
SPECIFICATION
colour
inside
Fire hazard testing –
Part 2-20: Glowing/hot wire based test methods – Hot-wire coil ignition (HWI)
test method test method – Apparatus, verification, test method and guidance

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IEC TS 60695-2-20 ®
Edition 4.0 2024-11
REDLINE VERSION
TECHNICAL
SPECIFICATION
colour
inside
Fire hazard testing –
Part 2-20: Glowing/hot wire based test methods – Hot-wire coil ignition (HWI)
test method test method – Apparatus, verification, test method and guidance
INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
ICS 13.220.40; 29.020 ISBN 978-2-8327-0038-9
– 2 – IEC TS 60695-2-20:2024 RLV © IEC 2024
CONTENTS
FOREWORD . 4
INTRODUCTION . 2
1 Scope . 8
2 Normative references . 8
3 Terms and definitions . 9
4 Principle . 11
5 Apparatus . 12
5.1 Test chamber . 12
5.2 Heater wire . 12
5.3 Power supply and test circuit . 13
5.4 Test specimen fixture . 13
5.5 Test specimen winding and pressing . 13
5.6 Conditioning chamber . 15
5.7 Timing device . 15
5.8 Micrometer . 15
5.9 Measuring scale . 15
6 Test specimens . 15
6.1 Test specimen preparation . 15
6.2 Test specimen dimensions . 15
7 Conditioning . 17
7.1 Requirements . 17
7.2 Test specimen conditioning . 17
7.3 Heater wire conditioning . 17
7.4 Test conditions . 17
8 Test procedure . 17
8.1 Verification of the heater wire . 17
8.1.1 General . 17
8.1.2 Apparatus Test circuit for the verification . 17
8.1.3 Determination of the test current, I . 18
c
8.1.4 Calculation of test current, I . 19
c
8.2 Determination of time to ignite, IT . 20
9 Observations and measurement . 23
10 Evaluation of test results – Assigning an HWI PLC classification . 24
11 Test report . 24
Annex A (informative) Guidance on how to effectively wind the heater wire on to the
test specimen . 25
Annex B (normative) Hot Wire coil Ignition test (HWCT HWI) – Performance level
category (PLC) classes . 26
B.1 General . 26
B.2 Reporting a classification . 26
Annex C (informative) Calibration curve to determine test current (I ) in a spreadsheet
c
program . 27
Annex D (informative) Precision data .
D.1 General .
D.2 Results of preliminary inter-laboratory round robin .

D.3 Evaluation of test results .
Bibliography . 33

Figure 1 – Test fixture arrangement (example) . 11
Figure 2 – Test specimen winding pattern . 12
Figure 3 – Test specimen fixture (example). 13
Figure 4 – Heater wire winding device (Example) . 14
Figure 5 – Test circuit arrangement for the heater wire calibration verification . 18
Figure 6 – Heater wire marking and connection . 19
Figure 7 – Calibration curve . 20
Figure 8 – Decision tree . 23
Figure A.1 – Illustration of type a) behaviour . 25
Figure A.2 – Illustration of type b) behaviour . 25
Figure C.1 – Calibration curve in a spreadsheet program used to determine I . 27
c
Table 1 – Nominal thickness tolerances . 16
Table 2 – Heater wire verification lengths . 18
Table B.1 – Hot Wire coil Ignition test (HWCT HWI) – Performance level category
(PLC) classes . 26
Table D.1 – Results for material 1A .
Table D.2 – Results for material 1B .
Table D.3 – Results for material 2A .
Table D.4 – Results for material 2B .
Table D.5 – Results for material 3 .
Table D.6 – Results for material 4 .
Table D.7 – Occurrence of PLC classes in the round robin .

– 4 – IEC TS 60695-2-20:2024 RLV © IEC 2024
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
FIRE HAZARD TESTING –
Part 2-20: Glowing/hot wire based test methods –
Hot-wire coil ignition (HWI) test method –
Apparatus, verification, test method and guidance

FOREWORD
1) The International Electrotechnical Commission (IEC) is a worldwide organization for standardization comprising
all national electrotechnical committees (IEC National Committees). The object of IEC is to promote international
co-operation on all questions concerning standardization in the electrical and electronic fields. To this end and
in addition to other activities, IEC publishes International Standards, Technical Specifications, Technical Reports,
Publicly Available Specifications (PAS) and Guides (hereafter referred to as "IEC Publication(s)"). Their
preparation is entrusted to technical committees; any IEC National Committee interested in the subject dealt with
may participate in this preparatory work. International, governmental and non-governmental organizations liaising
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Standardization (ISO) in accordance with conditions determined by agreement between the two organizations.
2) The formal decisions or agreements of IEC on technical matters express, as nearly as possible, an international
consensus of opinion on the relevant subjects since each technical committee has representation from all
interested IEC National Committees.
3) IEC Publications have the form of recommendations for international use and are accepted by IEC National
Committees in that sense. While all reasonable efforts are made to ensure that the technical content of IEC
Publications is accurate, IEC cannot be held responsible for the way in which they are used or for any
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4) In order to promote international uniformity, IEC National Committees undertake to apply IEC Publications
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any IEC Publication and the corresponding national or regional publication shall be clearly indicated in the latter.
5) IEC itself does not provide any attestation of conformity. Independent certification bodies provide conformity
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6) All users should ensure that they have the latest edition of this publication.
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Publications.
8) Attention is drawn to the Normative references cited in this publication. Use of the referenced publications is
indispensable for the correct application of this publication.
9) IEC draws attention to the possibility that the implementation of this document may involve the use of (a)
patent(s). IEC 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, IEC 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 https://patents.iec.ch. IEC
shall not be held responsible for identifying any or all such patent rights.
This redline version of the official IEC Standard allows the user to identify the changes
made to the previous edition IEC TS 60695-2-20:2021. A vertical bar appears in the margin
wherever a change has been made. Additions are in green text, deletions are in
strikethrough red text.
IEC TS 60695-2-20 has been prepared by IEC technical committee 89: Fire hazard testing. It is
a Technical Specification.
This fourth edition cancels and replaces the third edition published in 2021. This edition
constitutes a technical revision.
This edition includes the following significant technical changes with respect to the previous
edition:
th
a) Removed all text which was related to drip or dripping, since in this 4 edition only ignition
is taken into consideration for determination of the classification level;
b) Title changed: "Hot wire coil test method" is now "Hot wire ignition (HWI) test method".
The text of this Technical Specification is based on the following documents:
Draft Report on voting
89/1583/DTS 89/1593/RVDTS
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 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
at www.iec.ch/members_experts/refdocs. The main document types developed by IEC are
described in greater detail at www.iec.ch/publications.
Part 2 of IEC 60695 consists of the following parts:
Part 2-10: Glowing/hot-wire based test methods – Glow-wire apparatus and common test
procedure
Part 2-11: Glowing/hot-wire based test methods – Glow-wire flammability test method for end
products
Part 2-12: Glowing/hot-wire based test methods – Glow-wire flammability index (GWFI) test
method for materials
Part 2-13: Glowing/hot-wire based test methods – Glow-wire ignition temperature (GWIT) test
method for materials
Part 2-20: Glowing/hot-wire based test methods – Hot-wire ignition test method – Apparatus,
verification, test method and guidance
Part 2-21: Glowing/hot-wire based test methods – Fire containment test on finished units
A list of all parts in the IEC 60695 series, published under the general title Fire hazard testing,
can be found on the IEC website.
NOTE The following print types are used:
• Terms in bold in the text are defined in Clause 3.
Future documents in this series will carry the new general title as cited above. Titles of existing
documents in this series will be updated at the time of the next edition.

– 6 – IEC TS 60695-2-20:2024 RLV © IEC 2024
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.

INTRODUCTION
In the design of any electrotechnical product, the risk of abnormal heat and the potential
hazards associated with abnormal heat need to be considered. In this respect the objective of
component, circuit, and product design, as well as the choice of materials, is to reduce to
acceptable levels the potential risks during normal operating conditions, reasonably foreseeable
abnormal use, malfunction and/or failure. IEC 60695-1-10 [1] , together with its companion
IEC 60695-1-11 [2], provides guidance on how this is to be accomplished.
The primary aims of IEC 60695-1-10 [1] and IEC 60695-1-11 [2] are to provide guidance on
how:
a) to prevent ignition caused by an electrically energized component part, and
b) to confine any resulting fire within the bounds of the enclosure of the electrotechnical
product in the event of ignition.
Secondary aims of IEC 60695-1-10 [1] and IEC 60695-1-11 [2] include the minimization of any
flame spread beyond the product's enclosure and the minimization of the harmful effects of fire
effluents such as heat, smoke, toxicity and/or corrosivity.
The test method provided in this document applies to solid electrical insulating materials which
can provide test specimens. It applies to materials for which the test specimen does not deform
during preparation, especially during the winding of the test specimen with the heater wire as
described in 5.5.
Examples of deformation that render this test method inapplicable include:
a) bowing, in either a transverse or a longitudinal direction, or twisting of the test specimen
during the winding of the test specimen with the heater wire, to a degree visible to the eye,
or
b) visible indentation of the test specimen by the heater wire.
An informative classification system described in Annex B can be used for the preselection of
materials.
_____________
Numbers in square brackets refer to the bibliography.

– 8 – IEC TS 60695-2-20:2024 RLV © IEC 2024
FIRE HAZARD TESTING –
Part 2-20: Glowing/hot wire based test methods –
Hot-wire coil ignition (HWI) test method –
Apparatus, verification, test method and guidance

1 Scope
This part of IEC 60695, which is a technical specification, describes a test method that applies
to solid electrical insulating materials of which test specimens can be provided. The test
measures the time required to ignite a test specimen when it is affected by heat from an
electrically heated wire wound around the test specimen. If the test specimen drips, the time at
which this occurs is also recorded.
The test method can be used to provide classifications which can be used for quality assurance,
the preselection of materials of products as described in IEC 60695-1-30, or to verify the
required minimum classification of materials used in end products.
This basic safety publication is intended for use by technical committees in the preparation of
standards in accordance with the principles laid down in IEC Guide 104 and ISO/IEC Guide 51.
One of the responsibilities of a technical committee is, wherever applicable, to make use of
basic safety publications in the preparation of its publications. The requirements, test methods
or test conditions of this basic safety publication will not apply unless specifically referred to or
included in the relevant publications.
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 60695-1-30, Fire hazard testing - Part 1-30: Guidance for assessing the fire hazard of
electrotechnical products - Preselection testing process - General guidelines
IEC 60695-4:2012, Fire hazard testing - Part 4: Terminology concerning fire tests for
electrotechnical products
IEC GUIDE 104:2019, The preparation of safety publications and the use of basic safety
publications and group safety publications
ISO/IEC Guide 51:2014, Safety aspects – Guidelines for their inclusion in standards
ISO 291:2008, Plastics – Standard atmospheres for conditioning and testing
ISO 293, Plastics – Compression moulding test specimens of thermoplastic materials
ISO 294 (all parts), Plastics – Injection moulding of test specimens of thermoplastic materials
ISO 295, Plastics – Compression moulding of test specimens of thermosetting materials

ISO 13943:20082017, Fire safety – Vocabulary
ISO 16012:2004, Plastics – Determination of linear dimensions of test specimens
JIS C 2520:1999, Wires and rolled wires for electrical heating
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 13943:2017, some of
which are reproduced below for the user's convenience, as well as the following 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
abnormal heat
heat that is additional to that resulting from use under normal conditions, up
to and including that which causes a fire
[SOURCE: ISO 13943:2017, definition 3.1]
3.2
classification time
t t
A C
arithmetic mean of relevant times to ignite, IT and times to drip, DT, used for the purpose of
classification
3.3
combustion
exothermic reaction of a substance with an oxidizing agent
Note 1 to entry: Combustion generally emits fire effluent accompanied by flames and/or glowing.
[SOURCE: ISO 13943:2017, definition 3.55]
3.4
draught-free environment
space in which the results of experiments are not significantly affected by the local air speed
Note 1 to entry: A qualitative example is a space in which a wax candle flame remains essentially undisturbed.
-1 -1
Quantitative examples are small-scale fire tests in which a maximum air speed of 0,1 m⋅s or 0,2 m⋅s is sometimes
specified.
[SOURCE: ISO 13943:2017, definition 3.83]
3.5
end product
product which is ready for use
Note 1 to entry: An end product can be a component of another end product.
[SOURCE: IEC 60695-4:2012, definition 3.2.7]

– 10 – IEC TS 60695-2-20:2024 RLV © IEC 2024
3.6
fire effluent
all gases and aerosols, including suspended particles, created by combustion or pyrolysis and
emitted to the environment
[SOURCE: ISO 13943:2017, definition 3.123]
3.7
fire hazard
potential for harm associated with fire
Note 1 to entry: Alternatively, fire hazard can be a physical object or condition with a potential for an undesirable
consequence from fire.
[SOURCE: ISO 13943:2017, definition 3.131]
3.8
ignitability
ease of ignition
measure of the ease with which a test specimen can be ignited, under specified conditions
Note 1 to entry: Modified, notes to entry have been deleted
[SOURCE: ISO 13943:2017, definition 3.212]
3.8
ignition
sustained ignition (deprecated)
initiation of combustion
[SOURCE: ISO 13943:2017, definition 3.217]
3.10
molten drip
falling droplet of material which has been softened or liquefied by heat
Note 1 to entry: The droplets can be flaming or not flaming.
[SOURCE: ISO 13943:2017, definition 3.275]
3.9
preselection
process of assessing and choosing candidate materials, components or sub-assemblies for
making an end product
[SOURCE: IEC 60695-1-30, definition 3.5]
3.12
time to drip, DT
time elapsed after the start of a test when molten drips are first observed to fall from the test
specimen
3.10
time to ignite
IT
time elapsed after the start of a test when ignition of the test specimen is observed to occur

4 Principle
A rectangular bar-shaped test specimen is supported horizontally on a test fixture (an example
of a test fixture is shown in Figure 1 and Figure 3). The centre portion is wound with a coil of
heater wire as shown in Figure 2. A constant current is applied to the coil, which rapidly heats
up and the behaviour of the test specimen is observed. The time to ignite, IT and/or the time
to drip, DT shall be recorded.

Figure 1 – Test fixture arrangement (example)

– 12 – IEC TS 60695-2-20:2024 RLV © IEC 2024
Dimension in millimetres
Key
1 Test specimen
2 Heater wire
3 Electrical connection point
4 Distance to electrical connection points (≥ 35 mm)
5 Distance between all 5 windings (31,5 to 32,0 30,5 to 32,5 mm)
Figure 2 – Test specimen winding pattern
5 Apparatus
5.1 Test chamber
The test chamber consists of a laboratory fume hood/chamber shall have having an inside
volume of at least 0,5 m . The test chamber shall provide a draught-free environment whilst
allowing normal thermal circulation of air past the test specimen. The test chamber shall permit
observation of the test in progress. The inside surfaces of the walls shall be a dark colour. The
test chamber shall have an ambient light level not exceeding 20 lux. For safety and convenience,
it is desirable that this enclosure test chamber (which can be completely closed) is fitted with
an extraction device, such as an exhaust fan, to remove the fire effluent which may be toxic.
The extraction device, if fitted, shall be turned off during the test and turned on immediately
after the test. A positive closing damper may be needed.
5.2 Heater wire
The heater wire shall be a Nickel/Chromium wire (NCHW1 according to JIS C 2520), having a
nominal composition of > 77 % Ni and (20 ± 1 % Cr), having a nominal diameter of
(0,5 ± 0,016) mm and a length of 260 mm (+ 10 mm, – 0 mm).
-6
NOTE 1 NiCr (> 77 % Ni / 20 ± 1 % Cr) heater wire has a nominal cold resistivity of (1,08 ± 0,05) × 10 Ω⋅m.
NOTE 2 The Nickel/Chromium wire is also known as NCHW1 according to JIS C 2520 is also known as and
NiCr8020 according to DIN 17470 Werkstoffnummer 2.4869, or as specified in ASTM D3874.
NOTE 3 The length of wire may need to be adapted as follows: 280 mm (+ 10 mm, − 0 mm) for specimens with
thicknesses between > 3 mm and ≤ 8 mm; and 350 mm (+10 mm, −0 mm) for specimens with thicknesses between
> 8 mm and ≤ 13 mm.
5.3 Power supply and test circuit
A constant current DC power supply which can provide a constant current of at least 8 A and a
power of at least 150 W shall be used to energize the heater wire. The supply circuit shall have
the following capabilities.
a) There shall be a means for measuring the current through the test circuit current to within
with an accuracy of ± 0,2 %;
b) All electrical connections for the heater wire shall be capable of transmitting the test current
without significant loss and shall not mechanically affect the test specimen during the test.
c) A voltmeter shall be provided to measure the voltage using small probes as shown in
Figure 5.
The test circuit shall be provided with an on-off switch for cutting off the test circuit current (for
safety reasons).
NOTE An AC power supply is also suitable to drive the set constant current.
5.4 Test specimen fixture
An example of a test specimen fixture is shown in Figure 3. Two supporting posts positioned
70 mm ± 2 mm apart shall be provided with hold-down clamps to support the test specimen in
a horizontal and flat position.
Dimensions in millimetres
Key
1 Chamber surface
2 Test specimen fixture
3 Distance between the two supporting posts
4 Distance between the top and bottom of the supporting post
Figure 3 – Test specimen fixture (example)
5.5 Test specimen winding and pressing
The heater wire shall be wound around the test specimen centred along the longitudinal axis of
the specimen by hand or by using a winding device.

– 14 – IEC TS 60695-2-20:2024 RLV © IEC 2024
Five windings shall be applied using a force of (5,40 ± 0,02) N, equally distributed, with a pitch
of 6,35 ± 0,05 (6,3 ± 0,2) mm. In practice the five windings on the test specimen shall be evenly
spread over the test specimen which shall result in a distance of between 31,5 30,5 mm and
32,0 32,5 mm as shown in Figure 2. An example of a winding device is shown in Figure 4.

Figure 4 – Heater wire winding device (Example)
If winding is done manually, mark the test bar in the following way the first winding point at the
test bar shall be calculated according to Formula (1) and marked on the test specimen.
(125,0 mm – 30,5 mm)/2 (1)
For a test bar of 125,0 mm in length, knowing that the total distance between the 5 windings
should be between 31,5 30,5 mm and 32 32,5 mm, the first winding point shall start at:
(125-31,5)/2 47,25 mm from one of the test bar ends.
NOTE 1 Thin specimens can be wound by supporting the specimen on one side with a steel bar which has the same
size as the test specimen and a thickness at which the steel does not bend. The steel bar needs to be removed prior
to the test, taking care to ensure that the spacing of the windings on the specimen are not affected. When the
specimen is pliable and susceptible to indentation, the steel bar can also be used on both sides of the test specimen.
Deformation of the test specimens during preparation is to be kept to an absolute minimum.
Examples of deformation are:
– bowing, in either transverse or longitudinal directions, or twisting during the winding process
to a degree visible to the eye, or
– visible indentation of the wound wire into the edges of the test specimen.
The wound part of the wire should be in good contact with the test specimen.
This is achieved by pressing the wound test specimen between two stiff surfaces; pressing them
together with enough force sufficient to flatten the wire on to the test specimen surface.
NOTE 2 It has been found useful to use boards made of pinewood that are flat and smooth and having a minimum
thickness of 10 mm, and to apply a pressing force of approximately 150 N.
NOTE 3 See informative Annex A for guidance on properly wound test specimen.

5.6 Conditioning chamber
The conditioning chamber shall be maintained at 23 °C ± 2 °C at a relative humidity of
50 % ± 10 %.
5.7 Timing device
The timing device shall have a resolution of 0,5 s or less.
5.8 Micrometer
The micrometer shall have a resolution of
a) 0,01 mm or less for test specimens with a thickness of 0,25 mm or greater; and
b) 0,001 mm or less for test specimens with a thickness less than 0,250 mm.
5.9 Measuring scale
The measuring scale shall be graduated in millimetres.
6 Test specimens
6.1 Test specimen preparation
10 test specimens shall be prepared.
When tests are carried out on materials for preselection purposes, they shall be conducted on
uncoloured test specimens or on normally supplied coloured test specimens.
Test specimens shall be cut from a representative test specimen of the material (sheets or end
products) or shall be cast, or injection moulded in accordance with ISO 294, compression
moulded in accordance with ISO 293 or ISO 295, or transfer moulded to the necessary shape.
After any cutting operation, care shall be taken to remove all dust and particles from the surface
and cut edges shall be fine sanded to a smooth finish.
6.2 Test specimen dimensions
This test method applies to moulded or sheet materials available in nominal thicknesses of up
to and including 13 mm.
Test specimens shall be 125 mm ± 5 mm long by 13,0 mm ± 0,5 mm wide.
The preferred thicknesses for preselection purposes include 0,4 mm, 0,75 mm, 1,5 mm and
3,0 mm. Other thicknesses may be used by agreement between the interested parties and if so,
shall be noted in the test report.
Test specimens having thickness up to 3,0 mm shall be tested using the nominal 260 mm length
of wire.
For test specimens having thickness more than 3,0 mm and up to 8,0 mm the length of the wire
to be used shall be 280 mm.
For test specimens having thickness more than 8,0 mm and up to 13,0 mm the length of the
wire to be used shall be 350 mm.
NOTE 1 Some materials can be tested at lower thicknesses, depending on the rigidity of the test specimens. As
long as the test specimens do not deform during winding, the test method is applicable.

– 16 – IEC TS 60695-2-20:2024 RLV © IEC 2024
NOTE 2 The maximum thickness is not to exceed 3,0 mm, when using the nominal 260 mm length of wire. For
specimens with thicknesses between > 3 mm and ≤ 8 mm the length of the wire may need to be adapted to 280 mm,
and for specimens with thicknesses between > 8 mm and 13 mm the length of the wire may need to be adapted to
350 mm.
Thickness measurements are to be taken at the approximate center of the test specimen.
When test specimens are taken from end products, three measurements along that part of the
test specimen which is wound with heater wire, are to be taken. The arithmetic mean of the
three measured values is to be taken as the value of the thickness of the test specimen.
For rigid test specimens, thickness measurements shall be performed in accordance with
ISO 16012 as follows. Using a ratchet micrometer, close the micrometer at such a rate that the
change in reading on the scale or digital display can be easily followed. Continue the closing
motion until the ratchet clicks three times, the friction thimble slips, or the two contact surfaces
can be felt to be in full contact with the test specimen. Record the indicated reading. For flexible,
non-rigid, or elastic test specimens, a dial gauge micrometer may be used. The closing motion
shall be stopped when the pressure foot just contacts the test specimen.
NOTE 3 Other measuring devices equivalent to a micrometer may be used to measure thickness if found to be
satisfactory.
In order for test specimens to accurately represent a nominal thickness, the thickness shall
meet the tolerances given in Table 1.
Other measuring device can be used to measure the thickness of the test specimen, provided
that the device resolution complies with those specified in 5.8.
The thickness of the 10 specimens shall be measured to determine the test specimens set
nominal thickness and identify if any non-conform specimen is present. Each measured test
specimen thickness shall meet the tolerance provided in Table 1 for the specific thickness range.
Non-conform specimens shall be discarded and replaced by new conform specimens.
Table 1 – Nominal thickness tolerances
Thickness x Tolerance
mm mm
< 0,02 ± 10%
0,02 ≤ x < 0,05 ± 0,005
0,05 ≤ x < 0,1 ± 0,01
0,1 ≤ x < 0,2 ± 0,02
0,2 ≤ x < 0,3 ± 0,03
0,3 ≤ x < 0,5 ± 0,04
0,5 ≤ x < 0,6 ± 0,05
0,6 ≤ x < 3,0 ± 0,15
3,0 ≤ x < 6,0 ± 0,25
6,0 ≤ x < 13,0 ± 0,40
NOTE For example, to represent a thickness of 1,5 mm, all
tested specimens should measure between 1,35 mm and
1,65 mm.
7 Conditioning
7.1 Requirements
Unless otherwise required by the relevant material specifications or relevant product standards,
the following requirements shall apply.
7.2 Test specimen conditioning
10 test specimens shall be placed in a conditioning chamber for a minimum of 48 h at
23 °C ± 2 °C at a relative humidity of 50 % ± 10 % (see ISO 291:2008, Clause 6, Table 2,
Class 2).
7.3 Heater wire conditioning
10 pieces of un-annealed heater wires to be used for testing shall be placed in a conditioning
chamber for a minimum of 48 h at 23 °C ± 2 °C at a relative humidity of 50 % ± 10 %
(see ISO 291:2008, Clause 6, Table 2, Class 2).
7.4 Test conditions
All test specimens shall be tested in a laboratory atmosphere having a temperature of 15 °C to
35 °C and 75 % or less relative humidity.
8 Test procedure
8.1 Verification of the heater wire
8.1.1 General
Due to normal variations in metal alloys, it is essential that each spool of heater wire shall be
verified with respect to its energized resistance according to the following procedure. A
mathematical relationship exists between the supplied current and power dissipation, based on
performance under the verification procedure. Essentially, the voltage across a known length
of wire is measured over a range of current values to establish the power-current relationship
used to calculate the test current, I .
c
NOTE It has been found that the variation of electrical resistance within a given spool is not significant. However,
it is recommended advisable to perform the verification of the wire after a long period of non-use, or once a year.
This procedure should has been found to be good laboratory practice when performed in a draught-free
environment.
8.1.2 Apparatus Test circuit for the verification
A current supply shall be provided as described in 5.3 with a means to measure current. a
voltmeter shall be provided to measure the voltage using small probes as shown in Figure 5.
In addition to the test circuit described in 5.3, a voltmeter is used to measure the voltage over
the heating wire by means of small probes as shown in Figure 5.

– 18 – IEC TS 60695-2-20:2024 RLV © IEC 2024

Key
1 Current connection point
2 Voltage measuring probes
3 Test wire
Figure 5 – Test circuit arrangement for the heater wire calibration verification
8.1.3 Determination of the test current, I
c
Use a 260 mm long piece of heater wire. Mark it at 5 mm and 10 mm from each end. Connect
the heater wire to the current supply in such a way that 5 mm of wire is inserted, resulting in
250 mm wire in the circuit. Attach the small voltage-measuring probes to the heater wire at the
10 mm marks at each end, to measure the voltage to within ± 2 %, as shown in Figure 5.
Measure and record the length L of the wire between the voltage probes to an accuracy of
± 1 mm. The length of the wire between the voltage probes should be 240 mm.
NOTE 1 In Figure 5 the heater wire is straightened but it can also be inserted in a horse shoe shaped loop.
NOTE 2 When specimens with thicknesses > 3 mm and ≤ 8 mm are to be tested, calibration needs to be performed
by using 280 mm of wire. When specimens with thickness > 8 mm and ≤ 13 mm are to be tested, calibration needs
to be performed by using 350 mm of wire.
Determination of the test current shall be performed using heater wire length according to the
specimen thickness to be tested. For specimens having thickness up to 3,0 mm, heater wire
260 mm in length shall be used. For specimens having thickness more than 3,0 mm and up to
8,0 mm, heater wire 280 mm in length shall be used. For specimens having thickness more than
8,0 mm and up to 13,0 mm, heater wire 350 mm in length shall be used.
The heater wire shall be marked at 5 mm and 10 mm from each end. Connect the heater wire
to the current supply by using only the 5 mm of heater wire length for each end. Connect the
voltmeter measuring probes to the heater wire at the 10 mm marked point on each end. Measure
and record the length L of the heater wire between the voltmeter probes to an accuracy of
± 1 mm. The measured length L shall meet the value specified in Table 2.
Table 2 – Heater wire verification lengths
Heater wire length Measured length L
260 mm 240 mm + 0 mm / − 1 mm
280 mm 260 mm + 0 mm / − 1 mm
350 mm 330 mm + 0 mm / − 1 mm
The heating wire marking, and connection are shown in Figure 6.

Dimensions in millimetres
Key
1 Heater wire
2 Voltage measuring probes
3 Power supply connections
Figure 6 – Heater wire marking and connection
NOTE In Figure 5 and Figure 6 the heater wire is straightened but it can also be inserted in a horseshoe shaped
loop.
Apply current levels in increments of 1 A, starting from 1 A up to 8 A ± 1 %. Maintain the current
for a minimum of 10 s to achieve thermal equilibrium and record the current and voltage at each
level.
8.1.4 Calculation of test current, I
c
For each measurement, calculate the linear power density shall be calculated according to
Formula (2).
W = E * I / L (2)
where:
W = linear power density, W/mm;
E = measured voltage, V;
I = measured current, A;
L = measured length between voltage probes, mm.
Construct a calibration curve of current as a function of linear power density. The desired test
current I for the individual heater wire obtained from the calibration curve shown in Figure 6,
c
is that which corresponds to 0,26 W/mm ± 0,01 W/mm. Annex C describes how to determine
the test current I in using a spreadsheet program.
c
Plot the measured c
...