IEC TR 60943:1998+AMD1:2008 CSV

Guidance concerning the permissible temperature rise for parts of electrical equipment, in particular for terminals

IEC/TR 60943:1998+A1:2009 is intended for guidance in estimating the permissible values for temperature and temperature rise of component parts of electrical equipment carrying current under steady state conditions. The present report is intended to supply:
- general data on the structure of electric contacts and the calculation of their ohmic resistance;
- the basic ageing mechanisms of contacts;
- the calculation of the temperature rise of contacts and connection terminals;
- the maximum "permissible" temperature and temperature rise for various components, in particular the contacts, the connection terminals and the conductors connected to them;
- the general procedure to be followed by product committees for specifying the permissible temperature and temperature rise.
This consolidated version consists of the second edition (1998) and its amendment 1 (2009). Therefore, no need to order amendment in addition to this publication.

Guide concernant l'échauffement admissible des parties des matériels électriques, en particulier les bornes de raccordement

La CEI/TR 60943:1998+A1:2009 est destiné à servir de guide lorsqu'il s'agit d'estimer des valeurs admissibles pour les températures et les échauffements des parties conductrices de matériels électriques en régime établi. Le présent rapport se propose de fournir:
- les données générales sur la structure des contacts électriques et le calcul de leur résistance ohmique;
- les mécanismes fondamentaux du vieillissement des contacts;
- le calcul de l'échauffement des contacts et des bornes de connexion;
- les températures et échauffements maximaux admissibles pour différents organes de matériels, en particulier les contacts, les bornes de connexion et les conducteurs qui leur sont raccordés;
- la marche générale à suivre par le comité de produit pour spécifier les températures et échauffements admissibles.  Cette version consolidée comprend la deuxième édition (1998) et son amendement 1 (2009). Il n'est donc pas nécessaire de commander l'amendement avec cette publication.

General Information

• Status: Published
• Publication Date: 23-Mar-2009
• ICS: 01 - GENERALITIES. TERMINOLOGY. STANDARDIZATION. DOCUMENTATION
• ICS: 29.020 - Electrical engineering in general
• Technical Committee: TC 32 - Fuses
• Current Stage: PPUB - Publication issued
• Start Date: 24-Mar-2009
• Completion Date: 24-Mar-2009

Standards Content (Sample)

IEC/TR 60943
®
Edition 2.1 2009-03
TECHNICAL
REPORT
RAPPORT
TECHNIQUE
Guidance concerning the permissible temperature rise for parts of electrical
equipment, in particular for terminals

Guide concernant l’échauffement admissible des parties des matériels
électriques, en particulier les bornes de raccordement
IEC/TR 60943:1998+A1:2008

---------------------- Page: 1 ----------------------
THIS PUBLICATION IS COPYRIGHT PROTECTED
Copyright © 2009 IEC, Geneva, Switzerland

All rights reserved. Unless otherwise specified, no part of this publication may be reproduced or utilized in any form or by
any means, electronic or mechanical, including photocopying and microfilm, without permission in writing from either IEC or
IEC's member National Committee in the country of the requester.
If you have any questions about IEC copyright or have an enquiry about obtaining additional rights to this publication,
please contact the address below or your local IEC member National Committee for further information.

Droits de reproduction réservés. Sauf indication contraire, aucune partie de cette publication ne peut être reproduite
ni utilisée sous quelque forme que ce soit et par aucun procédé, électronique ou mécanique, y compris la photocopie
et les microfilms, sans l'accord écrit de la CEI ou du Comité national de la CEI du pays du demandeur.
Si vous avez des questions sur le copyright de la CEI ou si vous désirez obtenir des droits supplémentaires sur cette
publication, utilisez les coordonnées ci-après ou contactez le Comité national de la CEI de votre pays de résidence.

IEC Central Office
3, rue de Varembé
CH-1211 Geneva 20
Switzerland
Email: [e-mail]
Web: www.iec.ch

About the IEC
The International Electrotechnical Commission (IEC) is the leading global organization that prepares and publishes
International Standards for all electrical, electronic and related technologies.

About IEC publications
The technical content of IEC publications is kept under constant review by the IEC. Please make sure that you have the
latest edition, a corrigenda or an amendment might have been published.
ƒ Catalogue of IEC publications: www.iec.ch/searchpub
The IEC on-line Catalogue enables you to search by a variety of criteria (reference number, text, technical committee,…).
It also gives information on projects, withdrawn and replaced publications.
ƒ IEC Just Published: www.iec.ch/online_news/justpub
Stay up to date on all new IEC publications. Just Published details twice a month all new publications released. Available
on-line and also by email.
ƒ Electropedia: www.electropedia.org
The world's leading online dictionary of electronic and electrical terms containing more than 20 000 terms and definitions
in English and French, with equivalent terms in additional languages. Also known as the International Electrotechnical
Vocabulary online.
ƒ Customer Service Centre: www.iec.ch/webstore/custserv
If you wish to give us your feedback on this publication or need further assistance, please visit the Customer Service
Centre FAQ or contact us:
Email: [e-mail]
Tel.: [phone]
Fax: [phone]


A propos de la CEI
La Commission Electrotechnique Internationale (CEI) est la première organisation mondiale qui élabore et publie des
normes internationales pour tout ce qui a trait à l'électricité, à l'électronique et aux technologies apparentées.

A propos des publications CEI
Le contenu technique des publications de la CEI est constamment revu. Veuillez vous assurer que vous possédez
l’édition la plus récente, un corrigendum ou amendement peut avoir été publié.
ƒ Catalogue des publications de la CEI: www.iec.ch/searchpub/cur_fut-f.htm
Le Catalogue en-ligne de la CEI vous permet d’effectuer des recherches en utilisant différents critères (numéro de référence,
texte, comité d’études,…). Il donne aussi des informations sur les projets et les publications retirées ou remplacées.
ƒ Just Published CEI: www.iec.ch/online_news/justpub
Restez informé sur les nouvelles publications de la CEI. Just Published détaille deux fois par mois les nouvelles
publications parues. Disponible en-ligne et aussi par email.
ƒ Electropedia: www.electropedia.org
Le premier dictionnaire en ligne au monde de termes électroniques et électriques. Il contient plus de 20 000 termes et
définitions en anglais et en français, ainsi que les termes équivalents dans les langues additionnelles. Egalement appelé
Vocabulaire Electrotechnique International en ligne.
ƒ Service Clients: www.iec.ch/webstore/custserv/custserv_entry-f.htm
Si vous désirez nous donner des commentaires sur cette publication ou si vous avez des questions, visitez le FAQ du
Service clients ou contactez-nous:
Email: [e-mail]
Tél.: [phone]
Fax: [phone]

---------------------- Page: 2 ----------------------
IEC/TR 60943
®
Edition 2.1 2009-03
TECHNICAL
REPORT
RAPPORT
TECHNIQUE
Guidance concerning the permissible temperature rise for parts of electrical
equipment, in particular for terminals

Guide concernant l’échauffement admissible des parties des matériels
électriques, en particulier les bornes de raccordement

INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
COMMISSION
ELECTROTECHNIQUE
PRICE CODE
INTERNATIONALE
CQ
CODE PRIX
ICS 29.020 ISBN 978-2-88910-335-5
® Registered trademark of the International Electrotechnical Commission
Marque déposée de la Commission Electrotechnique Internationale

---------------------- Page: 3 ----------------------
– 2 – 60943 © IEC:1998+A1:2008
CONTENTS

FOREWORD .4
INTRODUCTION.6

Section 1: General
1 General.8
1.1 Scope and object.8
1.2 Reference documents.8
1.3 Definitions.9
1.4 Symbols.9

Section 2: Theory
2 General considerations concerning the nature of electric contact and the calculation and
measurement of the ohmic resistance of contacts.10
2.1 Electric contacts and connection terminals .10
2.2 Nature of electric contact.10
2.3 Calculation of contact resistance.12
3 Ageing mechanisms of contacts and connection terminals .16
3.1 General.16
3.2 Contacts of dissimilar metals.17
3.3 Oxidation ageing mechanisms .19
3.4 Results concerning ageing of copper contacts.21
3.5 Usage and precautions to be taken in the use of contact materials.24
4 Calculation of temperature rise of conductors, contacts and connection terminals .25
4.1 Symbolic representations .25
4.2 Temperature rise ΔT of a conductor with respect to the temperature T
s e
of the surrounding medium.27
4.3 Temperature rise ΔT o in the vicinity of the contact: temperature rise
o
of connection terminals.28
4.4 Temperature rise of the elementary contact points .28

Section 3: Application
5 Permissible temperature and temperature rise values.29
Θ .29
5.1 Ambient air temperature
a
5.2 Temperature and temperature rise of various equipment components .30
5.3 Temperature and temperature rise of conductors connecting electrical equipment .38
5.4 Temperature and temperature rise of connection terminals for electrical equipment –
Influence on connected conductors.39

---------------------- Page: 4 ----------------------
60943 © IEC:1998+A1:2008 – 3 –
6 General procedure to be followed for determining permissible temperature
and temperature rise .40
6.1 Basic parameters.40
6.2 Method to be followed for determining maximum permissible temperature
40
and temperature rise .

Annex A Numerical examples of the application of the theory and other data . 42
Annex B Physical characteristics of selected metals and alloys. 45
Annex C Physical characteristics of fluid dielectrics . 46
Annex D nformation on the reaction of contact metals with substances in the atmosphere . 47
Annex E Temperature rise of a conductor cooled by radiation and convection in the vicinity
of a terminal. 48
Annex F List of symbols used in this report . 57
Annex G Bibliography .59

---------------------- Page: 5 ----------------------
– 4 – 60943 © IEC:1998+A1:2008
INTERNATIONAL ELECTROTECHNICAL COMMISSION
__________

GUIDANCE CONCERNING THE PERMISSIBLE TEMPERATURE RISE
FOR PARTS OF ELECTRICAL EQUIPMENT,
IN PARTICULAR FOR TERMINALS



FOREWORD
1) The IEC (International Electrotechnical Commission) is a worldwide organization for standardization comprising
all national electrotechnical committees (IEC National Committees). The object of the 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, the IEC publishes International Standards. 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
with the IEC also participate in this preparation. The IEC collaborates closely with the International Organization
for Standardization (ISO) in accordance with conditions determined by agreement between the two
organizations.
2) The formal decisions or agreements of the 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 National Committees.
3) The documents produced have the form of recommendations for international use and are published in the form
of standards, technical reports or guides and they are accepted by the National Committees in that sense.
4) In order to promote international unification, IEC National Committees undertake to apply IEC International
Standards transparently to the maximum extent possible in their national and regional standards. Any
divergence between the IEC Standard and the corresponding national or regional standard shall be clearly
indicated in the latter.
5) The IEC provides no marking procedure to indicate its approval and cannot be rendered responsible for any
equipment declared to be in conformity with one of its standards.
6) Attention is drawn to the possibility that some of the elements of this International Standard may be the subject
of patent rights. The IEC shall not be held responsible for identifying any or all such patent rights.

The main task of IEC technical committees is to prepare International Standards. In
exceptional circumstances, a technical committee may propose the publication of a technical
report of one of the following types:
• type 1, when the required support cannot be obtained for the publication of an
International Standard, despite repeated efforts;
• type 2, when the subject is still under technical development or where for any other
reason there is the future but no immediate possibility of an agreement on an International
Standard;
• type 3, when a technical committee has collected data of a different kind from that
which is normally published as an International Standard, for example "state of the art".
Technical reports of types 1 and 2 are subject to review within three years of publication to
decide whether they can be transformed into International Standards. Technical reports of
type 3 do not necessarily have to be reviewed until the data they provide are considered to be
no longer valid or useful.
IEC 60943, which is a technical report of type 3, has been prepared by IEC technical
committee 32: Fuses.

---------------------- Page: 6 ----------------------
60943 © IEC:1998+A1:2008 – 5 –
This consolidated version of IEC 60943 consists of the second edition (1998) [documents
32/142/CDV and 32/148/RVC] and its amendment 1 (2008) [documents 32/187/DTR and
32/188/RVC].
The technical content is therefore identical to the base edition and its amendment and has
been prepared for user convenience.
It bears the edition number 2.1.
A vertical line in the margin shows where the base publication has been modified by
amendment 1.
Annexes are for information only.
The committee has decided that the contents of the base publication and its amendments will
remain unchanged until the maintenance result date indicated on the IEC web site under
"http://webstore.iec.ch" in the data related to the specific publication. At this date,
the publication will be
• reconfirmed,
• withdrawn,
• replaced by a revised edition, or
• amended.

---------------------- Page: 7 ----------------------
– 6 – 60943 © IEC:1998+A1:2008
INTRODUCTION
a) The temperature rise encountered in electrical assemblies as a result of the various losses
in the conductors, contacts, magnetic circuits, etc. is of growing importance as a result of
the development of new techniques of construction and operation of equipment.
This development has been particularly significant in the field of assemblies, where
numerous components dissipating energy (contactors, fuses, resistors, etc.), in particular
modular devices are found within enclosures of synthetic materials which are somewhat
impermeable to heat.
This temperature rise results in a relatively high temperature of the basic elements
constituting the electric contacts: a high temperature favours oxidation at the contact
interface, increases its resistance and thereby leads to further heating, and thus to an even
higher temperature. If the component material of the contact is unsuitable or insufficiently
protected, the contact may be irreparably damaged before the calculated useful life of the
equipment has expired.
Such temperature rises also affect connection terminals and the connected conductors, and
their effects should be limited in order to ensure that the insulation of the conductors
remains satisfactory throughout the life of the installation.
b) In view of these problems, this report has been prepared with the following objectives:
– to analyze the various heating and oxidation phenomena to which the contacts, the
connection terminals and the conductors leading to them are subjected, depending on
their environment and their arrangement;
– to provide elementary rules to product committees to enable them to specify permissible
temperatures and temperature rises.
c) Attention is drawn to the precautions to be taken for sets of components when parts are
grouped together in the same enclosure.
 The attention of users should be drawn particularly to the fact that the temperature rise of
terminals permitted by particular switchgear standards results from conventional situations
during type tests; these can differ appreciably from the situations met with in practice, which
have to be taken into account, particularly because of the temperatures permitted by the
insulation of the conductors which may be connected to the terminals under normal
conditions.
d) Attention is drawn to the fact that in the relevant product standards, the permissible
temperature and temperature rise for the external terminals are measured during
conventional type tests and therefore they may not reflect the actual situation likely to occur
in normal use.
Suitable precautions should then be adopted to avoid exposure to temperatures that may
affect the life of materials adjacent to the terminals of components.
In this case, it is essential to distinguish the concept of "external ambient temperature"
which prevails outside the enclosure from that of "the temperature of the fluid surrounding a
part" which comprises the external ambient temperature plus the internal temperature rise
due to the parts. These concepts, as well as other complementary concepts such as the
thermal resistance of an enclosure, are dealt with in clause 5 and explained by means of
numerical examples.
In order to facilitate complete calculation, this report links up the temperature of the fluid
surrounding a component to the external ambient temperature by the introduction of the
concept of "coefficient of filling" and gives a numerical example (5.2.3.2) which specifies the
values of the coefficient of filling to be used in several practical cases.

---------------------- Page: 8 ----------------------
60943 © IEC:1998+A1:2008 – 7 –
The quantities involved in calculating contact constriction resistance are subject to wide
variations due to the physical conditions and degree of contamination of the surface in
contact. By calculation alone, therefore, the contact resistance can be estimated to an
accuracy of no better than an order of magnitude.
More precise and more accurate values should be obtained by direct measurement on given
items of electrical equipment, because in practice it is often the case that other incalculable
degradation mechanisms predominate.
 This report is not meant to give guidance on the derating of components.
 It is strongly advised that the reference literature quoted at the end of this report be studied
before attempting to apply the data to a practical problem.

---------------------- Page: 9 ----------------------
– 8 – 60943 © IEC:1998+A1:2008
GUIDANCE CONCERNING THE PERMISSIBLE TEMPERATURE RISE
FOR PARTS OF ELECTRICAL EQUIPMENT,
IN PARTICULAR FOR TERMINALS

Section 1: General
1 General
1.1 Scope and object
This report is intended for guidance in estimating the permissible values for temperature and
temperature rise of component parts of electrical equipment carrying current under steady
state conditions.
This report applies to electrical power connections and materials adjacent to them.
This report is concerned with the thermal effects of currents passing through connections,
therefore there are no voltage limits to its application.
This report is only applicable when referred to in the appropriate product standard.
The extent and manner to which the contents of this report are used in standards is the
responsibility of individual Technical Committees.
Whenever "permissible" values are stated in this report, they mean values permitted by the
relevant product standard.
The present report is intended to supply:
– general data on the structure of electric contacts and the calculation of their ohmic
resistance;
– the basic ageing mechanisms of contacts;
– the calculation of the temperature rise of contacts and connection terminals;
– the maximum “permissible” temperature and temperature rise for various components, in
particular the contacts, the connection terminals and the conductors connected to them;
– the general procedure to be followed by product committees for specifying the permissible
temperature and temperature rise.

1.2 Reference documents
IEC 60050(441):1984, International Electrotechnical Vocabulary (IEV) – Chapter 441: Switch-
gear and controlgear and fuses
IEC 60085:1984, Thermal evaluation and classification of electrical insulation

---------------------- Page: 10 ----------------------
60943 © IEC:1998+A1:2008 – 9 –
IEC 60216-1:1990, Guide for the determination of thermal endurance properties of electrical
insulating materials – Part 1: general guidelines for ageing procedures and evaluation of the
test results
IEC 60364-4-42:1980, Electrical installations of buildings – Part 4: Protection for safety -
Chapter 42: Protection against thermal effects
IEC 60694:1996, Common specifications for high-voltage switchgear and controlgear standards
IEC 60721-2-1:1982, Classification of environmental conditions – Part 2: environmental
conditions appearing in nature. Temperature and humidity
IEC 60890:1987, A method of temperature-rise assessment by extrapolation for partially type-
tested assemblies (PTTA) of low voltage switchgear and controlgear
IEC 60947-1:1988, Low-voltage switchgear and controlgear – Part 1: General rules
1.3 Definitions
Definitions of terms used in this report may be found in the International Electrotechnical
Vocabulary. For the purposes of this technical report, the following terms also apply:
1.3.1
ambient air temperature Θ
a
the temperature, determined under prescribed conditions, of the air surrounding the complete
device [IEV 441-11-13]
NOTE For devices installed inside an enclosure, it is the temperature of the air outside the enclosure.
1.3.2
contact (of a mechanical switching device)
conductive parts designed to establish circuit continuity when they touch and which, due to
their relative motion during an operation, open or close a circuit or, in the case of hinged or
sliding contacts, maintain circuit continuity [IEV 441-15-05]
NOTE Do not confuse with "IEV 441-15-06 Contact (piece): one of the conductive parts forming a contact."
1.3.3
connection (bolted or the equivalent)
two or more conductors designed to ensure permanent circuit continuity when forced together
by means of screws, bolts, or the equivalent [3.5.10 of IEC 60694]

1.4 Symbols
A list of symbols used in this report is given in annex F.

---------------------- Page: 11 ----------------------
– 10 – 60943 © IEC:1998+A1:2008
Section 2: Theory

NOTE This theory applies to both "contacts" and "connections" as defined in 1.3.2 and 1.3.3. For convenience,
only the word "contact" only is used in this section to cover both applications.

2 General considerations concerning the nature of electric contact and
the calculation and measurement of the ohmic resistance of contacts
2.1 Electric contacts and connection terminals
Electric contact, in its simplest and most general configuration, results from contact
established between two pieces of (usually metallic) conducting material. In the case of
connection terminals, these are the terminal itself and the conductor which is connected to it.
The active zone is the contact "interface" which is the region where the current passes from
one piece to the other. It is in this area that the contact resistance occurs, causing heating by
Joule effect, and it is also where ageing occurs through chemical reaction with the surrounding
atmosphere.
2.2 Nature of electric contact
When one piece of metal is applied to another, contact is not made over the whole apparent
contact area, but only at a certain number of points called "elementary contacts".
The effective total cross-sectional area of these contacts is equal to the effective contact area
)
1
S if the possible presence of impurities is ignored (dust, etc.) at the contact interface.
a
There is also a fine layer of air or of oxide normally present, the effect of which upon the
contact resistance will be examined later (see 2.3).
In the following, for ease of calculation and for a better understanding of the contact
mechanisms, the simplifying assumption is made that there are n elementary contacts on the
apparent contact area, uniformly distributed, of average constant radius a (see figure 1). The
average distance between these elementary contacts is l.
The effective contact area is then:
2
S = n π a
a

──────────
)
1
For an explanation of the symbols used in this report, see annex F.

---------------------- Page: 12 ----------------------
60943 © IEC:1998+A1:2008 – 11 –
IEC 1 286/97

Figure 1 – Illustration of apparent contact and effective contact areas
The contact area S depends upon how hard the contacts are pressed against each other, i.e.
a
upon the force applied, the surface state of the contacts, and the hardness of the material
used.
For the forces normally found in electrical technology, the contact area is, in practice, the area
over which the force applied reaches the ultimate strength of the contact material characterised
by the "hardness" of that material.
In fact, the asperities on each of the two surfaces before they are brought into contact and
which are due to previous preparation of the surface are of small dimension (of the order of
1/100 mm) and are crushed even by small forces of the order of 0,1 N.
Assuming that the pressure exerted upon the contact area is equal to the contact hardness of
the metal (H), then the following equation is obtained:
F
= ξ H
S
a
However, this equation applies only for a contact force of F ≥ 50 N, in fact:
F
Sn==πa²
a
ξ H
where ξ is a dimensionless "coefficient of flatness" dependent upon the state of the surfaces in
contact, usually having a value of between 0,3 and 0,6 for normal forces, but which can be
much smaller after extensive polishing of the contact surfaces against each other.
As a result, the elementary contact radius a is given by the equation:
F
a =
(1)
πξ
nH

---------------------- Page: 13 ----------------------
– 12 – 60943 © IEC:1998+A1:2008

The number n of elementary contacts can be worked out approximately by the formula:
0,625 0,2
nn= H F (2)
k
–5
where n ≈ 2,5 × 10 (SI units)
k
The above expression gives only the order of magnitude of the number of elementary contacts.
–5
Values of n can differ significantly from the value estimated, for example between 0,5 × 10
k
–5
and 30 × 10 (SI units).
2.3 Calculation of contact resistance
Contact resistance is made up of two components:
a) constriction resistance, due to the drawing together of the lines of current as they pass
through the elementary contacts;
b) film resistance, corresponding to the film of oxide or of adsorbed molecules at the interface.
2.3.1 Calculation of the constriction resistance
Consider (see figure 2) an idealised elementary contact of radius a. If the electrical conductors
are large in relation to the elementary contact, the lines of current are hyperbolae with foci
located at the ends of the elementary contact diameter and the equipotential surfaces are
flattened ellipsoids of the same foci.

IEC 1 287/97
Figure 2 – Equipotentials and lines of current at an elementary contact point

---------------------- Page: 14 ----------------------
60943 © IEC:1998+A1:2008 – 13 –
The resistance R between the point of contact (heavy broken line in figure 2) and the semi-
(a,l)
ellipsoid of major semi-axis l (l being the average distance between neighbouring elementary
contacts and ρ the resistivity of the metal) is equal to half the contact resistance, and is written:
22
ρ la−
R = arctan
(a,l )
2.π a a
If l is large compared with a, which is the more common case:
ρ
R =
(a,ll)( /a→∞)
4a
since the constriction resistance is the sum of both halves
ρ
R = (3)
()e
2a
For an actual contact comprising n relatively widely spread elementary contact points, the
constriction resistance is thus:
ρ
R = (4)
e
2na
2.3.2 Calculation of the film resistance
The elementary contact points generally do not have a corrosion-free interface. Indeed, any
initially pure metal surface becomes covered with a molecular layer of oxygen, leading
...