IEC TR 60943:1998+AMD1:2008 CSV

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
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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
– 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

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
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

– 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.
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.
– 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.

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.

– 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

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.

– 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
)
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:
S = n π a
a
──────────
)
For an explanation of the symbols used in this report, see annex F.

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
– 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

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:
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 in a few
minutes to the formation of a homogeneous layer of oxide a few nanometres thick. If this layer
is sufficiently compact and uniform, it protects the metal to some extent, the oxidation can then
stop and the metal is "passivated"; this is particularly the case with aluminium and stainless
steel at ordinary temperatures.
For other metals (copper, nickel and tin in the presence of oxygen; silver in the presence of
sulphurous gases), the formation of this first layer of reaction product produced by oxidation or
corrosion slows up the subsequent reaction which nevertheless continues, but more and more
slowly.
For certain other metals (iron), the "oxidation" speed is more or less constant because the
surface is not protected by the layer formed.
The main formulae for surface chemical reactions giving the thickness s formed as a function
of time t and thermodynamic temperature T are contained in annex D for different metals.
They are derived from the general formula:
w
⎛ ⎞
sX=⋅exp− ⋅ t (5)
⎜ ⎟
⎝ ⎠
2kT
If the activation energy w is expressed in electronvolts, it is necessary to multiply w by 1,6021 ×
–19
10 J/eV. X is a constant and k is the Boltzmann constant.

– 14 – 60943 © IEC:1998+A1:2008
This thin layer of oxide does not present a purely ohmic resistance to the passage of the
current, such as could be evaluated by the formula:
ρ × length
cross-sectional area
The electrons can in fact pass through it by a "tunnel-effect" mechanism.
The "tunnel resistivity" σ (surface resistivity), which is used to characterize the conductive
o
properties of this layer, is expressed in Ωm (see table 1 for typical values). Tunnel resistivity
depends on the nature of the oxide (or other products of reaction with the atmosphere) and its
thickness. Its thickness generally does not exceed 10 nm.
If the layer of "oxide" covers the actual contact area S uniformly, the apparent resistance R
a
i
between the two faces is written:
σ
o
R =
i
S
a
In the case of n elementary contacts of radius a, the resistance R , due to the layer of oxide at
i
the interface, is expressed by the equation:
σσ
oo
R== (6)
i
total area in contact
n πa
Table 1 – Typical values of tunnel resistivity
σ
Metal State
o
Ω m
–12 –11
Copper New
2 × 10 to 3 × 10
–10
Oxidised
–12 –11
10 to 4 × 10
Tinned
–13 –12
Silver
4,6 × 10 to 4 × 10
–11
exceptionally up to 2,5 × 10
–11 –9
Aluminium 7 × 10 to 10
–13
The values obtained are low for new contacts. The minimum value of 4,6 × 10 for silver
corresponds to the limit thickness of two adsorbed mono-molecular layers of oxygen, i.e.
2 × 0,272 nm = 0,54 nm.
2.3.3 Expression of the total contact resistance
The contact resistance R is the sum of the constriction resistance R (equation (4)) and the
c e
film resistance R (equation (6)), i.e:
i
60943 © IEC:1998+A1:2008 – 15 –
σ
ρ
R=+ (7)
c
2na
naπ
If n and a in this equation are replaced by their values:
06,,25 02
–5
nn= H F with n ≈ 2,5 × 10 (SI units)
k k
F
a= with ξ = 0,45
nHπξ
we obtain the following expression for R :
c
ρπξ
0,,1875−−0 6 1
R=+HF HF
σξ
c o
2 n
k
This formula, applied to the different contact metals, gives the values of k and k shown in
1 2
table 2.
If one metal is thinly plated onto another, the hardness must be taken as that of the plating and
the resistivity as that of the base metal.
In the case of contacts of dissimilar metals, the overall resistance is the average of the
resistance calculated using the constants for each metal.
Table 2 – Typical values of contact resistance constants, calculated for relatively clean
–0,6 –1
surfaces (For substitution in: R = k F + k σ F )
c 1 2 0
Constriction resistance k Film resistance k
1 2
Metal
–6 6
× 10 × 10
Copper 90 247
Brass 360 450
Aluminium 130 135
Almelec 150 135
Silver 81 225
Tin 400 22,5
Nickel 420 585
Silvered copper 88 225
Tinned copper 57 22,5
Tinned aluminium 93 22,5
Silvered brass 310 225
Tinned brass 200 22,5
– 16 – 60943 © IEC:1998+A1:2008
2.3.4 Electrical resistance of contacts when new
Tinned copper contacts theoretically show the lowest resistance compared with other kinds of
contacts. However, this is only true provided two conditions are met: the layer of tin must be
sufficiently thin to prevent its resistivity from being involved, and sufficiently thick for the
hardness involved to actually be that of the tin. In practice, the resistivity obtained in the case
of new tinned contacts is comparable with that of silvered copper and slightly less than that of
copper. However, in the case of tinned contacts of the flexible type or those subject to
vibration, account must be taken of "fretting corrosion" phenomena on the layer of tin,
mentioned in 3.5.
Constriction resistance is particularly high in the case of tin and nickel, which rules out the use
of these materials in the solid state.
Film resistance is high in the case of nickel and nickel-plated copper, which may be admissible
in certain cases, bearing in mind the good corrosion resistance of nickel in corrosive
atmospheres (battery rooms, atmospheres containing H S etc.).
2.3.5 Measurement of contact resistance
Contact resistance measurement is useful either for development tests or as routine tests to
check production by comparison with a specimen which passed the temperature-rise test.
Contact resistance is usually measured by injecting a d.c. current through the junction (so as to
avoid effects of inductance), and measuring the resulting voltage drop across the junction.
For comparison purposes, it is important to measure the voltage drop at a defined location.
Measuring the contact resistance with a current much smaller than the normal current in
service could give incorrect values, in particular when spring-loaded contacts have been
operating on “no-load”.
In addition, the voltage of the test supply should be sufficient to break down any possible
surface layer, without exceeding the working voltage of the equipment under test. Care should
be taken to avoid errors due to thermo-electric effects.
3 Ageing mechanisms of contacts and connection terminals
3.1 General
The ageing of closed electric contacts not subjected to arc erosion (the case with terminals in
particular) is essentially due to the reaction of the metals with the surrounding environment at
the contact interface.
This reaction can be:
– of electrochemical origin (corrosion): as with bi-metallic contacts having incompatible
electrochemical potentials in the presence of significant humidity (> 50 % r.h.);
– of chemical origin: the oxidation being due to the ambient medium (oxygen in the air,
sulphurous vapours such as H S or SO ).
2 2
These two aspects are covered in this report.

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In addition, there are thermo-mechanical effects, involving stress relaxation, creep and
dimensional variations, which are also thermally activated, and have the effect of reducing
contact force and increasing contact resistance, but these are not included in this report. This
complex degradation process is in principle difficult to model, because it is dependent on
design and materials of manufacture. For certain devices, for example connectors, the effects
are so complicated and varied, that no general simple temperature-dependent degradation
curve exists.
3.2 Contacts of dissimilar metals

IEC 1 288/97
Figure 3 – Contact between dissimilar metals in the presence of humidity
(water adsorption)
Corrosion of contacts of dissimilar metals M and M will occur if the following conditions are
1 2
met:
a) different metals – The difference in electrochemical potential between terminals A and B
before contact must in practice be in the order of 0,35 V or more;
b) presence of an electrolyte – The film of water adsorbed on the surfaces in contact as a
result of ambient humidity can play this role;
c) presence of an oxidising agent – The term "oxidising" is taken here in the general sense of
transfer of electrons, whose presence is necessary to depolarise the cell formed and allow
the passage of current. Ambient air is sufficient;
d) contact closed, in order to conduct the corrosion current.
The potential differences appearing at the contact surfaces of M and M in figure 3 with the
1 2
contacts open are given in table 3.

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Table 3 – Voltages developed on bimetallic junction
Values in millivolts
negative pole
positive pole
Silver 0 150 170 190 190 210 230 250 260 330 470 480 510 560 710 720 770 770 790 1090 1100 1110 1590
Nickel 0 020 040 040 060 080 100 110 160 320 330 360 410 530 570 620 620 640 940 950 960 1440
Monel (30 % Cu)  0 020 020 040 060 080 090 160 300 310 340 390 540 550 600 600 620 920 930 940 1420
Cu/Ni (70/30)  0 0 020 040 060 070 140 280 290 320 370 520 530 580 580 600 900 910 920 1400
Copper   0 020 040 060 070 140 260 290 320 370 520 530 580 580 600 900 910 920 1400
Silver solder   0 020 040 050 120 260 270 300 350 500 510 560 560 580 880 890 900 1380
Bronzes*    0 020 030 100 240 250 280 330 480 490 540 540 560 860 870 880 1360
Red bronze    0 010 080 220 230 260 310 460 470 520 520 540 840 850 860 1340
Brasses*     0 070 210 220 250 300 450 460 510 510 530 830 840 850 1330
Stainless steel*     0 140 150 180 230 380 390 440 440 460 760 770 780 1280
Tin      0 010 040 090 240 250 300 300 320 620 630 640 1120
Sn-Pb eutectic      0 030 080 230 240 290 290 310 610 620 630 1110
Sn-Ag solder       0 050 200 210 260 260 280 580 590 600 1080
Lead       0 150 160 210 210 230 530 540 550 1030
Cast Iron        0 010 060 060 080 380 390 400 880
Mild steel        0 050 050 070 370 380 390 870
Al alloys*         0 0 020 320 330 340 820
Aluminium         0 020 320 330 340 820
Cadmium          0 300 310 320 800
Galvanised Fe          0 010 020 500
Zinc alloys*           0 010 490
Zinc           0 450
Mg alloy*             0
NOTE The above values are for guidance only. More exact values may apply for specific grades of metals and the value specified by the supplier should
be used, if available. Otherwise consult specialized textbooks.
* Typical values.
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Acceptable combinations to avoid corrosion should have potential differences less than
350 mV; the lower, the better.
It can be seen that the potential differences developed between dissimilar contacts of the
principal contact materials are low, apart from silver-tin and silver-aluminium combinations
which should be avoided, particularly in corrosive atmospheres.
3.3 Oxidation ageing mechanisms
Since each terminal or contact in fact consists of the joining of numerous small elementary
contact points, it is here that the corrosion mechanisms operate. There are two processes of
oxidation, both of which may take place simultaneously:
– the side surfaces of the elementary contact points are progressively attacked, reducing the
cross-section of the conducting area;
– the layer of oxide of surface resistivity σ gradually thickens
o
These two mechanisms are considered below.
3.3.1 Reduction in cross-section of the elementary contacts
IEC 1 289/97 IEC 1 290/97
Figure 4 – Elementary contact point Figure 5 – Oxidation of an
of radius a elementary contact point
On a non-oxidised contact an elementary contact point of radius a is considered (see figure 4).
The contact surface AA´ contains relatively little air, which is partly expelled by the closure of
the contact, and is sufficient only to produce slight oxidation.
By contrast, the side surfaces such as BC and B´C´ are exposed to the air and are subject to
progressive oxidation.
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As a result, the elementary contact radius gradually decreases and the contact resistance rises
(see figure 5).
In fact, the reduction in cross-section to which this type of oxidation leads is so slow that
several decades would be needed to bring about a major deterioration of the contact, even at
high temperatures. However, experience shows that this is not so in practice and that another
physical phenomenon must be involved; in fact, it is frequently found that contacts subjected to
current cycles deteriorate more quickly than those carrying a constant current. These cycles
result in differential thermal expansion at the contact area which leads to micro-movements of
the faces in contact with each other.
Because of these small relative movements, which may also be caused by electrodynamic
vibrations or mechanical shock, the contact width AA´ shown in figure 5 may be reduced to DD´
(see figure 6). The surfaces AD and D´A´ (initially protected) are now exposed to corrosion
and, when the contacts return to their initial position, the non-oxidised region in contact is very
small.
This apparently causes a considerable increase in the effect of oxidation at the point of
contact. The effects of micro-movement are thus equivalent in this case to an acceleration of
the oxidation.
This phenomenon is obviously more serious on electrically closed contacts (see 1.3.2) than on
tightened-down connection terminals.

IEC 1 291/97
Figure 6 – Influence of a relative micro-movement on the oxidation of an elementary contact
3.3.2 Growth in the layer of oxide at the contact interface
The second ageing mechanism is as follows (see figure 7).
It is assumed that, as a result of the contact movements (stress, vibration, shock) and through
diffusion through the interstices of the two surfaces (1) and (2), the oxygen has partial access
to these surfaces and creates an additional film of oxide between the two parts in contact,
which increases the surface resistivity of the layer of oxide at the interface and, consequently,
increases the contact resistance.

60943 © IEC:1998+A1:2008 – 21 –

IEC 1 292/97
Figure 7 – Oxidation of the opposite faces of a contact
If the contact surfaces were assumed to be freely exposed to the ambient air, the contact
resistance would very quickly (in a few hours) reach prohibitive values even at very low contact
temperatures. It is clear that the surfaces in contact offer each other mutual protection which
slows down the oxidation speed, the molecules of oxygen in this case only being able to diffuse
very
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