COPPER FOR BUSBARS
CHAPTER 6: JOINTING OF COPPER BUSBARS
David Chapman
June 2012
ECI Publication No Cu0171
Available from www.leonardo-energy.org
Publication No Cu0171
Issue Date: June 2012
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Document Issue Control Sheet
Document Title: Copper for Busbars - Chapter 6: Jointing of Copper Busbars
Publication No: Cu0171
Issue: 01
Release: Public
Author(s): David Chapman
Reviewer(s): Hans De Keulenaer
Document History
Issue Date Purpose
1 June 2012 Initial release
2
3
Disclaimer
While this publication has been prepared with care, European Copper Institute and other contributors provide
no warranty with regards to the content and shall not be liable for any direct, incidental or consequential
damages that may result from the use of the information or the data contained.
Copyright© European Copper Institute.
Reproduction is authorised providing the material is unabridged and the source is acknowledged.
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CONTENTS
Jointing of Copper Busbars ..................................................................................................................................... 1
Busbar Jointing Methods .......................................................................................................................... 1
Joint Resistance ........................................................................................................................................ 2
Bolting Arrangements ............................................................................................................................ 10
Clamped Joints ....................................................................................................................................... 12
Degradation Mechanisms ...................................................................................................................... 13
Fretting ................................................................................................................................................. 13
Conclusion ............................................................................................................................................................ 14
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JOINTING OF COPPER BUSBARS
Busbar joints are of two types; linear joints required to assemble manageable lengths into the installation and
T-joints required to make tap-off connections. Joints need to be mechanically strong, resistant to
environmental effects and have a low resistance that can be maintained over the load cycle and throughout
the life of the joint.
BUSBAR JOINTING METHODS
Efficient joints in copper busbar conductors can be made very simply by bolting, clamping, riveting, soldering
or welding. Bolting and clamping are used extensively on-site. Shaped busbars may be prefabricated by using
friction stir welding.
Bolted joints are formed by overlapping the bars and bolting through the overlap area. They are compact,
reliable and versatile but have the disadvantage that holes must be drilled or punched through the conductors
causing some distortion of the current flow in the bar. Bolted joints also tend to have a less uniform contact
pressure than those made by clamping but, despite these issues, bolted joints are very common ly used and
have proven to be reliable. They can be assembled on-site without difficulty.
FIGURE 1 - A TYPICAL BOLTED JOINT
Clamped joints are formed by overlapping the bars and applying an external clamp around the overlap. S ince
there are no bolt holes, the current flow is not disturbed resulting in lower joint resistance. The extra mass at
the joint helps to reduce temperature excursions under cyclic loads. Well-designed clamps give an even
contact pressure and are easy to assemble, but take up more space than a bolted joint and are more expensive
to manufacture.
FIGURE 2 - A SIMPLE CLAMPED JOINT
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Riveted joints are similar to bolted joints. They can be efficient if well made, it is difficult to control the
contact pressure. They cannot easily be dismantled or tightened in service and they are difficult to install.
FIGURE 3 - A RIVETED JOINT
Soldered or brazed joints are rarely used for busbars unless they are reinforced with bolts or clamps since
heating under short-circuit conditions can make them both mechanically and electrically unsound.
FIGURE 4 - A SOLDERED JOINT
Welded joints are made by butting the ends of the bars and welding. They are compact and have the
advantage that the current carrying capacity is unimpaired, as the joint is effectively a continuous copper
conductor. However, it may not be safe or desirable to make welded joints in situ. Welding of copper is
discussed in CDA Publication 98, Cost-Effective Manufacturing: Joining of Copper and Copper Alloys.
FIGURE 5 - A WELDED JOINT
The following sections apply to bolted and clamped joints.
JOINT RESISTANCE
In principal, a clamped or bolted joint is made by bringing together two flat surfaces under controlled (and
maintained) pressure, as shown in Figure 6.
FIGURE 6 – AN OVERLAPPED JOINT
The resistance of a joint is mainly dependent on two factors:
The streamline effect or spreading resistance, R
s, due to the diversion of the current flow through the
joint
The contact resistance or interface resistance of the joint, R
i
The total joint resistance, R
j, is given by:
This applies specifically to direct current applications. Where alternating currents are flowing, the changes in
resistance due to skin and proximity effects in the joint zone must also be taken into account.
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STREAMLINE EFFECT
When current flows through a joint formed by two overlapping conductors, the lines of current flow are
distorted and the effective resistance of the joint is increased since current flows only through a portion of the
material.
Provided that the width of both bars is the same, the streamline effect is dependent only on the ratio of the
length of the overlap to the thickness of the bars and not on the width. This is shown in Figure 7.
FIGURE 7 - STREAMLINE EFFECT IN OVERLAPPED JOINTS
The current density in the direction perpendicular to the bar, i.e. as current transfers from one bar to the
other, is highly non-uniform and is concentrated around the edges.
The resistance ratio e in Figure 7 is the ratio of the resistance of a joint due to streamline effect R
s, to the
resistance of an equal length of single conductor R
b
, i.e.
where:
a is the width of bar, mm
b is the thickness of bar, mm
l is the length of overlap, mm
is the resistivity of the conductor, mm
R
s
is the resistance of overlap section in to which contact resistance must be added)
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Hence:
From the graph it can be seen that the streamline effect falls very rapidly for l/b ratios up to two and then very
much more slowly for values up to ten. This means that in most cases the streamline effect has a limited
effect as the overlap is often much greater than five times the thickness in order to allow space for bolting or
clamping. There is no advantage in allowing very long overlaps; it is only necessary to allow enough space to
accommodate sufficient bolts to achieve the required contact pressure.
In the case of bolted joints, the bolt holes also reduce efficiency due to the s treamline effect. The resistance
ratio of a bolted overlap section can be estimated by:
where
d is the diameter of the holes
n is the number of holes across the width of the bars.
It follows that holes should be placed in-line along the length of the joint as shown in Figure 8; offsetting the
holes increases the resistance by increasing the disturbance of the current flow. In Figure 8a, the value of n is
2 while in Figure 8b the value of n is 4.
FIGURE 8 – BOLT PLACEMENT IN OVERLAPPED JOINTS
It has been found that the distortion effect in the tap-off of a T-joint is about the same as that in a straight
joint. Note that the current flow in the straight bar is disturbed by the presence of bolt holes.
It has been shown that the current distortion is reduced if the ends of the bars are angled at less than 45
degrees as shown in Figure 9. The initial joint resistance is reduced by 15%. Because the current flow is more
uniform, the development of localized hot spots is reduced, leading to a factor of 1.3 to 1.5 reduction in the
rate of increase in resistance under current cycling.
FIGURE 9 – OVERLAP JOINT BETWEEN BARS WITH ANGLED ENDS
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CONTACT RESISTANCE
There are two main factors that affect the actual interface resistance of the surfaces.
a. The condition of the surfaces
b. The total applied pressure.
CONDITION OF CONTACT SURFACES
In practice, an electrical contact between the solids is formed only at discrete areas within the contact
interface and these areas (known as ‘a‐spots’) are the only current conducting paths. The a -spots typically
occupy an area of the order of 1% of the overlap area.
Obviously, the larger the number of a-spots, the more uniform the current distribution across the joint area
will be. This can be encouraged by ensuring that the surfaces of the conductors are flat and roughened (which
removes the oxide layer and produces a large number of asperities) immediately before assembly. As the
contact pressure is increased the higher peaks make contact, disrupt any remaining surface oxide and form
metal to metal contact.
In some areas an oxide film may remain. Copper oxide films on copper form relatively slowly and are
semiconducting because copper ions diffuse into the oxide layer. When copper oxide films are compressed
between two copper surfaces, diffusion can take place in both directions so conduction takes place in both
directions. This is very different from aluminium; where the oxide is a very good insulator and forms within
microseconds of exposure to air.
Since the area of each a-spot contact is small, the current density is high leading to higher voltage drop and
local heating. In a well-made joint this heat is quickly dissipated into the mass of the conductor and the
temperature of the interface will be only slightly above that of the bulk material. However, if the contact
pressure is too low and the joint has deteriorated, local over -heating may be enough to induce basic
metallurgical changes including softening and melting of the material at the a-spot. At first sight this may
appear to be advantageous, however, as the joint cools the material contracts and fractures and is
subsequently liable to oxidise.
Since elevated temperature is the first symptom of joint failure, maintenance procedures should be
established to monitor the temperature of joints with respect to that of nearby bar using thermal imaging. If,
under similar load conditions, the differential temperature increases it may be a sign of early joint
degradation. As a first step, more intensive monitoring should be undertaken and if the trend continues,
remedial action taken.
It is not normally recommended that the surfaces of copper -to-copper joints are plated unless required by
environmental considerations. In fact, plating may reduce the stability of the joint because, as soft materials,
the plating may flow at elevated temperatures leading to reduced contact pressure.
However, to ensure a long service life a contact aid compound is recommended to fill the voids in the contact
area and prevent oxidation or corrosion. Many proprietary compounds are available or, if none are available,
petroleum jelly or, for higher temperatures, silicone vacuum grease may be used.
EFFECT OF PRESSURE ON CONTACT RESISTANCE
Joint resistance normally decreases with an increase in the size and number of bolts used. Bolt sizes usually
vary from M6 to M20 with either four or six bolts being used. The appropriate torque for each bolt size
depends on the bolt material and the maximum operating temperature expected.
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Contact resistance falls rapidly with increasing pressure, as shown in Figure 10 but above a pressure of about
30 N/mm² there is little further improvement. In most cases it is not advisable to use contact pressures of less
than 7 N/mm², with pressures above 10 N/mm² being preferred.
The contact resistance for a joint of a particular overlap area is obtained from Figure 10 by dividing the contact
resistance for 1mm
2
by the overlap area in mm
2
.
FIGURE 10 – THE EFFECT OF PRESSURE ON THE CONTACT RESISTANCE OF A JOINT
Contact pressure for both bolted and clamped joints is normally applied by tensioning one or more bolts. For
bolted joints, the pressure is applied around the bolt holes so using more bolts will result in a more even
pressure distribution. Large, thick, washers can be used to spread the load. For clamped joints, the load
transferred from the bolts, which are outside the width of the conducto rs, depends on the rigidity of the
clamps. Where the clamps are narrow, the pressure distribution provided by clamps can be quite uniform, but
for wider conductors the very rigid clamps required may be impractically large.
In everyday practice, contact pressure is impossible to measure and has to be inferred from the torque applied
to the bolts from the following equation:
where:
T is the tightening torque (Nm)
K is a constant, often referred to as the ‘nut factor’ – see Table 1
F is the force (kN)
D is the nominal bolt diameter (mm) - see Table 3.
The ‘nut factor’ depends on a number of factors including the coefficient of friction, the surface finish and
state of lubrication of the threads and other bearing surfaces. Table 1 gives typical nut factors for different
states of lubrication.
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TABLE 1 - NUT FACTORS FOR DIFFERENT STATES OF LUBRICATION
Bolt lubrication Nut factor
Dry 0.20 – 0.22
Contact aid compound 0.19 – 0.21
Boundary lubricant (Mo
2S) 0.15 – 0.16
It is important to control the rate of applying the torque as well as the final value ; bolts should be gradually
tightened in rotation.
The correct tightening torque must be carefully determined to provide sufficient initial contact pressure at
ambient temperature while not exceeding the proof or yield stress of the bolt material over the working
temperature range of the joint. Differential expansion between the bolt and bar materials results in an
increase or decrease in bolt tension (and therefore contact pressure) as temperature changes. Galvanised
steel bolts are often used with copper busbars but copper alloy bolts, e.g. aluminium b ronze (CW307G), are
preferred because their coefficients of expansion closely match that of copper resulting in a more stable
contact pressure. Copper alloy bolts also have the advantage that the possibility of dissimilar metal corrosion
is avoided and are also to be preferred where high magnetic fields are expected. Because these alloys do not
have an easily discernible yield stress, however, care has to be taken not to exceed the correct tightening
torque and the bolt stress over the working temperature range should not exceed 95% of proof stress.
Because of the strength of copper, deformation of the conductor under the pressure of the joint is not
normally a consideration.
Table 2 shows the proof stress and coefficient of thermal expansion of some typical bolt materials compared
to copper. It is clear from this table that the choice of bolt material will determine the thermal stability of the
joint.
TABLE 2 – PROOF STRENGTH AND COEFFICIENT OF THERMAL EXPANSION FOR COPPER AND TYPICAL BOLT MATERIALS
Material Proof strength
MPa
Coefficient of expansion
Per degree C
Copper (reference) Fully annealed - 50
Full temper - 340
16.5 x 10
-6
High tensile steel 700 11.1 x 10
-6
Stainless steel
316
414 15.9 x 10
-6
Aluminium bronze
CW307G
400 16.2 x 10
-6
Stainless steel
304
207 17.2·x 10
-6
Silicon Bronze
C651000
365 17.8·x 10
-6
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The increase in force is given by:
where:
a is the coefficient of expansion of the busbar conductor
b is the coefficient of expansion of the bolt
A
b is the bolt cross-sectional area
E
b is the elastic modulus of the bolt
E
a is the elastic modulus of the busbar
t is the thickness of the washer
a is the thickness of the busbar
A
w
is the apparent area under the washer
A
a is the apparent area of the joint overlap.
The change in bolt stress is proportional to so for a joint made with high tensile steel bolt the
tension will increase considerably ( = 5.5 x 10
-6
) while if the joint were made with CW307G bolts, the
tension will reduce only slightly ( = -0.3 x 10
-6
).
In any case, the joint must be designed so that the maximum tension in the bolts, at any temperature within
the working range, must be less than 95% of yield stress to avoid the risk of plastic deformation which would
ultimately lead to loosening of the joint and failure.
The stress in the bolt is calculated using the Tensile Stress area (see Table 3), not the nominal area.
TABLE 3 – TYPICAL THREAD CHARACTERISTICS
Size
Designation
Nominal
(Major)
Diameter
D
n
Nominal
Shank Area,
A
n
Pitch
(mm per
thread), p
Pitch
Diameter
d
p
Minor
Diameter
Area
A
s
Tensile Stress
Area
A
ts
M6 6.00 28.274 1.000 5.3505 17.894 20.123
M8 8.00 50.265 1.250 7.1881 32.841 36.609
M10 10.00 78.540 1.500 9.0257 52.292 57.990
M12 12.00 113.10 1.750 10.863 76.247 84.267
M14 14.00 153.94 2.000 12.701 104.71 115.44
M16 16.00 201.06 2.000 14.701 144.12 156.67
M20 20.00 314.16 2.500 18.376 225.19 244.79
M22 22.00 380.13 2.500 20.376 281.53 303.40
M24 24.00 452.39 3.000 22.051 324.27 352.50
M27 27.00 572.56 3.000 25.051 427.09 459.41
M30 30.00 706.86 3.500 27.727 518.99 560.59
M33 33.00 855.30 3.500 30.727 647.19 693.55
M36 36.00 1017.9 4.000 33.402 759.28 816.72
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If the design requirement is such that high tensile steel bolts must be used, the incremental force, F
supp
,may be
so high as to exceed 95% of the proof stress of the bolts. In these cases, disc-spring, or Belleville, washers
must be used. The height and the spring rate of the washer are selected to reduce the value of F
supp
according
to the following equation:
where:
h is the overall height of the disk-spring washer
K is the spring rate of the disk -spring washer.
In practice, the joint would be assembled normally with the required torque for the required contact pressure.
In service, as the joint temperature rises, the spring is compressed, limiting the increase in bolt tension to a
safe value.
FIGURE 11 - POSSIBLE BOLTING TECHNIQUES FOR COPPER BUSBARS
Changing the design of a bolted joint, for example by introducing a longitudinal slot (see Figure 12), can reduce
the contact resistance by 30 to 40%. The reduction in resistance is attributed to an improvement in the
uniformity of contact pressure in each ‘leg’ of the joint leading to increased contact area.
FIGURE 12 – JOINT WITH A LONGITUDINAL SLOT
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BOLTING ARRANGEMENTS
Although the required bolting arrangements should always be calculated for the circumstances of the
installation, many sources give recommendations. Those given in Table 4 have been used for many years and
are given here as a rough guide.
The recommended torque settings may be used for high-tensile steel (8.8) or aluminium bronze (CW307G,
formerly C104) fasteners with unlubricated threads of normal surface finish.
TABLE 4 - TYPICAL BUSBAR BOLTING ARRANGEMENTS (SINGLE FACE OVERLAP)
Bar width
mm
Joint
overlap
mm
Joint area
mm²
Number of
bolts
Metric bolt
(coarse
thread)
Bolt
torque
Nm
Hole size
mm
Washer
diameter
mm
Washer
thickness
mm
16 32 512 2 M6 7.2 7 14 1.8
20 40 800 2 M6 7.2 7 14 1.8
25 60 1500 2 M8 17 10 21 2.0
30 60 1800 2 M8 17 10 21 2.0
40 70 2800 2 M10 28 11.5 24 2.2
50 70 3500 2 M12 45 14 28 2.7
60 60 3600 4 M10 28 11.5 24 2.2
80 80 6400 4 M12 45 14 28 2.7
100 100 10000 5 M12 45 15 28 2.7
120 120 14400 5 M12 45 15 28 2.7
160 160 25600 6 M16 91 20 28 2.7
200 200 40000 8 M16 91 20 28 2.7
J OINT EFFICIENCY
The efficiency of a joint may be measured in terms of the ratio of the resistance of the portion of the
conductor comprising the joint to that of an equal length of straight conductor. It is possible to make joints
with an efficiency of greater than 100% - i.e. the resistance of the joint is lower than that of an equivalent
section of bar.
In terms of a complete busbar system, the proportion affected by joints is relatively small so that any
inefficiency of the joints has only a small impact on the overa ll performance. However, joint inefficiency is
important in two respects:
A joint with an efficiency of less than 100%, having a higher resistance, will run at a higher working
temperature and experience greater temperature excursions than the normal bar. This could have an
effect on the longevity of the joint and require more frequent maintenance
In switchgear cabinets there will be many joints close together ; less efficient joints will lead to excess
heating and higher voltage drops.
The resistance of a joint, as already mentioned, is made up of two parts, one due to the distortion of lines of
current flow and the other to contact resistance. The resistance due to the streamline effect at an overlap ped
joint is given by:
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where
e is the resistance ratio obtained from Figure 7
a is the width of bar, mm
b is the thickness of bar, mm
l is the length of overlap, mm
is the resistivity of the conductor, mm (17.24 for 100% IACS copper)
d is the diameter of the bolt holes, mm
n is the number of holes across the width of the bars. For clamped joints, the value of n is zero.
The contact resistance, R
i
, of the joint is:
where Y is the contact resistance of a unit area, obtained from Figure 10.
The total joint resistance, R
j
, is:
Since the resistance, R
b,
of an equal length of straight conductor is given by:
the efficiency of the joint is:
From this equation it is apparent that the most important factor is the reduction in cross section due to the
bolt holes, i.e. the term nd.
Taking the parameters for a 50mm wide busbar from Table 4,
The contact force, F, is given by (noting that there are two 12 mm bolts):
The area of the joint is 3500 mm
2
, so the pressure is
P= 10.7 N/mm
2
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From Figure 10, a Y value of 3000 is obtained.
For 10mm thick bar, the overlap to thickness ratio is 7 so that, from Figure 7, e = 0.55.
Substituting,
This joint has a resistance of 1.12 times that of a 70mm length of 50 mm x 10 mm copper bar, i.e. equivalent
to 78.4 mm of bar. The joint temperature will be slightly higher than that of the surrounding bar.
If the joint were redesigned with an overlap of 90 mm, using three in-line bolts at the same torque, the joint
efficiency becomes
The area of the joint is 4500 mm
2
, so the pressure is
P= 12.5 N/mm
2
From Figure 10, a Y value of 2600 is obtained.
For 10mm thick bar, the overlap to thickness ratio is 9 so that, from Figure 7, e = 0.55.
Substituting,
In this case, the joint has a resistance of 0.91 times that of a 90mm length of 50 mm x 10 mm copper bar , i.e.
equivalent to 82 mm of the bar. This joint will run at a slightly lower temperature that the surrounding bar.
CLAMPED JOINTS
The design criteria for bolted joints apply in principal also to clamped joints. However, some aspects require
particular attention:
The clamping plates must be designed to be the rigid enough to transfer the pressure without flexing.
Often, ribbed castings are used for this purpose.
The bolts which provide the joint pressure are at the periphery of the joint and will run at a
temperature somewhat below that of the bar. In some ‘wrap around’ lamp designs, the bolts will also
be physically shorter than the thickness of the stacked bars. The bolts will therefore expand less, and
joint pressure may rise excessively.
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DEGRADATION MECHANISMS
The deterioration of a connector proceeds slowly at a rate determined by the nature of different processes
operating in the contact zone and in the environment. This initial stage persists for a long time without
causing any noticeable changes because it is an intrinsic property of clusters of a‐spots that their overall
constriction resistance is not sensitive to small changes in their size. However, when the contact resistance
increases sufficiently to raise the local temperature, a self‐accelerating deterioration resulting f rom the
interaction of thermal, chemical, mechanical and electrical processes will be triggered, and the contact
resistance will rise abruptly. Hence, no deterioration will be noticeable until the final stages of the connector
life.
OXIDATION
Oxidation of the metal–metal contacts within the contact interface is widely accepted as the most serious
degradation mechanism occurring in mechanical connectors. Copper is not very active chemically and oxidises
very slowly in air at ordinary temperatures. As mentioned earlier (see ‘Condition of contact surfaces’),
cleaning and roughening the joint surfaces prior to assembly and the use of a contact aid will prevent
oxidation.
CORROSION
Corrosion is a chemical or electrochemical reaction between a metallic component and the surrounding
environment. It begins at an exposed metal surface with the formation of a corrosion product layer and
continues as long as reactants can diffuse through the layer and sustain the reaction. The composition and
characteristics of the corrosion product layer can significantly influence the corrosion rate.
Busbars are potentially affected by atmospheric, localized, crevice, pitting and galvanic corrosion. The most
important factor in all these corrosion mechanisms is the presence of water. In the presence of a
sulphur‐bearing atmosphere, tarnishing of the copper surface occurs because of sulphide formation from
hydrogen sulphide in the atmosphere. The growth of tarnished film is strongly dependent on the humidity,
which can reduce it if a low sulphide concentration prevails or increase it if sulfide concentration is high.
FRETTING
Fretting is the accelerated surface damage occurring at the interface of contacting materials subjected to small
oscillatory movements. Experimental evidence shows that amplitudes of <100 nm are sufficient to produce
fretting.
There is still no complete unanimity on the mechanisms of fretting, specifically with regard to the relative
importance of the processes involved. Nevertheless, based on the existing knowledge of the phenomenon, it
can be safely assumed that the following processes are present: (1) disruption of oxide film on the surface by
the mechanical action exposes clean and strained metal which will react with the environment and rapidly
oxidize, (2) the removal of material from the surfaces by adhesion wear, delamination or by shearing the
microwelds formed between the asperities of the contacting surfaces when the contact was made, (3)
oxidation of the wear debris and formation of hard abrasive particles that will continue to damage the surfaces
by plowing, (4) formation of a thick insulating layer of oxides and wear debris (a third body) between the
contacting surfaces.
The oscillatory movement of the contacting members can be produced by mechanical vibrations, differential
thermal expansion, load relaxation, and by junction heating as the load is cycled. Because fretting is
concerned with slip amplitudes not greater than 125 µm the movement it is ineffective in clearing away the
wear debris and accumulated oxides, and a highly localized, thick insulating layer is formed in the contact
zone, leading to a dramatic increase in contact resistance and, subsequently, to virtual open circuits.
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CREEP AND STRESS RELAXATION
Creep, or cold flow, occurs when metal is subjected to a constant external force over a period of tim e. The
rate of creep depends on stress and temperature and is higher for aluminum than for copper. Stress
relaxation also depends on time, temperature, and stress but, unlike creep, is not accompanied by dimensional
changes. It occurs at high stress levels and is evidenced by a reduction in the contact pressure due to changes
in metallurgical structure. The change from elastic to plastic strain has the effect of significantly reducing the
residual contact pressure in the joints, resulting in increased contact resistance, possibly to the point of failure.
THERMAL EXPANSION
The effect of temperature variation on contact pressure has already been discussed. Longitudinal expansion is
also important since it can lead to slip in the joint followed by loosening. It is important that long bars are
provided with a flexible element so that movement can take place elsewhere.
CONCLUSION
The quality of busbar joints is crucial to the long term reliability of a busbar system. It is important to take care
over the choice of joint design, the tightening torques, bolt types and the effect of temperature to ensure
reliability. In-service maintenance should include, ideally, thermal imaging of joints so that any problems can
be found before failure occurs.
