ALUMINUM AND COPPER CONDUCTORS
Author: R.M.B. Adair, B. A., B. A. I., Eng.
Indexing Terms: Electrical Connectors
The form and function are given of a hybrid type connector for use with aluminum and copper conductors. The connector employs the principle of mechanical advantage gained by utilizing interacting levers to generate a pressure connection. Reference is made to existing types and methods in use and comparisons are made between them, highlighting their relative strengths and weaknesses.
The mechanisms of electrical joint interface formation are examined and factors influencing the integrity and the longevity of such interfaces are ascertained.
Compatibility between dissimilar metals is examined in terms of relative electrochemical potential as well as physical parameters such as temperature coefficients of linear expansion. The relative merits of different surface coatings of the connector are discussed together with preparation techniques for aluminum contact surfaces.
The influence of elastic and plastic deformation of the hybrid connector type is evaluated with respect to joint interface integrity and stability, particularly in association with thermal expansion and contraction resulting from temperature changes, together with the long term effects of creep.
The factors surrounding the choice of attachment bolts are considered. The influence on axial loads generated, given a certain torque, is also examined with respect to threadform, size, lubrication and temper.
The basis and details are given of various accelerated lifetime test procedures for assessing connectors on aluminum and copper conductors. Comparisons are drawn between the tests, and those designed to produce an arduous thermal history are highlighted.
INDEX
1. INTRODUCTION
2. CYTOLOK
3. NATURE OF THE
GRIP
4. COPPER AND
ALUMINIUM
5. PERFORMANCE AND TESTING
6. APPENDIX
TECHNICAL SUBMISSION ON CYTOLOKâ
From the earliest days, terminating conductors has been a problem area. Generally, methods employed have been as follows:
(a) Mechanical “Set Screw” Termination’s:
In these, the conductor is located within the body of the connector and gripped by the action of a screw bearing down directly upon the conductor. Control should be exercised over the magnitude of the clamping force to prevent over or under clamping which can lead to damage to the conductor or hot joints respectively. A torque spanner should therefore be used to regulate the clamping force.
Potential exists here for errors.
The electrical performance of such joints tends to be somewhat inferior to other types of termination.
(b) Soldered or Sweated Termination’s:
Here the conductor is attached by means of a capillary weld. The soldering of the joint is facilitated by prior tin plating of the connector and/or conductor, both of which are usually of a copper or copper alloy, and require a flux. It is more difficult to solder aluminum.
Considerable heat is required, usually supplied by direct application of a naked flame which can often lead to conductor insulation damage and is certainly potentially dangerous in many circumstances.
The electrical performance of such joints is quite good, but there is an absolute temperature limitation due to the risk of the solder melting (at approx. 180oC ). This compromises the short circuit or fault current performance of the joint.
(c) Fusion Welded Termination’s:
In this type, a welded joint is formed between the metals of the conductor and connector.
The electrical and mechanical properties of this type of joint are good. However, the procedure involved in making a joint of this type is time consuming, requires special equipment, and is expensive.
(d) Crimping or Swaging:
This method was introduced about forty years ago.
A joint is formed by first inserting the conductor to be terminated into the cylindrical part of the connector, and then crimping the connector down onto the conductor by plastic deformation of the barrel.
Under ideal conditions, this results in a cold weld producing a joint of high integrity, with good electrical and mechanical performance.
Unfortunately special, purpose designed tools are needed to effect this, requiring accurately made dies specifically selected for a given type and size of connector.
These tools and dies are seldom, if ever, interchangeable between makes, often give trouble, can be difficult and time consuming to use and are extremely expensive.
The tools require maintenance and need periodic recalibration. If the organization using compression tools operates to a recognized Quality Assurance system, a requirement exists to apply checking and calibrating procedures to the tools and their associated dies. This is time consuming and consequently an added cost. For a given size of connector, specific dies must be used which results in potential for error. Clearly, incorrect die sizes will produce either an over or under crimped joint. Once made, this joint cannot be dismantled or re-used.
2. CYTOLOKâ
Now there is an entirely new way of terminating conductors which represents a radical departure from all the previously outlined methods. It has the genius of utter simplicity in its mode of application because no crimping machines, welding machines, blow lamps, torque spanners or other special tools are required.
A thoroughly secure joint, of high integrity, can now be produced with a CYTOLOKâconnector and a spanner, all in the time taken to tighten one bolt. This is because the CYTOLOKâ is a self crimping device utilizing the mechanical advantage obtained from two interacting levers.
The CYTOLOKâ can be dismantled if so desired, and then re-used.
The CYTOLOKâ is an aluminum alloy bodied electrical terminal connector consisting of two components, one being a hook shaped “female” component and the other a “male” component, the cylindrical head of which fits inside the hook of the female. The two components can rotate, one relative to the other, about a common axis running through the head of the male component.
A cable or conductor receiving bore exists in both components such that straight alignment of the bore is made when the connector is in the open position. Closing the connector results in a pre-determined mis-alignment of this bore which consequently grips a previously inserted conductor.
The male component exhibits symmetry about two planes, one of which is close to the horizontal. This allows the male component to be assembled to the female in two orientations. The cable receiving bore in the male component runs at an angle to its horizontal plane of symmetry and thus, when closed, two distinct levels of grip can be obtained on a conductor, depending upon the orientation of assembly. In this way, the CYTOLOK® can accommodate conductors of varying diameter. This is an important feature because conductors with a given nominal cross sectional area can possess a range of diameters resulting from differences in strand configuration. Where conductor strands are circular, spaces exist either side of the points of contact, whereas when strands are compacted, they fit together intimately leaving no inter-strand spaces, thereby reducing the overall diameter.
It can be clearly seen that the conductor within the body of the CYTOLOKâ undergoes slight deformation as the mounting bolt is tightened down, inducing a characteristic “S” shape into the cable.
The tail of the connector’s male component and that of the female component which are essentially acting as levers experience a bending stress resulting from the reaction between the connector and the conductor. This bending stress produces an ELASTIC deflection in both male and female components which, if not constrained by the mounting bolt, would spring apart.
This means that there is a considerable measure of resilience in the clamping effect of the connector.
This “spring” like grip is unique and no other type of connector possesses this feature. The principal advantages which arise out of the spring action include an ability to compensate for any creep or cold flow which might be exhibited by an aluminum conductor, an enhanced capacity to accommodate the thermal
expansion and contraction of conductors due to changes in operating temperature and the differences in the temperature co-efficients of linear expansion between unlike metals. The spring action also allows the maintenance of a firm clamping action under conditions of vibration.
The CYTOLOKâ is tested for use both with copper and aluminum conductors and for simultaneous attachment to either copper or aluminum bus-bars or terminals irrespective of the cable material.
There are two distinct considerations which have to be addressed in relation to copper and aluminum being brought into electrical and mechanical contact.
1) The differing temperature co-efficients of linear expansion shown by the two metals.
2) Their different electrochemical potentials.
i) Aluminum and its alloys expand at a rate approx. 1.4 times that of copper per unit
temperature rise. In the past, when attempts were made to install conventional copper
connectors onto aluminum conductors, difficulties were encountered because, under
elevated temperatures, the aluminum was constrained by copper surrounding it, leading
to creep. Upon subsequent cooling, the aluminum contracted away from the copper.
This process, repeated cyclically over a period of time, caused loosening of the joint and
subsequent failure.
Conversely, with an aluminum lug installed onto a copper conductor, the aluminum
surrounding the copper is free to expand under elevated temperature. The bulk stress in
the aluminum does not then rise and creep becomes less likely.
The use of a CYTOLOKâ on a copper conductor therefore not only corresponds to the
acceptable joint configuration of an aluminum connector applied to a copper conductor,
but also has the advantage of the inherent spring action of the device itself to
accommodate any movement due to differences in thermal expansion and contraction
between the two metals.
ii) Aluminum is a highly reactive metal but, like chromium, immediately forms an oxide layer on its surface when exposed to the atmosphere. This oxide layer, although thin (100-150 Å) is very stable and acts as a total and impermeable barrier between the base metal and the atmosphere. Consequently, further oxidation is prevented, the metal retaining its luster, and being generally resistant to corrosive attack.
When two dissimilar metals are brought into contact, for example aluminum and copper, and also subjected to the presence of an electrolyte, a galvanic cell usually results, which can lead to electrolytic corrosion.
The CYTOLOKâ derives protection against ordinary corrosive attack and against galvanic corrosion by virtue of the combination of the naturally occurring oxide layer just described and a further mobile coating designated EC90 which is applied to the surface of the connector before assembly.
This EC90 Process is one by which an ultra-thin transparent protective film (approx. 0.64 micron) is applied to the surface of the connector conferring upon it excellent water displacing, demulsification, and oxide formation inhibiting properties.
The solids contained in the protective film are solvent borne, and do not inhibit electrical contact.
During the process of forming an electrical connection, the brittle oxide layer which exists on the aluminum alloy surface is ruptured by the mechanical forces and the plastic deformation of the metal making up the cable connector. “Metal Bridges” then exist between metals of the conductor, connector and its mounting plate through which electrical charge will pass.
The very high, localized mechanical stresses which give rise to the formation of metal bridges prevent further oxidation or corrosion of those metal bridges by excluding the ingress of oxygen and ions, aided by the presence of the EC90 Process film which surrounds them.
The EC90 film is applied to the connector immediately after it emerges from an abrading and cleaning process, so as to limit the thickness of the oxide layer which builds up on the aluminum alloy surface. The mechanical rupturing of this layer is thus facilitated when the connection is subsequently made.
It is good design practice both for considerations of safety and reliability to over-engineer a connector to the extent that it will perform at least as well if not better than the conductor which it serves.
Performance is largely assessed on the basis of behavior in response to electrical load cycling tests. These are a form of proof test designed to artificially age a joint so that any deterioration likely to take place in the long term is induced in the short term and is thereby detectable during the course of the tests.
These tests vary somewhat in detail depending upon the National Test Standard concerned but usually entail between 500 and 2000 electrical load cycles and, in most cases, incorporate 3 short circuit overloads.
One electrical load cycle consists of a heating period followed by a cooling period. A moderately high current is passed of sufficient magnitude to cause a temperature rise in the conductor to between 120oC and 150oC which takes about 20 minutes. This is followed by the cooling period during which the temperature of the circuit under test is allowed to fall to 30oC or less.
One full cycle normally occupies a period of at least 50 to 60 minutes. A short circuit overload is rather different in nature. It is intended to reproduce the type of fault loading that can take place in unprotected circuits. Current densities in the region of 190 amps/mm2 are employed when the circuit contains copper conductors. This is sufficient to cause the temperature of the conductor to rise from ambient (at 20oC) to 250oC in one second. Large currents such as this, typically up to 45,000 amps, give rise to substantial electrodynamic forces, and test circuits require firm physical constraint to prevent disruptive movement.
An overriding consideration in assessing the merit of a joint is its electrical resistance, both in terms of its magnitude, and just as importantly, its stability.
For a given current flow, the rate at which heat is dissipated in the joint is proportional to its resistance. Low resistance therefore results in a reduced temperature of operation.
The CYTOLOKâ, by virtue of its design in being self crimping, contains more metal than conventional connectors. Not only does this provide the advantage of reduced electrical resistance, but it has a direct influence on the temperature of operation resulting from i) increased thermal inertia and ii) increased surface area. The low resistance and particularly the high thermal inertia of the CYTOLOKâ yield excellent performance under fault current conditions.
Because of the adiabatic nature of the process, although the conductor temperature may rise to 250oC in one second, the temperature of the CYTOLOKâ will rise to less than 100oC.
Under steady state conditions, the CYTOLOKâ acts as a heat sink because of its lower resistance combined with the higher surface area. Heat transfer to the surroundings is enhanced and it therefore tends to operate at a lower temperature than the conductor it serves.
All these factors, namely low resistance, high thermal inertia, high surface area, spring action, and low temperature of operation, result in a connector which forms an electrical joint of great stability and long term reliability, capable of meeting the requirements of all test standards to which it has been subjected.
Assessment of the mechanical performance of a connector is usually made by taking measurements of the tensile load which can be sustained by the joint between the connector and conductor.
Criteria for this class of connector are specified levels of tensile stress generally set at 40 N/mm2 for aluminum conductors and 60 N/mm2 for copper conductors.
CYTOLOKsâ meet or exceed these requirements to the extent that either certain conductors suffer tensile fracture leaving an undamaged portion still attached to the connector, or the maximum stress achieved exceeds the specified minimum stress by between 50% and 100%.
In conclusion, the CYTOLOKâ connector provides an elegant, unique new solution to an old problem by utilizing a simple but innovative idea, that allows ease, speed and convenience of installation, yet at the same time, yielding exemplary performance and reliability.
Robin M. B. Adair
Technical Director
CYTO (IRELAND) LIMITED
INDEX
1. INTRODUCTION AND BACKGROUND
2. CORROSION BEHAVIOUR OF ALUMINIUM AND OTHER RELEVENT
METALS
3. PRESENT REQUIREMENTS AND CODES OF PRACTICE
4. SUMMARY OF PUBLISHED RELEVANT LITERATURE
5.
ALUMINIUM/COPPER
JUNCTIONS
6.
ALTERNATIVE
COATINGS
7.
CONCLUSIONS AND
RECOMMENDATIONS
8.
REFERENCES
1. Background
and Introduction
Cyto Limited have invented, patented and manufacture a novel design of “cold-flow compensating, tool-less” Aluminum electrical connector.
This connector is an improvement over earlier designs of soldered, crimped or set screw termination’s since it is very easy to install without special skills or tooling. It is also designed to compensate for the natural cold flow that tends to reduce the contact pressure of Aluminum joints with time.
A present requirement (1,2) of some codes of practice is that Aluminum connectors “Be coated with an electrically conductive coating that will inhibit oxidation and corrosion”. Tin-plating is the most commonly used coating.
It is clear from other references (6,19) that Tin-plating of Aluminum connectors is unnecessary.
This report studies the fundamental mechanisms that govern the contacting of Aluminum with itself and other metals to form an electrically stable contact. The corrosion resistance of Aluminum is also examined.
The objective is to recommend cheaper alternatives to Tin-plating for Cyto connectors.
A secondary objective is to understand the conditions under which Aluminum connectors may be used with Copper cables.
Throughout this report, the word Aluminium is used to describe both pure Aluminum and the various families of Aluminum alloys (36,38) that have been developed specifically to be used as electrical conductors.
2. Corrosion Behavior of Aluminum and Other
Relevant Metals (3,4,5)
The propensity of a metal to react with oxygen at room temperature to form a surface oxide film may be deduced from the standard free energy formation of the metal oxide (DG in cal/mole). Hence:
Au203 39,000
Ag20 -2,586
------------------------------
Cu2O -34,980
NiO -51,700
FeO -58,400
-------------------------------
SnO2 -124,200
Cr2O3 -250,200
Al2O3 -377,600
As a simplification, the metals can be divided into three main groups.
First, the noble metals such as Gold (Au), Silver (Ag) and Platinum (Pt) which have little or no propensity to react with oxygen to form an oxide.
At the other extreme are the highly reactive metals, Aluminum (Al) and Chromium (Cr).
Why, then, do both groups tend to remain bright, shiny and metallic in appearance under normal, ambient atmospheric conditions?
The answer is that Al and Cr form, almost immediately, a very thin (c. 100-150Å), very stable oxide film which forms a total (but transparent) barrier between the metal and the atmosphere. This prevents further oxidation. The oxide on the metal is almost chemically perfect with virtually no voids or discontinuities.
Tin (Sn), like Al and Cr, achieves its corrosion resistance by forming a thin, transparent, protective oxide layer.
In the middle of the table are metals like Copper (Cu) and Iron (Fe). These too form an oxide in air but a thicker, less perfect oxide which, in time, will tarnish the surface of the metal, removing its metallic luster.
The difference between Cu and Fe oxides and the oxides on the surface of Al and Cr is that they are less chemically perfect and contain voids and imperfections through which metal or oxygen “ions” can pass to allow a thicker oxide to grow.
From this, it can be seen that, under most conditions, Aluminum is highly resistant to atmospheric corrosion, not because it is unreactive but because it is shielded from the atmosphere by a very thin but highly protective surface film.
In this respect, it is unlike copper which forms a far less protective surface film and is, hence, far more susceptible to oxidation and corrosive attack.
Gold is unlike both. It does not have a surface oxide film but gains its protection because it is naturally unreactive.
3. Present
Requirements and Codes of Practice.
The requirements of the U.S. Underwriters Laboratory (1) and the British Institution of Electrical Engineers (2) are quite specific. Both require that Aluminum connectors be protected from corrosion by more than their own oxide film.
Other references (8,14,19,31) make no mention of such requirement whilst some (6,23) state or imply that no such coating is necessary for Aluminum connectors.
It seems likely that the requirement to tin-plate Aluminum connectors is a hangover from the time when most connectors were made from copper (for which a protective coating is essential). Also, in earlier times, when most connectors were soldered, a tin surface would simplify the soldering process.
Since the tin-plating of Aluminum does no harm and since most of the earlier field experience has been gained with tin-plated junction surfaces, it seems likely that the codes of practice play safe and retain the requirement.
The UL Code (1) recognizes this by removing the requirement for plating on the mating surfaces of certain designs of connector.
Turning to the requirement for protection from corrosion during transit (as (a) above), under most conditions adequate protection can be achieved by following the accepted codes of packaging practice (34). If it is likely that the connectors might be stored in particularly corrosive conditions e.g. on board a ship, then an unplated Aluminum connector might be protected by a suitable corrosion inhibiting oil (35). These oils will not prevent the formation of a good electrical contact with a connector of the Cyto design.
In service protection from corrosive attack (( c ) above ) will not prove a problem with unplated Aluminum given dry ambient conditions, since Aluminum is well protected by the thin Aluminum oxide (Al O ) film which forms naturally on the surface.
Under severe corrosive conditions, e.g. sea water, buried connectors etc., some additional protection is necessary and a number of solutions, by painting or encapsulation, are described in the literature (10,14, 33,29). It should be emphasized that this additional protection would probably be necessary even if the connector was already plated.
From the foregoing, it is clear that the function of an Aluminium connector is not impaired by using it in the bare, unplated condition.
4. Summary
of Relevant Published Literature.
There is strong evidence from published literature that under all but severe corrosive environments, the plating of Aluminum connectors is unnecessary.
Even under severe corrosive conditions, electroplating is not really relevant since it will probably be inadequate ( 20 ) and some additional form of protection of the finished joint would be required ( 10, 14, 33).
A protective coating (additional to the existing protective oxide film) on Aluminum serves three possible functions:-
(a) In transit, protection of the connector prior to use.
(b) Improvement of the electrical contact between the connector and the conductor to which it is mated.
(c) In service protection from corrosion for the outer (non-mated) surfaces of the connector.
The most important fundamental consideration is clearly (b) since the existence of a highly insulating oxide film on the surface of bare Aluminum might inhibit its performance by increasing contact resistance. In this respect, however, Aluminum is a little different from tin which also forms a thin insulating oxide film.
In both cases (but particularly in the case of Aluminum) considerable work has been done to study the mechanism by which the brittle Aluminum oxide ruptures under mechanical stress allowing metal bridges to form, which create very low contact resistance and hence the free passage of electrical current (6,12,15,16,17,20,21,23,24,26 ).
Once metal to metal contact has been established across the bridges, further oxidation cannot take place unless the bridges are broken by creep or differential thermal expansion (if the metals are dissimilar).
The Cyto design of connector, which maintains an essentially constant pressure, eliminates the possibility of breakage due to creep.
Differential expansion problems can be eased by roughening the mating surfaces prior to contact and by the use of a mating compound, such as petroleum jelly ( 11,12,13,16,17,21,22,25,33,37 ).
It has thus been demonstrated overwhelmingly that bare, unplated Aluminum provides a viable electrical contact surface ( 6,7,8,11,12,13,15,17,18,22,25,26,28).
5. Aluminum/Copper
Junctions.
There is considerable discussion in the literature about electrical contact between aluminum and copper (3,17,20,22,28,35).
This arises because of the fear of galvanic corrosion ( 3,22 ) that can occur in damp, corrosive environments arising from the difference in electrochemical potential between Aluminum and Copper.
The position can be summarized by the following table:-
Element Normal Potential (v)
Gold ( Au ) +1.5
Silver ( Ag ) +0.8
Copper ( Cu ) +0.34
Tin ( Sn ) -0.14
Nickel ( Ni ) -0.25
Iron ( Fe ) -0.44
Chromium ( Cr ) -0.71
Aluminum ( Al ) -1.66
The more positive the normal potential then the more noble is the metal.
The greater the difference in potential between two dissimilar metals, the greater is the propensity for galvanic corrosion.
It is, however, very clear from the literature ( 36 ) that, provided sensible precautions are taken and correctly designed connectors are used, Copper and Aluminum can be jointed and can achieve good electrical contact with a good service life. More specifically, properly designed Aluminum terminal connectors are suitable for use in joining Copper cables to either Aluminum or Copper busbars or termination’s.
The Underwriters Laboratory indicate that an Aluminum terminal connector design is suitable for use, with either Aluminum or Copper junctions, by marking clearly with a stamping on the connector of: AL-CU (AL7CU or AL9CU).
One important consideration in joining Copper and Aluminum is the difference in the coefficient of thermal expansion of the two metals. A Copper connector on an Aluminum cable will force the cable to flow away from the connector during the expansion period, thus causing a loose joint during the contraction period. The resulting “hot joint” will lead to premature failure. This was responsible for the banning of the use of Copper connectors with Aluminum cables in the early days of their use in the USA, when the cause of the problem was not fully understood.
Aluminum connectors, on the other hand, are approved by building codes, the UL and other authorities in the USA for use with Copper cables. The reason is that, although the Aluminum will expand off the Copper cable, no permanent metal flow will take place.
Connectors of the Cyto design improve the position when used with Copper cables, since the design in “resilient grip” maintains a constant pressure on the cable during the expansion period.
Many other references are concerned about the rupture of “metallic bridges” that may occur during differential thermal expansion and recommend that both surfaces be roughened prior to mating, particularly when no corrosion protective coating or plating is present.
In the literature, references will be found to roughened lugs ( 25 ), bimetallic strips ( 22 ) or jointing compounds all of which may be used, in appropriate circumstances, when joining Copper to Aluminum. There are some references also to Copper “pigtails” on the end of copper cables prior to the use of Copper
termination’s ( 28 ). The use of these “pigtails” seems to occur most commonly when no Aluminum connectors are to hand or if the equipment contains built in Copper termination’s, which do not accept CU-AL rated terminal connectors.
When using Aluminum connectors of the Cyto type with Copper cables, some references recommend the “tinning” of the Copper cable and others recommend the use of a suitable jointing compound ( 37 ) such as petroleum jelly.
Under moist conditions, it is clear that galvanic corrosion may take place and this should be avoided by external protection of the joint by encapsulation, painting or lacquering, as has already been described.
6. Alternative
Coatings.
Given that tin-plating is not strictly necessary but that some form of protection is still required by the UL code, then a number of alternative surface coatings might be considered:-
(i) There is a large amount of published literature about “electronic” connectors as used, for example, with PC boards. Here a thin Gold, Cadmium, Silver or Palladium electroplate is not uncommon. (Clearly much too expensive for the Cyto application.)
Some microcrystalline waxes ( 7 ) have been developed as alternatives and these could be used with connectors of the Cyto type.
(ii) Anodizing of Aluminum ( 31 ), essentially the formation of Al O film, is an established means
of corrosion protection for Aluminum. Anodizing could be used to protect the outer surfaces
of Cyto connectors, providing that the electrical contact surfaces were screened during the
anodizing process.
(iii) Electroless nickel plating has been described in the literature for use with Aluminum ( 27, 32 )
and meets the requirements of the UL code. This would, however, provide very little cost
saving when compared to Tin electroplating.
(iv) There are a number of proprietary, corrosion inhibiting oils (35) which do not significantly inhibit electrical conduction but add a strong measure of additional corrosion protection particularly for “intransit” protection.
It must be emphasized the “bare” aluminum is naturally covered with a thin, self-healing, protective oxide film, which inhibits most corrosive attack. Additional protection is unnecessary under most circumstances.
If additional protection is to be provided then ( iv ) above is the most cost effective solution and if a coating must be provided then ( ii ) above is the most cost effective.
7. Conclusions
and Recommendations
7.1. Excellent electrical contact can be achieved with bare (unplated) Aluminum surfaces in contact with themselves or other metals.
7.2. Electrical connectors of the Cyto design (with the cold flow compensating feature) are ideally suited for use with unplated contacting surfaces.
7.3. Jointing compounds such as petroleum jelly improve the life and stability of the joint.
7.4. Roughening the metal surfaces of Aluminum and other contacting metal improves the joint.