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Basic Factors:
The basic factors which influence the design
and performance of pressure wire connec-
tions are as follows:
1. Creep
2. Surface Oxide
3. Corrosion
A fourth factor, known as thermal effects, is
also a consideration, but due to the technical
nature and length of the topic, it will not be
discussed in this publication.
At the outset it should be pointed out that
these factors give rise to much more difficult
problems in connections involving aluminum
conductors than those encountered in cop-
per to copper connections.
Creep is the cold flow of the metal under
pressure, and it continues until the pressure
reduces to a value at which any further creep
is negligible. Creep properties depend on the
particular metal or alloy and on its hardness;
alloys having less creep than pure metals,
and harder metals have less creep than soft
metals. In a typical connection, the conduc-
tors are generally of pure metal and often of
soft temper and therefore, subject to consid-
erable creep. In addition, the condition is
further exaggerated when aluminum is the
conductor as compared to copper, since its
creep rate is many times that of copper.
Effect of Creep:
Figure 2 shows typical
curves of total contact resistance plotted
against total contact force. Curve A shows
how the contact resistance continually
decreases with increasing contact force.
When the full contact force F1 is reached, the
contact resistance reaches the low value of
R1. In general, the full tightening force on a
connector greatly exceeds the maximum
force for which there is no appreciable creep.
Therefore, the force will gradually settle down
to a value after which there will be no further
significant creep. Fortunately, however, the
resistance does not climb back up along
curve A, the tightening curve, but instead it
follows a new curve B, the relaxing curve,
along which the resistance changes very little,
until the force relaxes to a value such as F2.
Admittedly, the point of "no appreciable
creep" is difficult to define. For pure metals,
especially in the soft state, there is always
some creep, even at very low pressures at
room temperature. However, we do know
that the pressure required to produce the
same creep rate is several times greater for
copper than for aluminum. Thus, to permit
the same contact force F2 for aluminum and
copper, the contact area A required for alu-
minum can be expected to be considerably
greater than that required for copper. This
explains why the contact areas for connec-
tors for aluminum must be considerably
greater than for copper, and why many light
duty connectors for copper are entirely inad-
equate for aluminum, even when specially
plated and when recommended compounds
are used on the contact surfaces.
Relaxation of pressure due to
creep, or for any other reason, would be a
much more difficult factor in a pressure con-
nection were it not for the relationship of
contact pressure to contact resistance on the
relaxation curve as shown in Figure 2. It is
frequently observed that some time after the
bolts of a clamp type connector are tight-
ened, the bolt tensions are relaxed apprecia-
bly. The question arises as to whether it is
necessary to retighten the bolts to the
original torque value. In a properly designed
connector, retightening is unnecessary since
the contact resistance should increase very
little due to the relaxation of pressure, as
shown by the relaxation curve of Figure 2.
This fact is largely responsible for the
successful operation of a compression con-
nector. The application of the compression
tool applies very high pressure, establishing
very low contact resistance. The removal of
the compression tool releases a very large
proportion of this pressure, and creep further
relaxes this pressure. Fortunately, the con-
tact resistance increases very little due to
this pressure relaxation.
Contact Force:
The previous analysis shows
that the total contact force largely deter-
mines the contact resistance. Thus, to
achieve the desired low value of contact
resistance, the proper size and number of
bolts in a clamp type connector must be
supplied, and the compression tool must
apply the proper force to a compression con-
nector. In addition, the connector must be
designed with sufficient structural strength,
contact area, and resilience, to assure that
the contact force cannot relax beyond the
point where contact resistance begins to rise
appreciably, as shown in Figure 2.
The contact of pure metallic surfaces cannot
be assured in practical connections. Surface
contamination must be expected, especially
surface oxidation. These surface films are
insulators as far as contact resistance is con-
cerned, and they must be broken to achieve
metal to metal contact to make an adequate
electrical connection. The difficulty of break-
ing the film depends on the nature of the film,
its thickness, and the metal on which it is
Copper oxide is generally broken down by
reasonably low values of contact pressure.
Unless the copper is badly oxidized, good
contact can be obtained with very little or no
Silver oxide is even more easily broken down
by the contact pressure; and since silver
oxide forms less readily at elevated tempera-
tures, silver contact surfaces are preferred
over copper when used for higher tempera-
tures. For this reason, it is considered good
practice to silver plate copper contact
surfaces that must operate at temperatures
over 200° C.
On the other hand, aluminum oxide is a hard,
tenacious, high resistance film that forms
very rapidly on the surface of aluminum
exposed to air. In fact, it is the toughness of
this film that gives aluminum its good corro-
sion resistance. The oxide film that forms
after more than a few hours is too thick and
tough to permit a low resistance contact
without cleaning. The aluminum oxide film is
transparent so that even the bright and clean
appearance of an aluminum connector is no
assurance that the low contact resistance
can be attained without cleaning.
In addition to the necessity for cleaning the
oxide from aluminum, the surface should be
covered with a good connector compound to
prevent the oxide from reforming. Common
practice is to clean the surface with a wire
brush or emery cloth. The compound should
be applied immediately after cleaning, or the
compound should be put on first and the
surface scraped through the compound.
Present practice is to scratch brush dry and
to apply the compound immediately there-
after. This allows a more thorough job of
cleaning the conductor.
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