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Ilsco
Engineering Handbook for Electrical Connectors
A brief summary of
comparative properties of metals suitable for
current carrying applications and required features
of the connector follows.
Silver, Copper, Gold,
Aluminum and Magnesium have comparable properties,
for current carrying applications and required
features of the connector. The above mentioned pure
metals all contain relative electrical conductivity
which can be defined by Percentage of Volume
Conductivity. Silver has 108.3% Volume Conductivity.
Copper contains 100% Volume Conductivity. Gold
assumes 73.4% Volume Conductivity. Aluminum has
64.9% Volume Conductivity. Magnesium contains 38.0%
Volume Conductivity.
The commercial use of
copper and aluminum for electrical applications are
obvious, in terms of economic justifications. The
accelerated use of aluminum becomes even more
obvious when realized that it requires twice the
amount of copper by weight, to carry a specified
amount of current. At the unit price per pound of
prime metal it is readily seen why aluminum is
increasing in electrical application.
As the next logical
step, an examination of available alloys and forms
of aluminum should be made to determine the optimum
choice for the manufacture of electrical connectors.
Aluminum is available
in a variety of suitable alloys and forms which
allow for the optimum choice for the manufacture of
electrical connectors. The following Aluminum Alloys
are defined by Form, Typical Yield Strength, Minimum
Yield Strength, Elongation, and Percent of
Conductivity. 6061-T6, in the Extrusion Form, has a
Typical Yield Strength of 40-45,000 psi, Minimum
Yield Strength of 35,000 psi. Elongation of 12, and
40% Conductivity. 6063-T6, in the Extrusion Form,
has a Typical Yield Strength 22,000 psi, Elongation
of 3.5, and 39% Conductivity. 356-T6, in the Sand
Cast Form, has a Typical Yield Strength of 24,000
psi, Minimum Yield Strength of 24,000 psi,
Elongation of 3.5, and 39% Conductivity. AXS 679
(380A), in the Die Cast Form, has a Typical Yield
Strength of 21,000 psi, Minimum Yield Strength
21,000 psi, Elongation of 3, and 25% Conductivity.
During the initial
analysis of these materials, reference was made to
the experience over many of the effect of stress on
tensioned overhead conductors. It is well documented
that in order to assure mechanical stability of
these conductors, over long periods of time, it is
necessary to design the line so that the maximum
stress will not exceed 50% of the conductors rated
breaking strength. In the case of aluminum
conductors the alloy most commonly used is EC-H19
having a yield strength, will result in the
conductor remaining mechanically stable throughout
its life.
The method of testing
used to confirm this premise is noted later.
Additional factors of
design were recognized as necessary to achieve a
stable and reliable connector.
Requirements of
aluminum connectors for use with aluminum or copper
conductors.
1. Adequate strength of
the connector to prevent creep loss in the
connection from exceeding the creep loss of the
conductor.
2. Strong enough
clamping force (torque) to keep the connector
operating temperature at a level below the
operating temperature of the maximum size
conductor.
3. High enough
conductivity to provide adequate efficiency
(minimum of 40%). |
To illustrate the
significance of these requirements, the
Terminal Life Cycle Table presents an examination of terminal
temperature in relation to clamping force. The
horizontal dotted line indicates the temperature of
the conductor at maximum rise. The tightening
characteristic curve shows a lowering of terminal
temperature rise with an increased clamping force.
Terminal temperature, measured in degrees
centigrade, is used in our illustration as a measure
of connector resistance. As the tightening
characteristic curve approaches the dotted conductor
temperature line at the same current value, the
connector and conductor resistance approximate each
other.
The curves labeled
Relaxation Characteristic, merely indicate the
anticipated progression a connector would follow to
failure, once clamping force has been reduced to a
level where terminal resistance can no longer be
maintained at a low level. Point A on each curve
represents that point where resistance is
sufficiently high to cause an elevated operating
temperature of the connector which will then
progress to ultimate connector failure.
Proceeding with the
assumption that stress limitation is a most critical
factor in connector design it was then necessary to
select and test the alloy materials which evidence
the most favorable conductivity/yield
strength/economic factors. The initial selection of
aluminum alloy 6061-T6 has long since been
determined to be the best available material from
which to fabricate connectors for use with both
aluminum and copper conductors.
Before proceeding further a
definition of terms used, to assure understanding of
the basis for the conclusions reached later in the
paper, are listed.
Physical Property of Aluminum
Tensile Strength
The maximum tensile load which a material is capable
of withstanding under gradually and uniformly
applied loading, divided by the original
cross-sectional area in the minimum plane
perpendicular to the direction of loading. Commonly
the term is taken to mean the same as "ultimate
tensile strength" or the less accurate "breaking
strength".
Yield Strength
The stress at which material exhibits a specified
permanent set. The value of set used for aluminum
and its alloys is 0.002 inch per inch or 0.2%. For
the aluminum alloys the yield strength in tension
and compression are approximately the same.
Elongation
The increase in distance between two gage marks that
results from stressing the specimen in tension to
fracture.
The distance between
gage mark measurements start at 2.000 before the
applied load. When distance between gage marks
measures 2.004, level of tensile load indicated on
gage is recorded as material specimen's yield
strength.
To record elongation,
tensile loading is continually applied until
specimen fractures, at which time two pieces are
mated and distance between gage marks accurately
measured. The resultant dimension divided by the
original increment provides the value expressed in
%, of the materials ability to stretch under load,
or its elongation.
Elastic Limit
The stress value below which no permanent set or
permanent deformation takes place; the highest
stress which will permit return to original shape
upon removal of force causing the stress.
Elasticity
The ability of a material to return to its original
shape and size upon removal of a load below the
elastic limit.
Creep
A precise unit of measure disclosing the increase in
dimension of a unit specimen having a specified area
= A, an applied force = W with resultant stress =
W/A. The initial increment of measurement L, is
effected by three factors resulting in amount of
increase or creep, stress, time and temperature.
To determine and
express the creep rate for a given specimen in terms
of inches of creep, per inches of original gage
length, per hour, the factors of stress and
temperature must be maintained constant. A change in
either or both, will result in a change in the creep
rate.
Cold Flow
As compared to Creep, cold flow has no units of
measure. The best description of cold flow relating
to application within the electrical industry, is an
excessively high rate of creep i.e. normal creep
rate static load condition would be expressed in a
fraction of an inch per inch of length. Cold flow,
conversely if possible to measure it in definable
terms, would be expressed in terms of inches of
movement per inch of length. Cold flow then can be
expressed as movement of appreciable magnitude
occurring at a stress level in a very short length
of time at an ambient temperature. Neither time or
temperature are critical in assessing the effecting
force of cold flow.
It is significant to
realize that it is an absolute necessity to have
cold flow of the conductor within a bolted connector
to develop the desired low resistance contact,
required for electrical/mechanical stability of the
connection. So is it necessary to have cold flow of
both the conductor and connector in the making of a
compression connection. In these two instances a
mechanical union of the two components is made by
means of an externally applied force to assure both
electrical and mechanical reliability. In the case
of a soldered or welded connection this component
union is made metallurgically.
Creep Loss
The unit of measurement, expressed in % of the
initially applied mechanical force to a connection,
divided by the resultant measured force after the
unit has been subjected to controlled loadings of
temperature and time. This value is relatively easy
to determine for bolted connectors by accurately
measuring applied torque before and after applying
control factors of time and temperature. The
resultant loss of torque is the creep loss
experienced within the connection assembly.
It is difficult and
inaccurate to project anticipated creep loss within
a compression assembly since there is no way of
effecting a measurement of the initially applied
force and the resultant force remaining on the
connection after the mechanical or hydraulic
compression tool is removed from the connector.
The maximum desired
creep loss in a connector/conductor assembly stated,
is that portion of the total creep component
represented by the creep component of the conductor
alone.
This brings us to the
threshold of the discussion of design parameters
used by ILSCO in the manufacturer of aluminum
connectors.
The Tolerable Creep
Limits for Aluminum Connectors can be defined in
terms of Stress Area, Initial psi and result in a
percentage of Creep Loss after a one-hour heat run
in an oven at 212° F. Aluminum 6061-T6 with a Stress
Area of .332, has initially applied mechanical force
of 11,500 psi and after a one-hour heat run in an
oven at 212° F, has a 13.3% Creep Loss. Another
section with a Stress Area of .249, at 15,400 psi,
reaches 13.3% Creep Loss. The section with Stress
Area of .166, at 23,100 psi, reaches 13.3% Creep
Loss. The section with Stress Area of .132, at
29,100 psi, reaches 25% Creep Loss.
To best understand the importance of the function of
creep components in a connection assembly, recall
the definition of cold flow wherein it is stated
that it is necessary to the formulation of an
electrically/mechanically stable connection to have
cold flow of the conductor. Since cold flow is by
definition a high rate of creep and since creep
occurs only where constant stress is applied; the
connection will undergo creep movement as a result
of load cycling. It is mandatory that the connector
be so designed that all creep within the assembly
occur in the conductor.
The determination of
tolerable creep limits for aluminum connector design
were established through the test described below.
This test established
the maximum stress which can be imposed on an
aluminum connector made from Alloy 6061-T6 as being
23,000 psi which is approximately 1/2 typical yield
strength. The value of creep loss for the first
three connectors were identical; 13.3% whereas the
fourth exhibited a 25% creep loss. The explanation
is simply that the loads applied to the section
modules of each of the first three test connectors
resulted in a stress level below the elastic limit
of the material and thereby causing no creep
movement in the connector. The conductor on the
other hand must be physically moved, and this
conductor movement resulted in a redistribution of
strand displacement and resultant redistribution of
load. The 13.3% creep loss therefore can be stated
as being that movement occurring within the
conductor.
The Fourth sample,
however, evidenced in higher creep loss, resulting
from creep in the conductor and the connector since
the connector was stressed beyond its elastic limit
and contributed to the total creep of the assembly.
The confirmation by actual test of this limitation
of stress to 1/2 the materials yield strength has
been utilized in the design of all connectors
manufactured by ILSCO.
Preparation of Wire
When an aluminum cable is installed, certain
procedures should be followed to ensure a good
connection.
1. The insulation
should be stripped with a whittling motion to
prevent the cable from being nicked.
2. The cable should
then be cleaned with a wire brush. This removes
the oxides from the surface of the conductor.
3. An oxide inhibitor (De-Ox) is recommended to be applied to aluminum
conductor immediately prior to installation.
4. For mechanical connectors, the set screw should be tightened.
After a few seconds, the set screw should be
re-tightened to ensure a good connection. For
compression connectors, the lug should be
crimped around the conductor using the proper
tool.
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Instructional Information
Proper Installation of Electrical Connectors
Mechanical Connector Installation
1. Select proper size
2. Strip conductor, per installation instructions
3. Clean exposed wire
4. Apply De-Ox®
5. Insert wire into connector
6. One conductor per opening unless indicated
7. Tighten connector to recommended torque
Compression Connector Installation
1. Select proper size
2. Strip conductor, per installation instructions
3. Clean exposed wire
4. Insert Wire into connector - De-Ox® pre-loaded in connector
5. Compress connector per manufacturer recommendations
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