Página 798 - BURNDY

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Contact Compounds:
Petrolatum or No-
Oxid are good contact surface compounds
for aluminum, but BURNDY
a petroleum type compound containing zinc
dust, has the additional advantage of assist-
ing in the breaking down of the contact
resistance. How this is accomplished is not
certain, but it appears that the zinc particles
of PENETROX A probably act as current
bridges in the breaks in the oxide film. For
more complete information about the PENE-
TROX™ line of compounds, refer to the
Accessories Section of this catalog.
Interstrand Resistance:
The high contact
resistance due to the oxide on the strands of
an aluminum cable may be responsible for a
poor distribution of current among the
strands on the cable. Thus, the outer strands
may carry much more than their share of the
current and overheating of the cable may
result. Tests have shown that even on new
cable this effect of interstrand resistance can
be considerable unless a good contact
compound is used. The clamping action
tends to break down the oxide and force the
compound between the strands. This is par-
ticularly true of compression connectors due
to the very high unit pressures developed.
The most effective way to break down inter-
strand resistance of aluminum cable is to use
compression connectors filled with a com-
pound having zinc particles. Then, when the
end of the cable is inserted in the connector,
the compound is forced between the strands
where it very effectively breaks down the
interstrand resistance upon application of the
compressive force.
Plating Aluminum:
Plating the contact sur-
faces of aluminum connectors will prevent
the formation of aluminum oxide. Electro-tin,
cadmium and zinc platings have been used
for this purpose. However, the use of a plat-
ed aluminum conductor, does not make it
less necessary to scratch brush the alu-
minum conductor, nor does it reduce the
need for a good contact compound.
Additional problems are introduced due to
the plating on aluminum which render it of
very doubtful value over the proper use of
bare aluminum. This will be more fully dis-
cussed later.
The electrical conductivity and mechanical
strength of an electrical connection must
remain stable under the deteriorating influ-
ences of the environment. This deterioration
is corrosion. It is the electrolytic action of
moisture and other elements of the atmos-
phere in conjunction with the metals of the
connection. If the conductors and connectors
are of copper or a corrosion resistant copper
alloy, corrosion is usually a minor factor.
However, it is a very vital factor if aluminum
is involved.
If moisture can be kept away from the con-
nection, corrosion will not be a factor. The
electrical connection of a high voltage splice
on insulated cable is generally free from
corrosion since the taping must be moisture-
proof. Similarly, taping may be used to avoid
corrosion on bare cable, provided it excludes
moisture. It is difficult to get a good tape seal
to the conductor itself, especially on strand-
ed cable. If moisture does penetrate the tap-
ing, it will not dry out as readily as if the joint
were untaped. Various plastic materials are
available today for covering low voltage con-
nections or for bare conductor connections
on high voltage. Unless such coverings are
completely moisture-proof, it is better to
rely on installation with a good contact
compound, using a connector designed to
resist corrosion.
Galvanic Action:
Whenever dissimilar metals
are in the presence of an electrolyte, a differ-
ence in electric potential is developed. One
metal becomes the cathode and receives a
positive charge. The other becomes the
anode and receives a negative charge. When
these metals are in contact, an electrical cur-
rent will flow, as in the case of any short-cir-
cuited electric cell. This electrolytic action
causes an attack of the anodic metal, leaving
the cathodic metal unharmed. The extent of
the attack is proportional to the strength of
the electrolytic current, which in turn is pro-
portional to the electric potential
difference developed.
The magnitude of the potential difference
generated between two dissimilar metals can
be seen by the position of these metals in the
electrolytic series. Figure 3 is such a series.
When two metals are in contact in an elec-
trolyte, the one higher up in this series is the
anode, the corroded metal, while the one
lower is the cathode, the protected metal.
The further apart the metals are in this series,
the greater the electrolytic potential differ-
ence, and the greater the attack to the
anodic metal.
Note that copper and aluminum are quite far
apart in the series, copper being cathodic
and aluminum anodic. Hence, when alu-
minum and copper are in contact in an elec-
trolyte, the aluminum can be expected to be
severely attacked.
Crevice Corrosion:
Electrolytic attack can
also occur between like metals due to a
phenomenon known as oxygen concentra-
tion cell or crevice corrosion. Since oxygen is
necessary for corrosive action, a variation in
the concentration of oxygen where a metal is
exposed to an electrolyte will generate a
difference of potential, and cause a corrosive
attack in the oxygen starved area. Thus,
since an electrolyte in a deep crevice is freely
exposed to the air at the outside, the con-
centration of oxygen will be greatest at the
mouth of the crevice. Then corrosion can be
expected to occur in the crevice remote from
the surface. Crevice corrosion can be pre-
vented if the crevice is filled with a com-
pound to exclude moisture. Thus, within the
contact groove of an aluminum connector
containing an aluminum conductor, there will
be numerous crevices in which corrosion will
take place unless a good connector com-
pound is applied during installation. Copper,
being a more noble metal, appears to be
much less subject to crevice corrosion.
Corrosion Testing:
The effectiveness of an
electrical connection to resist corrosion can
be tested in the laboratory under conditions
designed to greatly accelerate the natural
corrosive conditions of actual service. The
most widely accepted means is the standard
salt spray chamber. In this chamber the
specimens are placed in a salt fog made by
atomizing a 20% salt solution at 100 deg. F.
Electrical as well as other manu-
facturers and utility companies, have done a
great deal of testing and a considerable area
of agreement has been reached. There are,
recommended practices. The problem is
concerned with aluminum and aluminum to
copper connections since the effect of corro-
sion on copper to copper connections is far
less serious. Let us study the recommended
Aluminum to Aluminum Connections:
joining aluminum to aluminum conductors,
there is little disagreement that an aluminum
bodied connector is the proper choice, since
this obviously eliminates the galvanic corro-
sion of dissimilar metals. However, even in
this case, care must be taken to prevent
crevice corrosion and to select an alloy of
aluminum for the connector body that is free
from cracking due to stress corrosion.
Aluminum to Copper Connections:
Similarly, for joining aluminum to copper
conductors, an aluminum bodied connector
is the best choice since it prevents galvanic
corrosion of the aluminum conductor, the
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