Bond mating surfaces with areas as large as practical are desirable for several reasons. Large surface areas
maximize the cross-sectional area of the path for current and correspondingly maximizes the total number of
true metallic contacts between the surfaces. In addition to the obvious advantage of decreased bond resistance,
the current crowding which can occur during power fault conditions or under a severe lightning discharge is
lessened. Such current crowding produces a higher effective bond resistance than is present during low current
flow. The increased bond resistance raises the voltage drop across the junction to even higher values and adds
to the heat generated at the junction by the heavy current flow. Large bond areas not only lessen the factors
which contribute to heat generation, they also distribute the heat over a larger metallic area which facilitates
its removal. A further advantage of a large bond is that it will probably provide greater mechanical strength
and will be less susceptible to long term erosion by corrosive products because only a small portion of the total
bond area is exposed to the environment.
7.4.2 Direct Bonding Techniques.
Direct bonds may be either permanent or semi-permanent in nature. Permanent bonds may be defined as those
intended to remain in place for the expected life of the installation and not required to be disassembled for
inspection, maintenance, or system modifications. Joints which are inaccessible by virtue of their location
should be permanently bonded and appropriate steps taken to protect the bond against deterioration.
Many bonded junctions must retain the capability of being disconnected without destroying or significantly
altering the bonded members. Junctions which should not be permanently bonded include those which may be
broken for system modifications, for network noise measurements, for resistance measurements, and for other
related reasons. In addition, many joints cannot be permanently bonded for cost reasons. All such connections
not permanently joined are defined as semipermanent bonds. Semipermanent bonds include those which utilized
bolts, screws, rivets, clamps and other auxiliary devices for fasteners.
In terms of electrical performance, welding is the ideal method of bonding. The intense heat (in excess of
involved is sufficient to boil away contaminating films and foreign substances. A continuous metallic
bridge is formed across the joint; the conductivity of this bridge typically approximates that of the bond
members. The net resistance of the bond is essentially zero because the bridge is very short relative to the
length of the bond members. The mechanical strength of the bond is high; the strength of a welded bond can
approach or exceed the strength of the bond members themselves. Since no moisture or contaminants can
penetrate the weld, bond corrosion is minimized. The erosion rate of the metallic bridge should be comparable
to that of the base members; therefore, the lifetime of the bond should be as great as that of the bond
Welds should be utilized whenever practical for permanently joined bonds. Although welding may be a more
expensive method of bonding, the reliability of the joint makes it very attractive for bonds which will be
inaccessible once construction is completed. Most metals which will be encountered in normal construction can
be welded with one of the standard welding techniques such as gas, electric arc, Heliarc and exothermic.