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Achieving (and Maintaining) Electrical Connection Tightness With
trilingual summary
Electrical
apparatus and system reliability depend upon the integrity of the
connections between conductors. Such terminal points are of many
types, but of most concern to maintenance workers are those involving
threaded fasteners that compress conducting surfaces together.
Acceptably low contact resistance (and therefore both voltage drop
and heat production within the connection) requires adequate fastener
torque at installation and thereafter.
Most such terminations involve either mating of flat surfaces (such
as bus bars) or the squeezing of wire within a socket or saddle as a
fastener is tightened. A properly tightened connection of either
type may become loose in service, for several reasons. One is
repeated expansion and contraction of component parts caused by
heating and cooling as current flow varies. Vibration is also a
concern. Another is corrosion, resulting in high resistance and
localized heating that leads to still further corrosion. Yet
another--particularly with aluminum--is cold flow that causes
compressive stress in the joint to gradually relieve itself.
Disagreement exists on the correct value of initial torque. Many
electricians have been taught to tighten bolts almost to the
breaking point. Others have used arbitrary increases beyond values
recommended by equipment suppliers. Although many manufacturers now
mark fasteners with a design torque value, that is not universal,
and electrical codes require only compliance with markings that do
exist. Despite those differences, authorities do agree that
calibrated torque wrenches or screwdrivers should always be used to
tighten electrical connections.
Infrared thermography is the most useful technique for determining
when a fastener has loosened. However, contrasting opinions exist
concerning remedial action. Some technologists advocate periodic
re-tightening of fasteners to some original design level, or to a
value based on their own experience. Others believe that such a
practice tends to overstress fasteners, and that (based on in-plant
tests) a better practice is to disassemble the joint, make sure
mating surfaces are flat and clean and that fasteners are in good
condition, then re-assemble it using the proper torque.
From “Achieving
(and Maintaining) Electrical Connection Tightness," published in Electrical Apparatus
January 2006
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