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Achieving (and Maintaining) Electrical Connection Tightness

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.

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