Screw Terminal
A screw terminal presses the conductor against the contact surface with the pressure created by a screw. Contact quality depends directly on the applied torque; under-tightening creates high resistance, while over-tightening crushes the wires and causes a loss of cross-section. In a vibrating environment it requires periodic checking.
The preload created when the screw is tightened decreases over time for three reasons: copper slowly creeping under pressure, differences in thermal expansion, and vibration loosening the screw. As the preload falls, the contact resistance rises, the connection heats up, and the heating accelerates the creep. This self-feeding cycle is among the known causes of panel fires.
- Torque: use the value given by the terminal manufacturer and apply it with a torque wrench; a generic torque table is not reliable.
- Retightening: after the first run, typically within a few days to a week, check the connections once more.
- End preparation: use a ferrule on a stranded end; on a solid end, when you see the screw mark deepening, shorten the end and renew it.
The advantage of screw terminals is that they scale up to large cross-sections and can be found anywhere. Adding an annual tightening check to your maintenance plan largely closes the one real weakness of this type.
Context in which this term is used: Installation, redundancy and field continuity