Loose Terminal
The loss of contact pressure in a screw or spring connection. The contact resistance rises, heat is produced at that point, the heat loosens the connection further and the process feeds itself and can end in burning. It is the single most common cause of installation-related faults.
The mechanism is a vicious circle. The resistance of a loosening contact rises; since the heat produced at the same current is directly proportional to resistance, the point heats up. The heated metal expands, and when it cools it does not return exactly to its former position, so the pressure drops another notch.
- Causes: inadequate initial tightening, a stranded core connected without a ferrule and flattening over time, vibration, temperature cycling.
- Symptoms: yellowing or darkening on the terminal body, a smell of melting plastic, increasing voltage drop under load, intermittent operation.
- Prevention: fitting ferrules to stranded cores, following the torque given by the manufacturer with a torque wrench, preferring spring terminals in vibrating environments.
- Check: re-tightening all power connections some time after the first run.
The most reliable way to tell that a terminal has loosened is to measure the voltage between its two sides under load. Shaking it by hand or looking at the temperature is supportive but not sufficient.
The context in which this term is used: Installation, redundancy and field continuity