Terminal Tightening Torque
Tightening the terminal screw to the value specified by the manufacturer. An under-tightened connection creates contact resistance, heats up under load and loosens further over time. Over-tightening, on the other hand, strips the thread or cuts the conductor strands. It is the most common cause of heating in high-current DC connections.
The effect of contact resistance grows quickly. A transition resistance of only 5 milliohms produces 4.5 W of heat at 30 A. The heated connection expands, contracts as it cools, and this cycle loosens the screw further. In the end darkening, melted plastic and voltage drop appear.
Correct practice:
- Terminate the cable end with a ferrule or lug of the right size; do not clamp a bare stranded end directly.
- Tighten the screw to the torque stated by the manufacturer; if no value is given, tighten in steps with a torque wrench and check.
- Check again a few weeks after first commissioning and periodically thereafter.
- In vibrating installations use a lock washer or thread locker.
The most practical inspection method is to compare the temperature of the terminals with a thermal camera or a non-contact thermometer while the system is running at full load. A connection noticeably hotter than the others is either loose or oxidised.
The context in which this term is used: DC Motor Driver Selection Guide