Cable Ferrule
A cable ferrule is a thin metal sleeve crimped onto the end of a stranded conductor before it enters a terminal. It turns the spreading strands into a single cylinder, distributes the screw pressure evenly over the whole cross-section and keeps the strands from being crushed and broken under the screw. Types with a coloured plastic collar are colour coded by cross-section.
When a stranded end without a ferrule is pushed into a screw terminal, the screw crushes only the top few strands; the strands underneath still carry current, but the contact pressure is low. Over time these strands loosen, contact resistance rises and the terminal starts to heat up. A ferrule removes this mechanism.
- Correct size: the ferrule cross-section must match the cable; strands move inside an oversized ferrule, and an undersized ferrule will not take all the strands so cross-section is lost.
- Correct tool: a ferrule crimping tool with a square or hexagonal die is used; a ferrule squashed with plain pliers gives an uncontrolled contact.
- No stray strands: the stripping length must equal the ferrule length, and no bare strand may stick out of the sleeve.
Most faults inside a panel are loose, overheated terminals; using ferrules noticeably reduces this failure. When connecting to the screw supply terminals of AXI drivers and remote controls, always terminate stranded ends with a ferrule.
Context in which this term is used: Installation, redundancy and field continuity