Hi dudz, Panic is right with the provided information about cable structure.
I'd give you a couple of mechanical advices beside the electrical ones already mentioned.
1. If you're fitting the axis 2-6 of your Kuka robot, use a torsion cable, not just a flexible cable.
2. if you're using a kuka linear track, then you need a flexible cable for the cable carrier you must use.
3. The cable manufacturer igus, provides ready to connect kuka cables, assambled with the connector you need. This helps you saving assambling time, reduces cables storage and they come tested so you'd avoid the problem you mention here.
4. Most important part, this tool has helped me along the last three years with automation: https://echain-robot-equipment-configurator.igus.tools/start?l=en&c=En. I don't think there's a better way to prevent your robot cables from failing than this tool.