When stripping cables or wires, you need to use the proper tool for the job, like a good quality wire stripping tool (or a knife when working with larger conductors), and you must be careful not to nick or damage the wire.
Ensure you are using the correct hole in the stripping tool for the gauge of wire you are stripping. Damaging the wire creates a weak spot, and if the wire is bent back and forth a few times, it can crack or break the wire at the point where it has been nicked.
The following video demonstrates how easy it is to break a nicked wire.
Many electrical problems can be traced back to a poor or broken splice in conductors, and many times the problem is a broken wire resulting from damage caused to a conductor from improper stripping methods. Even if a nick in the wire doesn’t result in a complete break, compromising the integrity of the wire can impede the flow of electricity at that point.
Remember – Safety First!
Terry Peterman, the Internet Electrician