FWIW, I have been using iPhones since the iPhone 4 model with a variety of Apple, and non-Apple, charging cords. In that time, I've never experienced one catching on fire ... but, have on occasion, had a cord exhibit wear such that it exposed the wiring inside. Of course, I would stop using that cord and get a replacement.
Is it possible for a cord to be defective? Sure, but this would be more common with "lower quality" cords. For example, if a cord is poorly constructed, it could offer a point of resistance against the current running through it. This resistance would cause that point to "heat up". Potentially, then if the temperature gets high enough it could start melting the plastic outer shell to the point it may combust ... but, this is rare ... unless other factors are introduced.
Like:
- The cord being pulled out by pulling the cord itself and not at the connector ends.
- Accidentally dropping the phone while it is still connected to its charging cord.
- Coiling up the cord, incorrectly, too many times when not in use.
- Bending the cord at an extreme angle.
- Using a poorly made power adapter which would supply too much current that the iPhone's charging circuit cannot control.
As you can see, cords can be damaged by use as well. If the cord is defective to start with, this would only accelerate it failing sooner ... leading to a potential fire.
