Failure analysis: So how did this happen?
Facts:
- About 2 weeks ago the original receptacle was showing signs of overheating
- Receptacle was replaced
- Cord and plug was inspected
- The fire started a few seconds AFTER the heater was turned OFF.
The cord is the standard over molded 2 conductor.
Probably due to the loose receptacle connection and subsequent overheating, the plastic in the plug started to carbonize internally thus creating a current path between the terminals, but not visible externally. As long as the heater was on, the current path was through the heater with very little current flowing between the terminals of the plug due to the relatively low resistance of the heater vs. the current path resistance between the plug terminals. (see Ohms Law for parallel resistors). Once the heater was turned off, the only current path was then between the terminals. And this immediately caused the plug to become a heater. Interestingly enough, the current stayed low enough not to trip a 15 Amp breaker.
Conclusion: The cord was damaged due to overheating. Likely due to the loose receptacle connection over time.
Prevention: In the future if a receptacle needs to be replaced due to signs of overheating, the mating over molded plug needs to be replaced also. You can't see the hidden damage inside of the plastic plug. I would recommend a good quality back wired plug. Don't plug space heaters or other high current draw devices into loose receptacles. I would recommend replacing the cheap over molded plugs on high current draw devices with a quality plug right out of the box. I will be doing this from now on.