Be careful with engine block heaters

I wouldn't blame the block heater, I would blame the extension cord. If you are just using a residential electric cord plugged into a regular wall outlet that was most likely the problem. I was an electrician for 48 years. I saw this at a trucking company I worked for. There was no strain relief for the cord and over a period of time they started arching which caused the outlet and extension cord to melt. I finally convinced them to install twist lock plugs and that solved the problem.