When your electric heating pad smells like burning plastic it's often a sign that the heating pad is overheating or malfunctioning.
An electric heating pad should not smell of burning plastic or emit any odor when operating and if it is emitting a burning plastic smell or other odor it's a sign that something is wrong.
Some reasons for an electric heating pad to smell of burning plastic are problems with the heating element, bad wiring or other bad internal components.
If your electric heating pad is smelling like burning plastic you should unplug the heating pad and stop using it.
The heating element in the heating pad or other internal components of the heating pad can be overheating and causing the plastic casing or insulation of the heating pad to emit a burning odor.
An electrical fault, short circuit or other electrical fault can cause the wires to overheat and melt the plastic insulation in the heating pad or even a foreign object like a piece of plastic could have become lodged near the heating element and is melting.
Even dust buildup on the heating pad could result in a burning smell when the heating pad is turned on.
It's never normal for a heating pad to emit a burning plastic smell or other burning smell.