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Arcing inside the unit is most likely caused by a buildup of food particles inside the cooking area and in some models, inside a section called the front cavity lip. You will need to clear all food out of the microwave, including the front cavity lip. For models that have the cavity lip, there is a microwave part you should invest in called a front cavity lip protector. Installing this microwave part can prevent future arcing inside your microwave.
This microwave part is inexpensive, under ten dollars for some models. Be sure to unplug the unit, clean it completely, and install the lip protector while wearing rubber-soled shoes.