Sparking, known as arcing when it happens in a microwave, occurs when a metallic surface such as foil reflects electromagnetic waves from the magnetron on their way across the cooking cavity. Thoroughly cleaning the microwave and checking the materials you place on the turntable often helps to eliminate cases of arcing. A burning smell indicates that the oven might need cleaning; food scraps and residues heat up and slowly carbonise if they're left on the walls of the oven.
Dirt in the Microwave
Open the microwave, remove the turntable and thoroughly clean the walls with a nylon scourer and soapy water. Pay particular attention to any food splashes or stains to reduce arcing and smoke. Wash the turntable --- both the glass plate and plastic support --- in a sink of hot, soapy water and dry them before placing both back in the microwave.
Incorrect Cooking Materials
Do not microwave metal foils, plates with gold or reflective trim, cutlery or shiny ovenware. Arcing often occurs by mistake when a fork gets left in a meal or when a piece of foil clings to the edge of a plate. Carefully examine any plates for reflective trim that can cause arcing.
Incorrect Microwave Setup
Remove the metal cooking rack if you own a combination microwave and convection oven. Leaving the metal cooking rack in place in microwave mode causes arcing. The rack is intended for use with convection, broil or toast modes only.
Immediately stop and unplug the microwave if you notice sparks anywhere apart from the cooking cavity. Sparks and smoke coming out of the vents are often the result of a serious electrical fault, and you should immediately stop using the appliance until it can be examined by a professional technician.