If a flame sparked in the middle of your microwave while heating a plate of string beans, it is a serious safety concern and you should discontinue using the microwave immediately. This is not a normal occurrence, and it indicates a malfunction or a potential defect in the microwave.
The exact cause of the fire can vary, but some possible reasons could include:
Overheating: Microwaves are designed to heat food by emitting microwave radiation that excites water molecules in the food, generating heat. If the microwave's internal temperature rises too high due to a malfunctioning thermostat or other internal components, it could cause a fire.
Electrical issue: Faulty wiring or a damaged electrical component could cause a short circuit, leading to sparking and potentially a fire.
Insulation breakdown: Microwaves have specialized insulation to prevent the microwave radiation from leaking out. If the insulation is compromised or damaged, it could lead to arcing and sparking inside the microwave.
Regardless of the cause, the best course of action is to stop using the microwave immediately, unplug it, and have it inspected by a qualified technician or contact the manufacturer for assistance. Do not attempt to use the microwave again until it has been repaired or replaced by a professional.
It's also essential to have a fire extinguisher readily available in your kitchen and know how to use it in case of emergencies like this. Safety should always be a top priority when dealing with potential fire hazards.