My brother unexpectedly use a metal plate in heating left over food
My brother use metal plate in microwave And its sparks,
"Most Frequent Causes for Microwave is sparking or arcing
The waveguide cover might be damaged and shorting out. Try removing the waveguide cover and see if the problem resolves. If the waveguide cover is burnt or will not stay in place, replace it.
The paint on the interior of the microwave might be chipped. If the paint is chipped, the interior metal will be exposed, causing the microwave to spark or arc. Check the interior of the microwave for any spots where the paint has chipped away. If there are any damaged spots, touch them up with microwave oven paint.
The paint on one of the rack supports might be chipped. Inspect the rack supports for any spots where the paint has chipped. If there are any damaged spots, repaint them with microwave-safe paint. If the rack support is burnt, replace it."