Microwaving food on a porcelain plate with a metallic rim can indeed be dangerous and should be avoided. While the shopkeeper may have claimed it was microwave-safe, the presence of any metal, including a metallic rim, can lead to sparking and potential hazards in the microwave.
Metal in a microwave can act as an antenna, attracting and concentrating electrical currents. When microwaves interact with the metal, it can cause electric arcing (sparks), which can damage the microwave and even start a fire.
Even if you've used the porcelain plate with the metallic rim a few times without any apparent damage to the food, it's still not safe. The sparks indicate that the metal is reacting to the microwaves, and while you might not notice immediate harm to the food, there could be uneven heating or damage to the microwave over time.
To ensure microwave safety, only use microwave-safe dishes and cookware that are explicitly labeled as such by the manufacturer. Microwave-safe dishes are typically made of materials like glass, ceramic, or microwave-safe plastics, which do not interact with microwaves in a harmful way.
If you suspect that your microwave has been damaged due to sparks from using inappropriate dishes, it's essential to stop using it immediately and have it inspected by a qualified technician before using it again. Your safety and the safety of others should always be a top priority when using electrical appliances like microwaves.