All appliances eventually fail. Why are there no cases of people getting burned by a microwave ovens?

Engineers / designers generally work to the idea that things should 'fail-safe'. This means that if something breaks, it breaks in a way which leaves the machine in a 'safe' condition. Fuses and circuit breakers open, rather than stay closed.

Switches tend to fail in a way that shuts off the power.

Microwaves have multiple switches. Switches have a much longer life than the magnetron, so the chance of all of the switches failing while the microwave still works is very, very low. If the microwave keeps running when you open the door, most people have the sense to close the door rather than stick their hand in.

The failure you describe is remotely possible. But most people still have a healthy respect for microwave ovens. If one didn't switch off when you opened the door, I don't think you'd reach in to it.

The energy density in a 'consumer' microwave is low enough that if exposed, you'd withdraw before any significant damage could occur.

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