5 yr old microwave has worked perfectly - until now. Put small amount of food on a safe plate in for 45 seconds. Plate suddenly cracked, was extremely hot - and food wasn't even warmed.

It will be fine. They aren't made for sustained cooking on high power. Popcorn is dry but oiled, so it gets very hot and burns. You can use the glass carousel plate or replace it. Depending how big the crack is, any water in the crack will finish it off. A new glass tray is $57. A new Microwave the same is $319. Samsung are one of the better makes.

https://www.google.com/search?q=samsung+me18h704sfs+replacement+glass+plate

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it is megnatron plate which broke. You can replace that plate yourself . Also can can work without if safely.

Things brake and parts fail.

may be an incorrect setting capacity, try to reset time and amount capacity of food to be reheated.

What usually happens to make the plate break or shatter is heat build up in one place, caused by spillage of food stuffs that have "welded" themselves to the plate or a scratch on the plate allowing water to enter and when it gets hot explodes the plate.
There, from what you have explained and the information you have supplied, no reason to doubt the machine. A new plate and it should work again with no problem.
Hope that puts your mind at rest.

Tin foil or steel plates in a microwave will burn it out. I suspect it is or has burnt out. Never put anything steel in a microwave.

Sparks in microwaves are caused by metal things in there. What kind of plate - any metal or gold leaf decoration on it?
Try a microwave safe dish.
Make sure you do not use metal items in microwaves.
I do not know the model number of your nuker, is it a new one or old? How long have you had it, and has it always done this?

I don;t think your plate is micorwave safe. Put the plate and a glass measuring cup with water in the microwave for 1 minute, if the plate is hot and the water is not the plate is not microwave safe. IF the water is hot and the plate is not. I don't know what the problem is

Was the ceramic plate part of the microwave or a plate form the kitchen?

If it wasn't the microwave plate, it should be fine. You should be able to be sure it's all clean and use it as usual.

A microwave oven should of course never be left running unattended, especially when using any paper products.

Paper plates and paper towels can sometimes have small amounts of impurities which can ignite.

This is covered on page 1 in your owner's manual, which can be downloaded here.

We're happy to help you with free advice and we'd appreciate your thoughtful rating of our answer.

The cooling fan is a must!

Otherwise, the magnetron will be ruined, and that drives up the repair cost.

If you or a friend decide to work on it, we have *critical* safety, disassembly, and troubleshooting info at our site, and our link is at our listing here on FixYa: http://tinyurl.com/yzjozk

You can order parts here: https://www.mcappliance.com/partorder.aspx

To contact Magic Chef about getting an *authorized* servicer, see: http://www.mcappliance.com/contactus/contactus.asp

We're happy to help and we appreciate your thoughtful rating of our answer.