how to tell if a microwave fuse is bad
 

At the in-laws' house, operating the toaster and microwave at the same time blows the breaker. Been that way for decades. Today, after resetting the breaker, the microwave won't wake up; the digital screen is blank and it doesn't respond to the touch panel. I removed the case and found an 18A/250V fuse along the main wire. It has an opaque sleeve with a painted black stripe near one end. DVM reads 0.0 Ohms immediately then settles to 0.3 Ohms within 5 seconds. Is this a good fuse?

Sixto
MB-less

Would you not check for continuity? Looks like there is if there is some resistance as opposed to Infinite.

If it failed, you'd likely have infinite resistance. 0.3Ω could just be the lead resistance of the meter or the meter probes not making perfect contact with the fuse ends. How expensive are the fuses? You can always try to replace it.

The microwave works again with the same fuse. Maybe something had to cool...

Sixto
MB-less

Had that happen to me. Microwave had to cool down. Took longer and longer to cool down till it wouldn't work. Found out what was bad and it took more to buy the part than the microwave was worth.

spdrun is right. more than likely, if you shorted the leads together you would see that same 0.03. some meters have the ability to be zeroed while shorting the leads together.

There is more then a few sites on the Internet that help you troublshoot Appliances.

Of course different makes of Microwave Ovens are different. But, on My particular Microwave I had the Plastic Block that the Fuse Holder is mounted to melt enough to warp and that was enough to disconnect the Fuse.
At the same time the Buss Type Fuse Metal tips were corroded.
In My case I was able to fix it.

This is one of the places I have used.
Microwave Repair Help - Free troubleshooting and videos - RepairClinic.com

Also check the Wall Socket and if it is a GFI (not sure of exact Name) Type Wall Socket they have their own Circuit Breaker in them. The GFI Wall Socket Circuit breakers as the get older seem to trip once in a while with out an overload.
I had to replace one after 7 years use because it kept tripping more and more often. After it was replace I had no more issues.

Both appliances together demanded more current than the service wires to the plug could handle without blowing the circuit breaker out. So there was a voltage drop to the microwave. Lower available voltage/more current demanded.

This probably created some overheating of internal components. The average microwave draws 1000 watts or ten amps. A toaster perhaps 750 watts or 7.5 amps.

The cheapest fix is to fish a 12/2 wire to the plug and increase the breaker to a twenty amp one with this change. Or make sure that the two appliances are never used simulatainiously on the same circuit.

This is only sane though if the other plugs on the same circuit are separated as you do not want any remaining 14/2 on a twenty amp circuit. Today the plug by code for the microwave alone is separate. A fifteen amp circuit can service a microwave. Also refrigerators by current code have to be on a separate dedicated circuit as well.

Plugs on kitchen counters have had various code changes required over the years. If the present situation is continued the microwave might eventually be damaged. I suspect you might have triggered some self recovering thermal device in the microwave.

My Microwave is 1500 watts.

The smallest Circuit Breaker on My House is 20 Amps and I think that is the same on My Mothers House built in 1955.

These are just observations:
I have had several cases at My own and My Mothers House were Breakers in the Fuse Box becaom Old or ? and they would trip even if there was no over load. Typically when I find them the Switch lever is somewhere between on and off (but it still cut off the Electricity).

That is different then when they turn off during a over load and the Switch Lever goes all the way to off.

Installing a new Circuit Breaker fixed the above issue.

Agreed that a circuit breaker can become tired. I do not play with microwaves and did not realize some where 1500 watts.

I guess though you can buy them in many power configurations though. Just never gave it any thought. I have been putting them on self standing plugs with fifteen amp breakers. I better really start to read the backplates on the microwaves more.

It was not meant to be a criticism.
On most stuff there is a Tag that has the Watts or Amps. Also sometimes it is on the Box.

There is also large Capaciter inside that can shock you even if the Oven is un-plugged.
You either avoid it or go in the Internet and find out how to discharge it safely if you don't know how to do that.