After a kitchen remodel, my 1650 W microwave was plugged into a dedicated outlet connected to a 120/240 v circuit breaker(not tandem). Another circuit with four kitchen outlets is connected to the same breaker. My question is whether either circuit will ever be exposed to the full potential 240 volts, or whether the circuits are necessarily limited to 120 volts because they are separate. After only a few days, the microwave plugged into the dedicated line died. I am wondering if this is because it was exposed to the higher voltage or some other reason. A label in the machine says it should never be used on a circuit with more than 120 volts. Do I need to get the electrician to make changes or did the microwave die of natural causes?
A two pole breaker can be used to create a multi-wire branch circuit. It should have a 3 wire cable with a bare ground. The two hot legs would have 240 volts between them and each will have 120 volts to the neutral.
If the neutral has a bad connection toward the origin of the circuit it is possible to get strange voltages as the loads are now in series with each other between the two hot legs. So you are splitting 240 volts between the loads if the neutral is disconnected. For instance, this could cause the microwave or other loads to have 180 volts while the other loads have 60 volts. This could cause damage to your appliances.
Have the voltage on the wiring checked and have it checked for bad connections before using it any more.
Get an electrician to make the changes, as you know those wires should be on single pole breakers.
More than likely, microwave died of natural causes. It wasn't afforded more voltage in that setup, just not the correct protection to protect your house from potential fire hazard. The springshock built into the breaker takes a little more physical force to trip in case of fault.
This sounds like a multi wire branch circuit 2 hotts sharing 1 neutral each outlet that is wired with 1 hot and neutral will only have 120v with recent code changes this should be a common trip double pole breaker but in years past the breakers just needed to be on separate legs. If it was wired wrong with 2 hotts to 1 outlet there would be 240v and most appliances would go up in smoke. If both breakers were on the same leg the neutral Would be overloaded but that would not cause the microwave to fail unless the neutral opened.
Nothing wrong with that
A multi-wire branch circuit is perfectly allowed and is safe. You do need to follow the rules for such circuits.
There are many ways to wire a MWBC. One common way is as a "tee", where the two hots go separate directions at a certain point, and the neutral splits at that point and goes both ways. Often this is at or before the first outlet.
One of the most important rules is that neutrals must be pigtailed. This only applies "before (or at) the tee". Often with receptacles and whatnot, there are two neutral "screws", and you put the supply on one, and the feed downstream on the other. You can't do that with an MWBC. If you remove that receptacle, you will disconnect the neutral for both sides!
If a neutral is broken on an MWBC, it means the 120V circuits on one leg are now in series with the 120V circuits on the other. The voltage on both sides will add up to 240V, but it will not be 120V and will wobble all over the place depending on load. This will tend to fry everything.
In other words, you must make sure your neutral connections are absolutely tip-top.
In the age of cheap products, a microwave failing does not indicate a lost MWBC neutral. However, my guess is that the MWBC comes to the microwave receptacle, one side (e.g. Red) serves only that receptacle, the other side (black?) and neutral continues beyond to serve the counter receptacles. Surely the first receptacle is a GFCI. If the installer was a hack, he put both the supply and load neutrals into backstabs in the receptacle instead of pigtailing. And the line-side backstab failed under heavy load, because that's what backstabs do. That would cause the neutral problem.
So out of an abundance of caution, I would check the neutral connection both on the microwave receptacle and the service panel. If it's not pigtailed, pigtail it.
If you really want to, you can tear out the 12/3 and replace it with 12/2/2 and run redundant neutrals. But I wouldn't. MWBCs are fine if you do them right.
GFCIs are tricky
GFCI devices compare the current on the hot vs neutral wire; they must be equal, or to be more precise, they must add up to zero. (The current flows in opposite directions). If they do, then all of the current is accounted for and none is leaking through a human.
There's not a problem putting a GFCI on a MWBC, you must simply use a 2-pole GFCI device (typically a breaker) that adds current in 3 wires - the two hots and neutral. That is expensive though.
If the MWBC is wired as a "tee", with the neutral shared to a certain point then two subcircuits going off in different directions, a plain 120V GFCI can be used after the "tee". This is a very common configuration on kitchens.