Yes, you can substitute a 250 volt fuse in place of a 125 volt fuse. You just can't go the other way around.
As far as 1.5 amperes in place of 3 amperes, well, you've got a problem there because the fuse will blow at about 1.5 amperes instead of 3 amperes, so the intended load will not be met.
All of this, of course, assumes that the physical size of the fuse is the same.
Bottom line is never substitute fuses.
No, use a fuse rated for the proper voltage.
Electrically yes it is safe but the pin configuration will be different.
You need to define what you mean by 2.0 and 1.6.
A fuse rated for 250 v is fine for 240 v. The fuse might be a cartridge fuse, so it must fit the available socket, or it might be fuse wire that has to be fitted in the holder correctly.
Yes you can do that as long as it will fit in the fuseholder. It is acceptable to replace a lower Voltage Rated fuse with a higher but NEVER replace a higher with a lower. There are several things to consider when replacing a fuse: 1) The minimum voltage rating of the fuse is equal to or higher than the voltage of the circuit you are placing it in 2) The Amperage rating of the fuse does not exceed the capacity of the circuit you are trying to protect. Always replace a fuse with one that has the same amperage rating. 3) The interrupting rating of the fuse is sufficent for the capacity of the circuit supplying the fuse. If replacing a fuse always use one with an interrupting capacity equal to or greater than the fuse you are replacing.
No, a 250-Amp fuse would be much larger than a 25-amp fuse. A fuse is generally designed to have a thermal runaway and melt the connection when there is too much current for too long.
a 6.3 amp 125 volt fuse a 6.3 amp 250 volt or 300 volt is fine with similar time delays depending on application it may be easier to find a 10A it should still protect most of the components
I think in most applications the difference between 6.3 amps and 7 amps is meaningless. However, if your equipment calls for 6.3 amp protection, 7 amps may allow your equipment to burn up before the fuse blows.
If the voltage is not right your radio will definitely get messed up.
Assuming the wiring is sized for 12 amps, you can replace your fuse with any 12 amp fuse or smaller and with a voltage rating at or above what you expect to connect to it. The amp rating protects the wire, so you cannot go above what the wire can handle. The voltage rating is the max voltage that it can safely protect, so you cannot use a fuse with a lower voltage rating than you expect to connect to.
4 amp, 250 volt (ASC) is what the fuse says on my blender
Yes, that is safe.