Isolated power supply to the DIY projects
An isolation transformer is just a normal transformer that has the same input voltage as output voltage. It is also usually fused or has a circuit breaker. You can make one by putting two transformers back-to-back such as 120:24 and 24:120. It's not usually a great thing to do unless your current requirements are modest and you have no other option. Good isolation transformers often have one or two screens between the primary and secondary windings.
As well as preventing your distribution box circuit breaker (or fuse) from blowing, that also helps prevent some kinds of electrical shocks because you need to touch two different points to be shocked rather than one point and earth.
If you plan on working with mains voltage a lot, a variac that allows you to set mains voltage higher as well as lower than the input PLUS an isolation transformer can be good. Variacs typically do not provide isolation (they are variable autotransformers).
A trick I've used sometimes with mains-powered circuits is to use a circuit breaker or fuse and add a (say) 100W incandescent bulb in series. The bulb has a low resistance for modest current, but increases greatly with higher currents so it acts as a PTC current limiter. If there is an issue, the fault current is much less than it would be through a typical circuit breaker.