Using 120V AC through a Breadboard (good or terrible idea?)

[paulwece]

Thanks,

But what is it about a phone charger that provides "physical & electrical separation" though? Ultimately power from 120VAC still gets transformed into 5VDC inside the phone charger so one can still consider the charger as part of an larger "overall" circuit. Just curious.

[lastchancename]

In an approved transformer/plug pack/adapter, the separate windings of the transformer provide electrical isolation, the plastic case and it's design create physical protection, the low voltage output is obvious (in conjunction with the physical & electrical isolation)
Proper grounding may also be needed if the reference or earth potential is required  in your project.

[Paul__B]

the separate windings of the transformer provide electrical isolation

Specifically, there is the requirement that there be two separate and distinct layers of insulation between the primary winding and the secondary winding - in every direction - thus double-insulated".