Whenever I breadboard mains powered stuff I stay away from the typical white strip breadboard... that's only for low-voltage, low current, < 30V or so, < 500mA or so.
When I make up mains powered projects, I use copper clad board as a base, and wire it up Manhattan style, punching out circle rings for standoffs and can stack them two or three high if I need more clearance. I earth ground the main board, everything else floats above it and is point to point wired.
I've used this style for prototyping offline switchmode PSU's with DC busses up to 400V no problem. Just make sure to maintain clearance and creapage distances appropriate for the voltages at hand.
When I was younger and wired up 10KV filters and matching networks, I built it with 1.25" x 3" tall ceramic insulator standoffs screwed onto aluminum plate as a base, sliver plated copper tubing for point to point wiring, and just maintained proper spacing for the wiring, caps, inductors, etc, which were also mounted on ceramic standoffs. Everything was spaced well enough apart that there was no flashover. I actually learned this HV way first, and brought that style down to my lower-voltage work (120V, 400V, 600V)