whatA power grid in the planner's honest cartoon: triangles generate, dots consume, and power divides itself over every parallel path by Kirchhoff's laws, with no dispatcher steering it. Line thickness is loading. Lose a line and its flow reroutes instantly; any line pushed past capacity flashes, trips in turn, and the failure cascades. Consumers stranded without a generator are crossed out.

whyThe grid is built the way real ones are: N minus one safe, its capacities sized to survive the loss of any single line, and the page verifies that at load by trying every one. The claim: safety against one failure says almost nothing about two. The right pair of lines, often far apart, takes down most of the grid, and over hundreds of random pair-trips the blackout sizes come out heavy-tailed: nine in ten cost nothing, and the rest run from a few percent to three quarters of the load. The histogram collects them on log axes. Click one line, then another.

howDC power flow: a Laplacian solve per island, generators rescaled to what each island can serve, load shed where nothing reaches. Capacities are the envelope of flows over all single-line outages plus five percent, which makes N minus one safety true by construction; the page still checks it honestly. Cascades settle step by step, each overloaded set tripping together. One integer seeds the lattice, the thinning, the generators, and every random pair.