  We study the size distribution of power blackouts for the Norwegian and North
American power grids. We find that for both systems the size distribution
follows power laws with exponents $-1.65 \pm 0.05$ and $-2.0 \pm 0.1$
respectively. We then present a model with global redistribution of the load
when a link in the system fails which reproduces the power law from the
Norwegian power grid if the simulation are carried out on the Norwegian
high-voltage power grid. The model is also applied to regular and irregular
networks and give power laws with exponents $-2.0\pm0.05$ for the regular
networks and $-1.5\pm0.05$ for the irregular networks. A presented mean field
theory is in good agreement with these numerical results.
