  We analyze a 19-year time series of North American electric power
transmission system blackouts. Contrary to previously reported results we find
a fatter than exponential decay in the distribution of inter-occurrence times
and evidence of seasonal dependence in the number of events. Our findings
question the use of self-organized criticality, and in particular the sandpile
model, as a paradigm of blackout dynamics in power transmission systems.
Hopefully, though, they will provide guidelines to more accurate models for
evaluation of blackout risk.
